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1 1 INTRODUCTION Unit Structure : 1.0 Objectives 1.1 Introduction 1.2 Security Trends 1.3 OSI Security Architecture 1.4 Security Attacks 1.5 Security Services 1.6 Security Mechanisms 1.7 Questions and References 1.0 OBJECTIVES After going through this unit you will be able • To understand the important and basic concepts of computer and network security • To study and practise various cryptography techniques • To identify and analyse security attacks, vulnerabilities and threats • To learn various security domains to protect assets 1.1 INTRODUCTION Use of Internet is growing exponentially in today’s world and with this global internet use, users are worried about various security parameters like what to protect, how to protect, what are various vulnerabilities, threats and attacks etc. In today’s digital globalization, the need of information security is in high. Currently, data security professionals are in more demand and definitely its importance will be growing day by day. We will focus on security policies, services and mechanisms, various types of attacks on network, data and on assets of the organization. We will also discuss various measures to be taken to adapt and monitor preventive measures. Information Security • Information security is a process or method of protecting information and information systems from unauthorised access, interruption, data alteration, disclosure, recording or inspection of information. • The information can be any form such as data over the cloud, social media data, data of any customer or supplier or any user, phone data etc. Network Security • Network security is a process or method of protecting networking infrastructure from an unauthorized access and risks and maintaining integrity and confidentiality of data and networks. munotes.in

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2 Information and Network Security • The whole world has transformed into digitization and every business wants to provide security services which protect their stakeholder’s data. • Network security helps to protect sensitive information from various attacks and also protects your assets. 1.2 SECURITY TRENDS • Implementation of digitalization in every aspect of life is not only making each one of us dependent on it but also warning us data security is a primary goal to maintain privacy. • Constant changes in technologies have shifted network security trends as virus, worms, adware, phishing, data breaching are considered to be normal activities. A. IOT a. Internet of Things (IOT) refers to physical devices connected to internet and share data. Eg. Fitness trackers, smartwatches, voice assistants etc. b. Open communication between various devices lead to unknown attacks or software bugs. B. Ransomware a. Ransomware attacks are rising. b. Hackers use ransomwares to hide their malicious code into it and earn financial gains. c. Exploits have been utilized by mobile malware with the help of social engineering tricks which can easily get access or control on infected device. C. Rise of Artificial Intelligence a. The use of Machine Learning and Artificial Intelligence is growing rapidly in the industries as it plays very important role in network security. b. Though practical implementation of AI and ML is a paramount in developing automated security systems and threat detection, there is a risk as ML and AI may get exploited by attackers as attacker are getting smarter. D. Mobile security a. Mobile threats such as spyware, vulnerabilities with android operating system, DDOS attacks, spam SMS, stealing of data causing harm to individual as well as organization which is continuously evolving. munotes.in

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Introd uction b. In this era of digital transformation intruders are constantly searching exposures and new ways to target assets of the organization. E. Cloud computing a. Though cloud offers scalability, efficiency and low cost but they could also be a prime target for cyber criminals. b. Misconfigured cloud can cause unauthorised access, data hijacking, cloud maintenance, unsecured network, weak passwords and some accidental or intentional threats. F. Social engineering and phishing a. SMS phishing, likejacking, clickjacking, SIM jacking and duplicate websites are also the threats which is worrying to users as these tricks give access to private and sensitive information to attackers. G. Social networking attacks a. Attackers try to connect to target by using social media as their medium and by spreading social network. b. Usually, cyber criminals spend a lot of time in social networking creating fake links or offers and targeting people and networks and gaining access to personal information. 1.3 THE OSI SECURITY ARCHITECTURE It is very essential to evaluate security needs of an organization effectively and to assess and select security policies accordingly. To set up parameters for information and network security appropriately for an organization, there is a need of defining security requirements properly and various methodologies towards satisfying it. OSI Security Architecture was developed as an international standard. For Information and network security, organizations need to developed security features for their services and products related to these services and mechanisms. OSI security architecture is a systematic way of defining and providing requirements of the security and it emphasises on following concepts:-

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4 Information and Network Security • Security attacks : - The actions which comprise information security of an organization • Security mechanisms :- The practises that are designed to identify, prevent and recover from a security attacks. • Security services :- Services which improve systems and data processing and transferring security and also planned to counter security attacks. 1.4 SECURITY ATTACKS Security attack is an unauthorized access to sensitive data to expose, steal or damage it. There are two types of security attacks :- • Passive attack • Active attack
1.3.1 Passive attack • Attacks which attempt to observe or make use of information but don’t affect resources of the system are called as passive attacks. • Inspection of data transmission and gaining access to data • These attacks do not modify contents of original message • So, these attacks are very difficult to detect • And that’s why it is always better choice to prevent these kinds of attacks rather than identifying and correcting them • These attacks are threats against data confidentiality Protective measures :- o Avoid disclosing sensitive and personal information over social media or online as attackers make use of them for hacking the network Security AttackPassive attackRelease of message contentTraffic AnalysisActive attackMasquerdingReplay attackModificationDenial of Service (DOS)munotes.in

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Introd uction o Use encryption for data masking so that information is unreadable by an intruder • These attacks are sub categorized into following two types :- o Release of message contents o Traffic analysis Release of message content • Unauthorized access of data • Interception of data • Disclosure of confidential information • Threat against confidentiality • To prevent such attacks, encryption technique can be used for data masking.
Traffic analysis • Observing online data traffic pattern • Analysing frequency and length of data packets • Threats against data confidentiality

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6 Information and Network Security Active attack • Attacks which attempt to modify resources of the system or their operations are called as active attack • Data alteration or modification • False stream creation • More harmful as compared to passive attacks • Threats against integrity and availability • Preventing theses attacks are very difficult as it involves high scale of software and physical vulnerabilities • The victim gets notified about this type of attack as it involves data modification Protective measures :- o Use of strong password is recommended o Use of one time password for authentication during communication o Use of session keys to prevent access once session time out. • Active attacks are classified into four types :- o Masquerading o Replay attack o Modification o Denial of Service (DOS) Masquerading • A system or a user posing as a false identity to gain access or to modify information • Also known as spoofing • One entity pretends to be other entity

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Introd uction Replay attack • Passive capturing of data packet and its transmission to produce authorized effect • In order to get benefit, repeating some actions on data and reusing captured data. This action gets performed little later than the original time Data Modification • Some part of the message is modified or the message is delayed or restructured to create unauthorized effect • Alteration in packet header eg destination address field will redirect packet to some other destination Denial of Service (DOS) • This uses authorized users from using services • Attacker intends to target on a specific system • Interruption in entire network by disabling the network • Flooding the messages on targeted system to lower down its performance • Prevents regular use of services • Incapacitating the server or targeted machine • Sometimes targeted system will be completely collapsed or shut down due to packet over flooding • Usually reported such attacks in internet related services
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8 Information and Network Security
Difference between Active attack and passive attack Active Attack Passive Attack Message modification Message observation Data packets changed Data packets unchanged Threats against integrity and
availability Threats against confidentiality Detection of attacks are
possible Prevention of such attacks are possible Resources can be changed No effect on resources Influences system resources Information or data is reviled With the help of passive
attacks, data is gathered to
make an attack Information collection is possible with
the help of chat, personal communication
or through social media Prohibition of such attacks
are very difficult Easy to prevent 1.5 SECURITY SERVICES • Services which are provided to enhance security of data processing and information sharing of an organization is called as security services. • These services are intended to mitigate security attacks. • These services make use of one or many security mechanisms. • These services are defined by x.800. • These services are provided by communicating open systems’ protocol layer • Services ensures sufficient security for systems and data communication processes.
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Introd uction • Processing or communication services provides protection to system resources Security services or principles of security are classified as follows :- Authentication • Providing the assurance of origin of identity • Establishing proof of identity • Guarantees identity proof of sender and receiver before carrying out any communication • Violating authentication is fabrication • Fabrication means failing to prove authentication • Receiver needs to check for actual sender’s identity as what is claimed by him • There can a secret key shared between parties involved in communication • Authentication can make use of digital signature • Authentication can be verified by third party also. o Peer entity  In logical connection to assure the identity of the entities connected o Data origin  In connectionless transmission to assure identity of the sender as it is claimed  Not useful while duplication or modification of data  Useful in e-mail system Security ServicesAuthenticationPeer EntityData OriginAccess ControlConfedentialityConncetionConnectionlessSelective FieldTraffic FlowAvailabilityIntergrityConnection with recoveryConnection without recoveryConnectionlessNon repudiationOriginDestinationmunotes.in

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10 Information and Network Security
Access Control • Prevention from unauthorized access to a resource • Policies that define who can access what • Limiting and controlling access of host and its applications. • If the users or services are authenticated then access controls are allotted to them. o Role management  Roll of a user  It defines which user can access what resources. o Rule management  It defines which resources can be accessible under what circumstances. • Access Control List (ACL) matrix needs to prepared which defines list of users, their roles and what they can access. Confidentiality • Assuring data privacy • Protecting unauthorized data access • Securing data transmission from passive attacks • Only intended sender and receiver should be able to see contents of the message. o Connection  Protection for all user’s data over a connection o Connectionless
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Introd uction  Protection for all user’s data over a single data block o Selective field  Confidentiality is made applicable on selective fields over a connection or on a single data block o Traffic flow  Protecting analysis of traffic flow including source, destination length, frequency or any other parameters of traffic • Data encryption is needed before data transmission so that intruder over the network won’t be able to read contents of the message. • Loss of confidentiality is known as interception.
Availability • Availability of system and its resources as and when required by an authorized entity • Depends upon proper management and system control of resources • Attack against availability is interruption

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12 Information and Network Security Integrity • Data received by the receiver is exactly same as send by the sender • Keeping data intact till it reaches to destination • Data transmission do not allow data alteration, insertion, deletion or replay • Related to active attacks so detection is more important rather than prevention • Attack against integrity is modification or alteration o Connection with recovery  Integrity of user data on a connection and discovering data alternation, insertion, deletion or replay if any  Reports stream modification and denial of service o Connection without recovery  Integrity of user data on a connection and discovering data alternation, insertion, deletion or replay if any but without data recovery o Connectionless  Integrity of user data on a connectionless communication and discovering data alternation, insertion, deletion if any.  Replay detection is possible in some cases
Non repudiation • Parties involved in communication can not refuse to sending or receiving of data
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Introd uction • Protection against refusion of identities by sender or receiver after transmitting a message • This principle of security is with respect to ownership of the message • Non repudiation can be proved with the help of authentication and integrity • Attack against non repudiation is repudiation o Origin  Message identity proof shared by the sender and after sending denying it o Destination  Message identity proof shared by the receiver and after receiving denying it
1.6 SECURITY MECHANISMS • Security mechanisms are the techniques or tools to implement security principles or security services in an organization • These mechanisms can be operated by itself, oneself, administrators or others to provide specific services • Security mechanisms are set of processes designed to detect, prevent and recover from various types of threats and attacks • Dependency on protocol layers or security services have divided security mechanisms into two categories o Specific security mechanism o Pervasive security mechanism
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14 Information and Network Security
Specific Security Mechanism • Mechanisms that are specific to any protocol layer or security services o Encipherment  Converting data to unreadable format through encryption to maintain confidentiality  For data transformation various mathematical algorithms are used  Data transformation and recovery depends on encryption keys  Difficulty level of encryption is dependent on mathematical algorithm and no of keys used  Techniques which are used in encipherment are steganography and cryptography o Digital Signature  Security achievement by appending original data with invisible digital data  Form of an electronic signature SpecificEnciphermentDigital signatureAccess ControlData IntegrityAuthentication ExchangeTraffic PaddingRouting controlNotarizationPervasiveTrusted FunctionalitySecurity LabelEvent DetectionSecurity Audit TrailSecurity Recoverymunotes.in

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Introd uction  Digital signature is added to document by sender and verified by receiver  Authenticating sender is been done by using this mechanism  Sender uses his own private key and corresponding public key will be share with receiver. Receiver on the other side uses this public key to assure authenticity of the sender who has claimed data has been sent by him  Authenticity and integrity of the data is been proved  Data forgery is protected o Access Control  Enforcing access permission for resources  Controls unauthorized access of data  Example is passwords, Pin numbers, usage of firewall  Defining role of the users and their permissions o Data Integrity  Assurance of keeping data intact during transmission  Original data is appended with a code which needs to be same during transmission and verified by sender and receiver to prove data integrity  Original data contents and appended check value is shared with receiver.  Receiver at the other end also computes new check value based on some predefined algorithm.  This newly created check value and the value which shared by sender is then compared.  If these both are same then data integrity is maintained  If these both values do not match then there is considered to be data modification while transmission of data  Detects unauthorized modification of data o Authentication Exchange  This mechanism involves identity of parties should be proved who involved in communication  Example is two-way handshaking mechanism used to establish connection  Entities involved in communication proves their identity to each other munotes.in

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16 Information and Network Security o Traffic padding  Insertion of bits into data gaps  Prevent against traffic analysis attack  Generation of fake instance of data, false communication o Routing control  Selection of specific secure routes for data transmission  Enables routing changes when security breach is suspected  Choosing and continuously changing various available routes between communicating entities  Preventing traffic analysis attack on a specific route o Notarization  Involvement of trusted third party  Interference of third party to ensure certain data exchange properties  Mediator between sender and receiver to reduce any chances of data conflict  Record maintenance of request from sender and receiver in case later denied  Prevents repudiation Pervasive Security Mechanism • Mechanisms that are not specific to any protocol layer or security services o Trusted Functionality  Supposed to be correct with respect to some criteria like as per security policies  Extending the scope or effectiveness of security mechanisms  Functionalities providing access to or directly access security mechanisms should be trustworthy o Security Label  Marking for a resource  Naming or assigning security attributes to the resource  Supplementary data associated with transferred data o Event Detection  Security related events detection  Detecting events associated with security violation munotes.in

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Introd uction o Security Audit Trail  Independent review and monitoring of system records and activities  Analysis capability of system controls  Ensuring compliance with existing policies and operational procedures  Discovering security breaches  Suggests changes in policies, controls and procedures if needed  Collection of data and its potential use to facilitate security audit o Security Recovery  Handles requests from mechanisms like management functions and event handling  Takes necessary recovery actions for applying set of rules 1.7 QUESTIONS 1. What is computer security and discuss its objectives? 2. Write a note on OSI security architecture. 3. State and explain types of active attacks. 4. Explain various principles of security. 5. List and briefly define categories of security mechanisms. REFERENCE FOR FURTHER READING • Cryptography and Network Security, Atul Kahate, Tata McGraw-Hill, 2013. • Cryptography and Network Security: Principles and Practice 5th Edition, William Stallings, Pearson,2010 • Syngress.The.Basics.of.Hacking.and.Penetration.Testing.Aug.2011 • Cryptography and Network, Behrouz A Fourouzan, Debdeep Mukhopadhyay, 2nd Edition,TMH,2011  munotes.in

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18 2 CLASSICAL ENCRYPTION TECHNIQUES Unit Structure : 2.1 Symmetric Cipher Model 2.2 Substitution Techniques 2.3 Transposition Techniques 2.4 Steganography 2.5 Questions and References 2.0 OBJECTIVES Symmetric encryption or one key encryption or conventional encryption is a type of cryptosystem where encryption and decryption process is done using same key. • An original message is called plain text. • Converted form of message is called cipher text. • Conversion of plain text to cipher text is called as encryption. • Conversion of cipher text back to plain text is called as decryption. • Study of techniques used for encryption is called as cryptography. • Schemes used for encryption is called as cryptographic system. • Tools or techniques used for message deciphering without knowing the details of enciphering is called as cryptanalysis. • Study of cryptography and cryptanalysis together is called as cryptology. 2.1 SYMMETRIC CIPHER MODEL Symmetric Cipher Model contains five components.

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Classical Encryption
Techniques • Plain text – Original data that is to be feed as input to the algorithm • Encryption algorithm – Various algorithms used to convert plain text into cipher text. These algorithms are classified as substitution and transposition algorithms. • Secret Key – The value which is input to an algorithm and independent of plain text and that algorithm. • Cipher text – Converted data that is received as output. Output is based on plain text and secret key used for algorithm. • Decryption algorithm – Reverse of an encryption algorithm. It used cipher text and secret key and generates plain text. Requirements of symmetric key cryptography • Use of strong encryption algorithm • Secret key should be kept private and secure and not shared to anyone else who are not involved in communication Characteristics of cryptography • Techniques used in encryption o Substitution – Each character of plain text is replaced by other character or symbol o Transposition – Rearrangement of characters of plain text o Multiple rounds of substitution and transposition is possible • Number of Keys used o If same key is used for encryption and decryption process by sender and receiver then process is called as  Symmetric key cryptography  Single key cryptography  Secret key cryptography  Conventional cryptography o If different keys are used for encryption and decryption process by sender and receiver then process is called as  Asymmetric key cryptography  Two key cryptography  Public key cryptography • Processing of plain text o Processing one block of input at a time and producing corresponding output block for each input block is called as block cipher. munotes.in

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20 Information and Network Security o Processing input in continuous format and producing corresponding output one element at a time as long as it goes is called as stream cipher.
Approaches for symmetric key cryptography attack • Cryptanalysis o Depends on nature of the algorithm used o Knowledge of basic features of plain text o Exploits features of an algorithm o Attempt to find plain text or secret key used o Finds weaknesses in cryptography system • Brute force attack o Trying every possible key for decryption process o Use of large keys fails this attack Basic approaches of cryptography are substitution and transposition techniques. Various methods involved under these techniques are as follows:- Cryptography Techniques Substitution Techniques Transposition Techniques 1. Caesar cipher
2. Monoalphabetic cipher
3. Playfair cipher
4. Hill cipher
5. Vigenere cipher/ Polyalphabetic
cipher
6. Vernam cipher
7. Additive cipher 1. Rail Fence
2. Columnar
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Classical Encryption
Techniques 2.2 SUBSTITUTION TECHNIQUES • The techniques in which letters of plain text are replaced by other characters, symbols or numbers are called as substitution techniques. • Plain text is considered as sequence of bits and substitution involves substitution of plain text bit pattern with cipher text bit pattern. Caesar cipher • Simplest substitution technique • Invented by Julius Caesar • Earlier known as substitution cipher • Replacement of each character of plain text by three positions down in the alphabet • Example :- A become D, B become E and so on • First row below indicates plain text and second row indicates cipher text Example • Plain Text :- WELL DONE TYCS • Cipher Text :- ZHOO GRQH WBFV Monoalphabetic Cipher • Mono means one i.e. one to one mapping between characters • Each plain text character will be replaced by different cipher text character • A become D, B become K, Z become I and so on • These replacing characters can be any set of random characters • But only one to one mapping is allowed • Plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z • Cipher: D K V Q F B J W P E S C X H T M Y A U O L R G Z N I Example • Plaintext: IF WE WISH TO REPLACE LETTERS • Cipher text: WI RF RWAJ UH YFTSDVF SFUUFYA
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22 Information and Network Security Playfair cipher • This technique uses keyword • Characters in keyword are arranged in 5x5 matrix row wise from left to right and from top to bottom • Repeated characters from keyword have to be written only once in matrix • Fill the rest spaces in matrix with remaining characters from A – Z which are not a part of keyword. • I and J can not be written separately. They need to write in same cell of matrix. For example :- Keyword BALLOON B A L O N C D E F G H I/J K M P Q R S T U V W X Y Z Encryption process • Break plain text into group of 2 characters o Eg Plain text :- MAROON o MA RO ON • If both alphabets are same add x after first alphabet o Eg Plain text :- BALLY o BA LX LY • If only one character is left add x at end o Eg. Plain text :- MORNING o MO RN IN GX • RULE 1 • If both characters from a group appears in the same row, replace with immediate right characters respectively. B A L O N C D E F G H I/J K M P Q R S T U V W X Y Z o Consider above matrix o Plain text :- ON munotes.in

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Classical Encryption
Techniques o Cipher text :- NB o Next character of O is N and Next character of N is B is same row • RULE 2 • If both characters from a group appears in the same column, replace with immediate below characters respectively. B A L O N C D E F G H I/J K M P Q R S T U V W X Y Z o Consider above matrix o Plain text :- FM o Cipher text :- MT o Character below F is M and character below M is T is same column • RULE 3 • If both characters forming a group are not in the same column or row, replace them with character on corners of rectangle. First character must be present on the same row. B A L O N C D E F G H I/J K M P Q R S T U V W X Y Z o Consider above matrix o Plain text :- CU o Cipher text :- GQ o Characters forming rectangle with C and U are G and Q respectively. We have to consider row wise. So G is in same row as C and Q is in same row as U. • Example munotes.in

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24 Information and Network Security • Keyword :- PLAYFAIR EXAMPLE P L A Y F I/J R E X M B C D G H K N O Q S T U V W Z Plain text – BALLOON Group of 2 characters BA – DP (Rule 3) LX (same characters can not be repeated) – YR (Rule 3) LO – AN (Rule 3) ON – QO ( Rule 1) Hill Cipher • Invented by L. S. Hill in 1929 • Divide input string into matrix of size n • Replace alphabets with numbers as below • Divide plain text characters into block size of 2 or 3 (preferable) • According to this block size consider matrix of 2x2 or 3x3 as secret key. • Secret key matrix can consist any numbers and should be same for each block • Matrix multiplication of secret key and plain text character conversion matrix • Resultant matrix mod 26 • Find the corresponding alphabets

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Classical Encryption
Techniques Vigenere Cipher / Polyalphabetic cipher • Consider plain text and keyword • If number of characters in keyword is less than number of characters in plain text then repeat keyword character in sequence. • Write a matrix of all alphabets 26x26 in sequence of characters • Find the mapping of one character of keyword to one character of plain text
• Plain text : STAY HOME • Keyword : TYCS STAY HOME TYCS TYCS Intersection of S to T from above matrix which is L Intersection of T to Y from above matrix which is R Cipher text :- LRCQ AMOW Vernam Cipher (One Time Pad) • One time pad (OTP) is one time shared key. • Covert plain text alpha bets into number like A= 0, B=1 and so on. • One Time Pad is set of random characters • OTP also has to be converted into numbers a b c d e f g h i j k l m n o p q r s t u v w x y z
---------------------------------------------------
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
C D E F G H I J K L M N O P Q R S T U V W X Y Z A B
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
H I J K L M N O P Q R S T U V W X Y Z A B C D E F G
I J K L M N O P Q R S T U V W X Y Z A B C D E F G H
J K L M N O P Q R S T U V W X Y Z A B C D E F G H I
K L M N O P Q R S T U V W X Y Z A B C D E F G H I J
L M N O P Q R S T U V W X Y Z A B C D E F G H I J K
M N O P Q R S T U V W X Y Z A B C D E F G H I J K L
N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
O P Q R S T U V W X Y Z A B C D E F G H I J K L M N
P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
Q R S T U V W X Y Z A B C D E F G H I J K L M N O P
R S T U V W X Y Z A B C D E F G H I J K L M N O P Q
S T U V W X Y Z A B C D E F G H I J K L M N O P Q R
T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
U V W X Y Z A B C D E F G H I J K L M N O P Q R S T
V W X Y Z A B C D E F G H I J K L M N O P Q R S T U
W X Y Z A B C D E F G H I J K L M N O P Q R S T U V
X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
Y Z A B C D E F G H I J K L M N O P Q R S T U V W X
Z A B C D E F G H I J K L M N O P Q R S T U V W X Y munotes.in

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26 Information and Network Security • Add Plain text values with OTP values • Find corresponding characters • If addition is more than 25, subtract 26 from it Additive cipher • Also called as shift cipher • Similar to modified version of Caesar cipher • Key is considered in numeric format • Plain text characters need to be shifted to the right according to specific positions • If key = 7, A  H, B  I and so on • Plain text :- S E C U R I T Y • Cipher text :- X J H Z Z N Y A 2.3 TRANSPOSITION TECHNIQUES • The techniques in which letters of plain text are rearranged or jumbled are called as transposition techniques. Rain fence cipher • Plain text is represented as sequence of diagonals • And then read as rows • Plaint text :- GOOD MORNING G O M R I G O D O N N • Cipher text :- GOMRIG ODONN
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Classical Encryption
Techniques Columnar cipher • Plain text is written in columns • Order of the columns to read characters need to be finalized • Read columns accordingly
Multicolumnar cipher • Plain text is written in columns • Order of the columns to read characters need to be finalized • Read columns accordingly • Write cipher text again in columns • And repeat above process multiple times by keeping order of columns same
• In another version one keyword will be given. • Number of characters in a keyword will be number of columns
•MulƟ columnar transposiƟon technique with four columns order 4,2,1,3:
•Order of column is 4, 2, 1, 3•WRROEOHMEOCMOOTMColumn 1Column 2Column 3Column 4EEOWOOORCHTRMMMOmunotes.in

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28 Information and Network Security • Order of reading columns is alphabetical order of keyword
2.4 STEGANOGRAPHY • The technique of hiding message to keep it secret into some image is called as steganography. • This technique provides secrecy. • Special softwares are available to perform this process. • It is a complicated process as it requires many steps to hide few bits of information into an image. • If the process or image is discovered, then this method is useless. • Example: people hide their secret messages with graphics images by replacing last two rightmost bits of each byte of the image with the two bits of the secret message. • The resulting image will not look so different and will carry secret message. • The receiver would perform the opposite by reading last two bits of each byte of that image file and reconstruct the secret message.

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Classical Encryption
Techniques 2.5 QUESTIONS 1. What are substitution techniques? Explain any one with the help of an example. 2. Explain Mono-alphabetic cipher with an example. 3. Explain Hill cipher with an example. 4. What is Polyalphabetic cipher? Explain with an example. 5. Explain Rail Fence cipher with an example. 6. Briefly define Caesar cipher with an example. 7. What are transposition techniques? Explain any one with the help of an example. 8. Define the following terms:- Brute force attack, cryptanalysis 9. Write a note on steganography. 10. Explain playfair cipher encrypt the plain text “SECRET MESSAGE” by using keyword “PROBLEM”. REFERENCE FOR FURTHER READING • Cryptography and Network Security, Atul Kahate, Tata McGraw-Hill, 2013. • Cryptography and Network Security: Principles and Practice 5th Edition, William Stallings, Pearson,2010 • Syngress.The.Basics.of.Hacking.and.Penetration.Testing.Aug.2011 • Cryptography and Network, Behrouz A Fourouzan, Debdeep Mukhopadhyay, 2nd Edition,TMH,2011  munotes.in

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30 3 BLOCK CIPHER AND THE DES Unit Structure : 3.1 Block cipher principles 3.2 DES 3.3 Strength of DES 3.4 AES 3.5 Multiple encryption and Triple DES 3.6 Block cipher modes of operations 3.7 Stream Ciphers 3.8 Questions and References 3.0 OBJECTIVES  For cryptography various types of symmetric key and asymmetric key algorithms are used.  These algorithms have two key aspects associated to them: – Algorithm Type: it defines what size of plaintext should be encrypted in each step of the algorithm. – Algorithm Mode: it defines the details of the cryptographic algorithm. 3.1 BLOCK CIPHER PRINCIPLES • In general, if we want to encrypt any book and we convert pages topic wise or content wise then it is considered as block cipher whereas when we convert pages character by character by character it is considered as stream cipher. • The conversion of plaintext to ciphertext is done in two basic ways based on which there are following two types of cryptographic algorithms: o Stream Ciphers o Block Ciphers
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Block cipher and the DES Stream Cipher • Conversion of one bit or one byte at a time • Example :- Vigenere cipher and Vernam cipher • Conversion procedure of stream cipher should must implement bit stream generator. • The main reason behind this is sender and receiver can produce cryptographic bit stream. • Generator of bit stream is key management algorithm. • It should produce a cryptographical strong bit stream. • Now, participants of communication need to share generating key so that each one can produce keystream.
Block Cipher • Plaintext is divided in to blocks and used to create cipher text of equal length. • Example :- Network based symmetric cryptographic applications • Blocks of data of predefined size is encrypted. • To connect these blocks sequentially, blocks are then chained together.
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Difference between Stream cipher and block cipher Block Cipher Stream Cipher Conversion of plain text into cipher
text by taking block at a time Conversion of plain text into cipher
text by taking one byte at a time Block size is 64 bits or more than
that Stream cipher uses 8 bits No complexities More complex Uses confusion and diffusion Uses only confusion Reverse process is difficult Reverse process is easy Algorithm modes are : - ECB
(Electronic Code Book), CBC
(Cipher Block Chaining) Algorithm modes are : - CFB
(Cipher Feedback), OFB (Output
Feedback) Works on transposition techniques Works on substitution techniques Slow process Fast process Eg Feistel cipher Eg. Vernam cipher
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Block cipher and the DES 3.2 DES (DATA ENCRYPTION STANDARD) • DES is also called as Data Encryption Algorithm. • It is a cryptographic algorithm. • This algorithm is used to provide security over three decades. • DES is basically a landmark in cryptographic algorithm. • DES is generally used in ECB (Electronic Code Book), CBC (Cipher Block Chaining), CFB (Cipher Feedback) mode. • It was invented in 1972 in United States. • This algorithm is used to give protection to systems and communication • It was previously identified as Lucifer. Working of DES • DES is a block cipher. • It encrypts input in block size of 64 bits each. • That means, 64 bits of plain text is consider as input for DES and which produces 64 bits of output which is cipher text. • The same algorithm and key are used for encryption and decryption with minor differences. • Key size of DES is 56 bits. • Actually, initial key size is 64 bits. • Before starting DES process, every 8th bit of the key is discarded to produce a 56 bits key.

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34 Information and Network Security  DES algorithm is based on following two fundamental concepts of cryptography :- – Substitution (also called as confusion) – Transposition (also called as diffusion)  There are 16 steps in DES.  Each step is called round.  Each round performs the steps of substitution and transposition.  These steps are: – During the first step, 64 bit of plain text block is assigned to an initial permutation [IP] function. – The initial permutation is implemented on plain text. – Above process produces two parts of the permuted block:  Left Plain Text (LPT)  Right Plain Text (RPT) – Now each of LPT and RPT goes through 16 rounds of encryption process. – LPT and RPT are combined. – A final permutation [FP] is implemented on the combined block. – The output of this process produces 64 bits of cipher text.

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Block cipher and the DES  Initial Permutation (IP): – It occurs only once before the first round. – Following IP table shows, how the transposition in IP should proceed. Bit position in the plain text box To be overwritten with the
content of this position 1 58 2 50 3 42 4 34 ……….. ……… 64 7 Table after transposition is as follows 58 50 42 34 26 18 10 2 60 52 44 36 28 20 12 4 62 54 46 38 30 22 14 6 64 56 48 40 32 24 16 8 57 49 41 33 25 17 9 1 59 51 43 35 27 19 11 3 61 53 45 37 29 21 13 5 63 55 47 39 31 23 15 7  After IP is completed, the resultant 64-bit permuted text block is separated into two half blocks.  Each block consists of 32 bits.  Left block is called LPT and right block is called RPT.  16 rounds are performed on these two blocks each.  Each round consists of following steps: o Key transformation o Expansion permutation o S-box substitution o P-box permutation o XOR and Swap Step 1  Key Transformation: – 56-bit key is available for each round. munotes.in

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36 Information and Network Security – During each round, from this 56-bit key, a 48-bit subkey is created using a process called key transformation. – 56-bit key is divided into two parts of 28-bits – These parts are shifted towards left by one or two positions. – For round number 1,2,9 or 16, shifting is done by one position and for rest of rounds shifting is done by two positions. – After shifting random 48 bits are selected. – This process is also known as compression permutation. Step 2  Expansion permutation: – 32-bit RPT is expanded to 48 bits as 48 bits key length is generated in step 1.  32-bit RPT is divided into 8 blocks of 4 bits each.  Next each 4-bit block is expanded to 6-bit block, by adding 2 extra bits, that is the first bit of the block 1 is the last bit of the block 8 and the last bit of the block 8 is the first bit of the 7th block  And thus 48-bit RPT is obtained.
– Above process results into expansion as well as permutation of input bits while creating output bits. – The expansion permutation is shown in the following table: 32 1 2 3 4 5 4 5 6 7 8 9 8 9 10 11 12 13 12 13 14 15 16 17 16 17 18 19 20 21 20 21 22 23 24 25 24 25 26 27 28 29 28 29 30 31 32 1 – 48-bit key is XORed with 48-bit RPT
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Block cipher and the DES – Result is considered for next step, which is S-box substitution.
Step 3 • S-box Substitution: – This step accepts 48 bits output from XOR operation of previous step and converts it into 32 bits using substitution technique. – Substitution process is performed by eight substitution boxes (S-boxes). – Each box takes 6 bits of inputs and produces 4 bits of output. – The 48-bit block text is then divided into 8 blocks of 6 bits each. – Each block is given to S-box.
– For conversion, S-box tables are used. – There are 8 S-box tables for 8 blocks. – Each table has 4 (0-3) rows and 16 (0 -15) columns.
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• Output of each S-box is combined to form a 32-bit block • This block is assigned to last stage of round, P-box Permutation. Step 4 • P-box Permutation: – Output of S-Box which is 32 bits are permuted using a p-box. – This process involves simple permutation
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Block cipher and the DES – Replacement of each bit with another bit as indicated in p-Box table without any expansion or compression. – This is called as P-box Permutation. – The P-Box is shown below. • For example, value 16 in first block specifies that bit at position 16 moves to bit at position 1 in the output. Step 5 • XOR and Swap: – LPT, which is of 32 bits, which we have not processed at all. – LPT is XORed with output of RPT. – Result of this XOR operation becomes the new RPT. – The old value of RPT becomes new LPT in process of swapping.
 Final Permutation: – Finally, at the end of all 16 rounds, Final Permutation is performed only once. – A simple transposition is based on Final Permutation Table. – Output of the Final permutation is 64-bit encrypted block. – The below is table for final permutation values.
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40 Information and Network Security 3.3 STRENGTH OF DES • Strength of DES is based on following two factors :- o Use of 56 bit key  56 bits key used in encryption  There are 256 such keys are possible  For these many number of keys brute force attack is impractical o Nature of algorithm  By exploiting characteristics of DES, cryptanalyst can perform cryptanalysis. 3.4 AES • The more popular and widely accepted and adopted symmetric encryption algorithm nowadays is Advanced Encryption Standard (AES). • At least six time faster than triple DES. • Plain text block size is 128 bits or 16 bytes • Key length can be 128, 192, 256 bits • AES processes entire block of data as a single matrix in each round using substitution and transposition. • The key which is provided as input is expanded into an array of forty four 32 bit words. • Four distinct words are considered as a key for each round. • This algorithm can be implemented on software and hardware both, so it seems to be the strongest security protocol. • Its higher key length size makes it more tough against hacking. • Its common application are e-commerce, wireless communication, financial transactions, secure data storage etc. • Four different stages are used for AES; three for substitution and one for permutation. • Byte Substitution (SubBytes) o 16 input bytes are replaced by looking into a S-box table. The result is in a matrix of four rows and four columns.
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Block cipher and the DES • Shiftrows o First row is not shifted. o Second row is shifted one (byte) position to the left. o Third row is shifted two positions to the left. o Fourth row is shifted three positions to the left. o The resultant new matrix consists of same 16 bytes but shifted with respect to each other. • MixColumns o Each column of four bytes is transformed using a special mathematical function. o This function takes input as four bytes of one column and results four completely new bytes. The output is another new matrix consisting of 16 different bytes. • Addroundkey o The 16 bytes of matrix are considered as 128 bits and are XORed with 128 bits of key.

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42 Information and Network Security 3.5 MULTIPLE ENCRYPTION AND TRIPLE DES Potential weakness of DES was brute force attack. So there was a need to design an algorithm which is completely new or make alterations in existing algorithm. Hence, Multiple encryption and triple DES arose. Multiple encryption • An encryption algorithm is implemented many times. • Plain text is converted to cipher text using any algorithm. • Cipher text generate in above step is considered as input for the second step. • Repeating above processes multiple times. Multiple DES  DES was prone to attacks due to advancement in computer hardware.  Since DES was a very capable algorithm it would be better to reuse DES rather than writing a new cryptographic algorithm.  Because of the above problems, variations of DES were introduced and are known as multiple DES. Double DES  Double DES makes use of two keys, let us assume K1 and K2.  Firstly, it performs DES process on original plain text using key K1 and generate cipher text.  DES process is repeated on cipher text generated in previous step with key K2.  The final result is encryption of encrypted text with original plain text encrypted twice with two different keys shown in figure below.  Weakness of double DES is Meet-in-the-middle attack.  This attack involves encryption from one side and decryption from the other side. Finally, matching results in middle somewhere. Triple DES  To enhance security of DES to an advance level triple DES was proposed.  This uses three stages on DES for encryption and decryption.
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Block cipher and the DES i. Triple DES with Two Keys-  Triple DES with two keys uses two keys, let us assume K1 and K2.  Key K1 used in first and third stage.  Key K2 used in second stage.  First plain text is encrypted using DES with key K1 and generate cipher text. This cipher text is then decrypted with key K2 and finally output of second step is encrypted again with key K1.  Thus having EK1(DK2(EK1(P)))EK1(DK2(EK1(P))) shown below.  This process is also called as ECE (Encrypt Decrypt Encrypt) mode.  This process proves protection from man in the middle attack.
ii. Triple DES with Three Keys-  Triple DES with three keys uses two keys, let us assume K1 K2 and K3.  First plain text is encrypted using DES with key K1 and generate cipher text.  This cipher text is then decrypted with key K2.  Output of second step is encrypted again with key K3.  Final Cipher text is the output of previous step. 3.6 BLOCK CIPHER MODES OF OPERATIONS  A basic algorithm is designed to be effective but then also various cipher modes which is also known as algorithm modes are implemented to make the algorithms more capable and efficient in secreting the patterns.
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44 Information and Network Security  An algorithm mode is the permutation of series of the basic algorithm steps on block cipher and some feedback received from the previous step.  There are 4 algorithm modes: – Electronic Code Book (ECB) – Cipher Block Chaining (CBC) – Cipher Feedback (CFB) – Output Feedback (OFB)
• Electronic Code Book (ECB) o The simplest mode of operation used for data block. o The plaintext is divided into 64-bit block each. o Same key is used to encrypt each block. o Codebook means there is a unique cipher text for every n bit block of plain text for assigned key. o If the message is longer than b bits, group the message into b bit block, and use padding for the last block if needed. o Advantages  Suitable for small amount of data  Transmission of data over network like phone lines, then such transmission will affect into blocks.  Faster and easy implementation o Disadvantages  If same b bit block is repeating then it will result into the same cipher text.  For lengthy message, ECB is not secure.
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Block cipher and the DES  No additional security measures are used so might be considered as weak
• Cipher Block Chaining (CBC) o To overcome the security issues with respect to ECB, this technique uses feedback mechanism. o Feedback method ensures if plain text block is repeating then also results into different cipher text block. o In this mode, before the encryption process starts, each plain text block is XORed with the previous cipher text block. o Each block is utilized to modify the encryption of the next block. o That means each block is rely on the corresponding current input plaintext block and also on the previous plaintext block. o The first step receives 2 inputs :-  First block of plaintext  Random block of text called Initialization vector (IV) o With the help of key, above two inputs are converted into cipher text block o Initialization vectors is a randomly produced set of characters used to make each message unique. o The second step makes use of second plaintext block XORed with the cipher text block generated in step 1 and uses the key for encryption to produce cipher text block 2. o The third step takes the third plaintext block XORed with the cipher text block of step 2 and uses the key for encryption to produce cipher text block 3. o This process lasts till all the blocks of plaintext are encrypted. o Advantages  Suitable for large amount of data  Maintains confidentiality
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46 Information and Network Security  Supports authentication  Security enhancement as compared with ECB due to use of XOR and IV o Disadvantages  Not better choice for interactivity as used might need to wait for arrival and processing of 64 bits block
• Cipher Feedback Mode (CFB) o This cipher mode is implemented for stream ciphers. o Stream cipher removes the need of padding a message and also operates in real time. o If stream is being transmitted then each character conversion can be done and use immediately for data transmission further. o In this mode data is encrypted in units which are smaller like 8 bits rather than using a fixed size block of 64 bits. o Used to encrypt any number of bits. o In this mode, like CBC, 64-bit Initialization vector(IV) is used. o Require use of Shift register is also. o The following four steps are used for this cipher mode o Assume that there are i-bits of plaintext taken at a time  Step 1: Initially, shift register is filled with 64-bit initialization vector (IV), and encryption algorithm is executed once to create 64 bits IV ciphertext.  Step 2: The left-most i-bits of encrypted IV is then XOR'ed with i-bits of plaintext which generates first portion of ciphertext (say C) and then C is transmitted to receiver.
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Block cipher and the DES  Step 3: Contents of shift register including IV are shifted left by i positions and empty rightmost i places of shift register are then filled with C.  Step 4: Steps 1 to 3 are repeated until all the plaintext units are encrypted. o Advantages  Size of plain text and cipher text is same.  More secure o Disadvantages  Due to complexities and it is more time-consuming process



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• Output Feedback Mode (OFB) o Similar to CFB. o Use of an initialization vector (IV). o Encryption of varying block sizes process includes output of the IV is fed into the next step of encryption in place of the ciphertext. o The XOR (exclusive OR) value of each plaintext block is produced independently of both the plaintext and ciphertext. o Changing of IV in same plaintext block produces different ciphertext. o Advantages  Used when there is no tolerance for error propagation  No chaining dependencies  Bit errors in transmission do not propagate o Disadvantages  Vulnerable  Less secure

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Block cipher and the DES 3.7 STREAM CIPHERS • Encrypts one byte of plain text at a time. • A key is given as an input to pseudorandom bit generator which produces a stream of random numbers. • Result of generator is called keystream which is then combined with plain text stream one byte at a time using XOR operation. • Similar to OTP (one time pad) technique. • Pseudorandom generator should produce a stream which repeats after longer period. The longer the period of repeat or longer the length of keyword, more time will be required for cryptanalysis. • If keystream contains more random numbers, more randomized cipher text it will produce and as a result cryptanalysis will be difficult. • To secure against Brute Force Attack, Key should be sufficient long enough minimum 128 bits is required. • Faster as compared to block cipher. • Less code is required as compared with block cipher.


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50 Information and Network Security 3.8 QUESTIONS 1. Explain different types of algorithm modes in details. 2. Explain with neat diagram working of DES algorithm. 3. Explain the process involved in AES. 4. Explain multiple DES. REFERENCE FOR FURTHER READING • Cryptography and Network Security, Atul Kahate, Tata McGraw-Hill, 2013. • Cryptography and Network Security: Principles and Practice 5th Edition, William Stallings, Pearson,2010 • Syngress.The.Basics.of.Hacking.and.Penetration.Testing.Aug.2011 • Cryptography and Network, Behrouz A Fourouzan, Debdeep Mukhopadhyay, 2nd Edition,TMH,2011  munotes.in

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51 4 PUBLIC KEY CRYPTOGRAPHY AND RSA Unit Structure : 4.1 Principles of Public – key cryptosystems 4.2 The RSA Algorithm 4.3 Questions and References 4.1 PRINCIPLES OF PUBLIC – KEY CRYPTOSYSTEMS • Revolution of Public Key cryptography changed the history and pattern of cryptography. • It is based on mathematical algorithms or formulas rather than using traditional methods of transposition and substitution. • Also known as Asymmetric key cryptography. • Use of two separate keys proved confidentiality and authentication • Security of this cryptography depends on key length and computations involved in breaking a cipher. • Drawback of symmetric Key cryptography is key distribution and mutual understanding for keys. Public Key Cryptosystem • Makes use of two different keys. • One key is used for encryption and only the other corresponding key should be used for decryption. • No other key can decrypt the message – not even the original (i.e. the first) key used for encryption. • Every communicating party requires just a key pair for communicating with any number of other communicating parties. • It should be impracticable to find decryption key provided with the knowledge of algorithm and encryption key.

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52 Information and Network Security Components of Public Key Cryptosystem • Plain Text o A normal readable message which is considered as input for the process of encryption • Encryption Algorithm o Sequence of steps which are used to convert plain text into cipher text • Public key and private key o Key pair which is selected for asymmetric key cryptography. o One key is used in encryption process and other will be used for decryption process. • Cipher Text o The resultant text which is produced after implementing encryption algorithm. • Decryption Algorithm o Sequence of steps which are used to convert cipher text back into plain text with the help of matching key Steps involved in Asymmetric key cryptography • Suppose user A wants to send a message to user B. • Then A and B should each have a private key and a public key. o A should keep her private key secret. o B should keep her private key secret. o A should inform B about her public key. o B should inform A about her public key. – When A wishes to send a message to B, A encrypts the message using B’s public key. – This is possible because A knows B’s public key. – A sends the message encrypted with B’s public key to B.
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Public key cryptography
and RSA – When B receives the message, B decrypts A’s message with B’s private key. – Only B knows her private key and the message could be decrypted only with B’s private key. – No one else or no other recipient would be able to make the sense of the message content even if one is able to intercept the message. – All participants involved in communication gets access to public keys and private keys are created by each participant locally. – Incoming communication is considered to be secure as long as private key is kept secure and protected. – A system can change private key and corresponding public keys can be generated and distributed.
Difference between Symmetric key cryptography and Asymmetric key cryptography

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54 Information and Network Security Applications • Encryption / Decryption – Encryption of the message by the sender by receiver’s public key • Digital signature – Sender uses his private key to sign message. It proves integrity and confidentiality of the message. • Key exchange – Session keys are exchanged to provide privacy. Requirements • It should be feasible for receiver to produce a pair of public key and private key. • It should be feasible for sender by knowing public key and message to be encrypted to produce cipher text. • It should be feasible for receiver B to decrypt cipher text using private key to recover plain text. • It should be infeasible for others to find or know the private key by knowing public key. • It should be infeasible for other to find plain text by knowing public key and cipher text. 4.2 THE RSA ALGORITHM • Invented by Rivest, Shamir & Adleman of MIT in 1977. • Widely used as public key cryptography. • Use of two keys :- Public and private key • One key which is known to be as public key is sent to other who are involved in communication and other key is kept secret. • Based on block cipher. • Plain text is encrypted in blocks. • Public and private key are based on random prime numbers. • Assume random prime numbers are P and Q. • N = P * Q • Sender and receiver should know the value of N. • Public key which is known as encryption key E is a number which should not be a factor of (P – 1) (Q – 1). • For encryption, Calculating Cipher Text (CT) with input of Plain Text (PT), munotes.in

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Public key cryptography
and RSA • CT = PTE mod N • For decryption, D is decryption key. • PT = CTD mod N • It should be possible to find N, E, D for all plain text. • It should be easy to calculate PTE mod N and CTD mod N. • It should be infeasible to calculate D, given values of E and N. Example • Select prime numbers. • P = 7, Q = 17 • N = P * Q = 7 * 17 = 119 • ( P – 1)( Q – 1) = 6 * 16 = 96 • 2, 3 and 4 are factors of 96. • So, let us consider 5 as E. • Assume, PT = 4. • CT = PTE mod N = 45 mod 119 = 1024 mod 119 = 72 • CT = 72 Security of RSA Possible attacks on RSA are :- • Brute Force o Trying all possible combinations to find keys • Mathematical attack o Factoring products of two primes o If value of P and Q are known using N then its easy to find private key. • Timing attack o Running time of decryption algorithm • Chose cipher text attack o Exploits properties of RSA • Short message attack o Attacker knows few blocks of plain text and tries to decrypt cipher text. o Padding of plain text before encryption to avoid such attacks munotes.in

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56 Information and Network Security • Cycling attack o Plain text is converted into cipher text continuously by counting iterations till the occurrence of plain text. • Unconcealed message attack o In few cases it is found that cipher text is same as plain text which means plain text is not hidden. 4.3 QUESTIONS 1. State and explain with example steps involved in RSA algorithm. 2. Using RSA algorithm, find the value of cipher text C, if the plaintext M = 5, p = 3, q = 11, d = 7. 3. What are the components of simple symmetric cipher model? 4. Differentiate between symmetric and asymmetric key cryptography. REFERENCE FOR FURTHER READING • Cryptography and Network Security, Atul Kahate, Tata McGraw-Hill, 2013. • Cryptography and Network Security: Principles and Practice 5th Edition, William Stallings, Pearson,2010 • Syngress.The.Basics.of.Hacking.and.Penetration.Testing.Aug.2011 • Cryptography and Network, Behrouz A Fourouzan, Debdeep Mukhopadhyay, 2nd Edition,TMH,2011  munotes.in

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57 5
KEY MANAGEMENT
Unit Structure :
5.0 Objective
5.1 Introduction
5.2 Types of keys
5.3 Public -Key Cryptosystems
5.4 Types Public -Key Cryptosystems
5.4.1 RSA algorithm (Rivest -Shamir -Adleman)
5.4.1.1 Algorithms for generating RSA keys
5.4.1.2 Example
5.4.1.3 RSA Encryption
5.4.1.4 RSA Decryption
5.4.1.5 RSA Analysis
5.4.1.6 RSA security
5.4.2 ElGamal Cryptosystem
5.4.2.1 ElGamal Analysis
5.4.3 Elliptic Curve Cryptography (ECC)
5.4.3.1 RSA and ElGamal Schemes – A Comparison
5.5 Key Manag ement
5.5.1 Why is Key Management Important
5.5.2 Types of Keys
5.5.3 How Key Management Works
5.6 Diffie -Hellman Key Exchange
5.6.1 PRACTICE PROBLEMS
5.6.2 Establishing a shared key between multiple parties
5.6.3 Why is the Diffie -Hellman key excha nge secure?
5.6.4 Authentication & the Diffie -Hellman key exchange
5.6.5 Variations of the Diffie -Hellman key exchange
5.7 Summary
5.8 Questions
5.9 Reference for further reading munotes.in

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58 Information and Network Security 5.0 OBJECTIVE
Strong data encryption requires encryption key management. Being able
to own and manage your encryption keys is crucial to meet
compliance standards and to satisfy regulatory sovereignty
requirements. Key management additionally ensures regulatory
compliance and secures data from risks posed by privileged users. An
effective key management solution also ensures that keys and their
policies can be stored in an appliance that remains in full control of
security teams, and not the storage administrators.
With the increasing dependence on cryptography to protect digital assets
and communications, the ever -present vulnerabilities in modern
computing systems, and the growing sophistication of cyber attacks, it has
never been more important, nor more challenging, to keep your
cryptographic keys safe and secure. A single com promised key could lead
to a massive data breach with the consequential reputational damage,
punitive regulatory fines and loss of investor and customer confidence.
5.1 INTRODUCTION
Cryptography lies at the heart of the modern business - protecting
electro nic communications and financial transactions, maintaining the
privacy of sensitive data and enabling secure authentication and
authorization. New regulations like GDPR and PSD2 , the commercial
pressure for digital transformation, the adoption of cloud technology and
the latest trends in IoT and blockchain/DLT all help drive the need to
embed cryptogra phy into virtually every application – from toasters to
core banking systems!
The good news is that modern cryptographic algorithms, when
implemented correctly, are highly -resistant to attack – their only weak
point is their keys. However, if a key is comp romised, then it’s game over!
This makes such cryptographic keys one of your company’s most precious
assets, and they should be treated as such. The value of any key is
equivalent to the value of all the data and/or assets it is used to protect.
5.2 TYPES OF KEYS
There are three primary types of keys that need to be kept safe and secure:
1. Symmetric keys – typically used to encrypt bulk data with
symmetric algorithms like 3DES or AES; anyone with the secret key
can decrypt the data
2. Private keys – the s ecret half of public/private key pairs used in
public -key cryptography with asymmetric algorithms like RSA or
ECDSA; anyone with the private key can impersonate the owner of
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59 Key Management systems or ge nerate a fraudulent digital signature that appears
authentic
3. Hash keys – used to safeguard the integrity and authenticity of data
and transactions with algorithms like HMAC -SHA256; anyone with
the secret key can impersonate the originator of the data/tr ansactions
and thus modify the original data/transactions or create entirely false
data/transactions that any recipient will believe is authentic
5.3 PUBLIC -KEY CRYPTOSYSTEMS
Public -key cryptography, or asymmetric cryptography, is an encryption
scheme that uses two mathematically related, but not identical, keys - a
public key and a private key. Unlike symmetric key algorithms that rely
on one key to both encrypt and decrypt, each key performs a unique
function. The public key is used to encrypt and the pri vate key is used to
decrypt.
It is computationally infeasible to compute the private key based on the
public key. Because of this, public keys can be freely shared, allowing
users an easy and convenient method for encrypting content and verifying
digital s ignatures, and private keys can be kept secret, ensuring only the
owners of the private keys can decrypt content and create digital
signatures.
Since public keys need to be shared but are too big to be easily
remembered, they are stored on digital certific ates for secure transport and
sharing. Since private keys are not shared, they are simply stored in the
software or operating system you use, or on hardware (e.g., USB token,
hardware security module) containing drivers that allow it to be used with
your s oftware or operating system.
Symmetric cryptography was well suited for organizations such as
governments, military, and big financial corporations were involved in the
classified communication.
With the spread of more unsecure computer networks in last fe w decades,
a genuine need was felt to use cryptography at larger scale. The symmetric
key was found to be non -practical due to challenges it faced for key
management. This gave rise to the public key cryptosystems.
The process of encryption and decryption is depicted in the following
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The most important properties of public key encryption scheme are −
 Different keys are used for encryption and decryption. This is a
property which set this scheme different than symmetric encryption
scheme.
 Each receiver possesses a unique decryption key, generally referred
to as his private key.
 Receiver needs to publish an encryption key, referred to as his
public key.
 Some assurance of the authenticity of a public key is needed in this
scheme to avoid spoofi ng by adversary as the receiver. Generally,
this type of cryptosystem involves trusted third party which certifies
that a particular public key belongs to a specific person or entity
only.
 Encryption algorithm is complex enough to prohibit attacker from
deducing the plaintext from the ciphertext and the encryption
(public) key.
 Though private and public keys are related mathematically, it is not
be feasible to calculate the private key from the public key. In fact,
intelligent part of any public -key cryptos ystem is in designing a
relationship between two keys.
5.4 TYPES PUBLIC -KEY CRYPTOSYSTEMS
5.4.1 RSA algorithm (Rivest -Shamir -Adleman)
The RSA algorithm is the basis of a cryptosystem -- a suite of
cryptographic algorithms that are used for specific securit y services or
purposes -- which enables public key encryption and is widely used to
secure sensitive data, particularly when it is being sent over an insecure
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61 Key Management RSA was first publicly described in 1977 by Ron Rivest, Adi Shamir and
Leonard Adleman of the Massachusetts Institute of Technology, though
the 1973 creation of a public key algorithm by British mathematician
Clifford Cocks was kept classified by the U.K.'s GCHQ until 1997.
In RSA cryptography, both the public and the pri vate keys can encrypt a
message; the opposite key from the one used to encrypt a message is used
to decrypt it. This attribute is one reason why RSA has become the most
widely used asymmetric algorithm : It provides a method to assure the
confidentiality, integrity, authenticity, and non-repudiation of electronic
communications and data storage.
Many protocols lik e secure shell, OpenPGP, S/MIME, and SSL/TLS rely
on RSA for encryption and digital signature functions. It is also used in
software programs -- browsers are an obvious example, as they need to
establish a secure connection over an insecure network, like the internet,
or validate a digital signature. RSA signature verification is one of the
most commonly performed operations in network -connected systems.
Algorithm
The RSA algorithm holds the following features −
 RSA algorithm is a popular exponentiation in a finite field over
integers including prime numbers.
 The integers used by this method are sufficiently large making it
difficult to solve.
 There are two sets of keys in this algor ithm: private key and public
key.
You will have to go through the following steps to work on RSA
algorithm −
Step 1: Generate the RSA modulus
The initial procedure begins with selection of two prime numbers namely
p and q, and then calculating their produc t N, as shown −
N=p*q
Here, let N be the specified large number.
Step 2: Derived Number (e)
Consider number e as a derived number which should be greater than 1
and less than (p -1) and (q -1). The primary condition will be that
there should be no common fac tor of (p -1) and (q -1) except 1
Step 3: Public key
The specified pair of numbers n and e forms the RSA public key and it is
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62 Information and Network Security Step 4: Private Key
Private Key d is calculated from the numbers p, q and e. The mathematical
relationship between the numbers is as follows −
ed = 1 mod (p -1) (q -1)
The above formula is the basic formula for Extended Euclidean
Algorithm, which takes p and q as the input parameters.
Encryption Formula
Consider a sender who sends the plain text message to someone whose
public key is (n,e). To encrypt the plain text message in the given scenario,
use the following syntax −
C = Pe mod n
Decryption Formula
The decryption process is very straightforward and includes analytics for
calculation in a systematic approach. Considering receiver C has the
private key d, the result modulus will be calculated as −
Plaintext = Cd mod n
Generating RSA keys
The following steps are involved in generating RSA keys −
 Create two large prime numbers namely p and q. The product of
these numbers wil l be called n, where n= p*q
 Generate a random number which is relatively prime with (p-
1) and (q-1). Let the number be called as e.
 Calculate the modular inverse of e. The calculated inverse will be
called as d.
5.4.1.1 Algorithms for generating RSA keys
We need two primary algorithms for generating RSA keys using Python
− Cryptomath module and Rabin Miller module .
1. Cryptomath Module
The source code of cryptomath module which follows all the basic
implementation of RSA algorithm is as follows −
def gcd(a, b):
while a != 0:
a, b = b % a, a
return b
def findModInverse(a, m): munotes.in

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63 Key Management if gcd(a, m) != 1:
return None
u1, u2, u3 = 1, 0, a
v1, v2, v3 = 0, 1, m
while v3 != 0:
q = u3 // v3
v1, v2, v3, u1, u2, u3 = (u1 - q * v1), (u2 - q * v2), (u3 - q *
v3), v1, v2, v3
return u1 % m
2. RabinMiller Module
The source code of RabinMiller module which follows all the basic
implementation of RSA algorithm is as follows −
import random
def rabinMiller(num):
s = num - 1
t = 0
while s % 2 == 0:
s = s // 2
t += 1
for trials in range(5):
a = random.randrange(2, num - 1)
v = pow(a, s, num)
if v != 1:
i = 0
while v != (num - 1):
if i == t - 1:
return False
else:
i = i + 1
v = (v ** 2) % num
return True
def isPrime(n um):
if (num 7< 2):
return False munotes.in

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64 Information and Network Security lowPrimes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47,
53, 59, 61,
67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137,
139, 149, 151,
157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227,
229, 233, 239, 241,
251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313,317,
331, 337, 347, 349,
353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431,
433, 439, 443, 449,
457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541,
547, 557, 563, 569,
571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643,
647, 653, 659, 661,
673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757,
761, 769, 773, 787,
797, 809, 811, 821 , 823, 827, 829, 839, 853, 857, 859, 863, 877,
881, 883, 887, 907,
911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997]
if num in lowPrimes:
return True
for prime in lowPrimes:
if (num % prime == 0):
return False
return rabinMiller(num)
def generateLargePrime(keysize = 1024):
while True:
num = random.randrange(2**(keysize -1), 2**(keysize))
if isPrime(num):
return num
The complete code for generating RSA keys is as follows −
import random, sys, os, rabinMiller, cryptomath

def main():
makeKeyFiles('RSA_demo', 1024) munotes.in

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65 Key Management def generateKey(keySize):
# Step 1: Create two prime numbers, p and q. Calculate n = p * q.
print ('Generating p prime...')
p = rabinMiller.generateLargePrime(keySize)
print('Generating q prime...')
q = rabinMiller.generateLargePrime(keySize)
n = p * q
# Step 2: Create a number e that is relatively prime to (p -1)*(q -1).
print('Generat ing e that is relatively prime to (p -1)*(q -1)...')
while True:
e = random.randrange(2 ** (keySize - 1), 2 ** (keySize))
if cryptomath.gcd(e, (p - 1) * (q - 1)) == 1:
break
# Step 3: Calculate d, the mod inverse of e.
print('C alculating d that is mod inverse of e...')
d = cryptomath.findModInverse(e, (p - 1) * (q - 1))
publicKey = (n, e)
privateKey = (n, d)
print('Public key:', publicKey)
print('Private key:', privateKey)
return (publicKey, privateKey)
def mak eKeyFiles(name, keySize):
# Creates two files 'x_pubkey.txt' and 'x_privkey.txt'
(where x is the value in name) with the the n,e and d,e integers
written in them,
# delimited by a comma.
if os.path.exists('%s_pubkey.txt' % (name)) or
os.pat h.exists('%s_privkey.txt' % (name)):
sys.exit('WARNING: The file %s_pubkey.txt or %s_privkey.txt
already exists! Use a different name or delete these files and re -run
this program.' % (name, name))
publicKey, privateKey = generateKey(keySize)
print()
print('The public key is a %s and a %s digit number.' %
(len(str(publicKey[0])), len(str(publicKey[1])))) munotes.in

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66 Information and Network Security print('Writing public key to file %s_pubkey.txt...' % (name))
fo = open('%s_pubkey.txt' % (name), 'w')
fo.write('%s,%s,%s' % (keySize , publicKey[0], publicKey[1]))
fo.close()
print()
print('The private key is a %s and a %s digit number.' %
(len(str(publicKey[0])), len(str(publicKey[1]))))
print('Writing private key to file %s_privkey.txt...' % (name))
fo = open('%s_privke y.txt' % (name), 'w')
fo.write('%s,%s,%s' % (keySize, privateKey[0], privateKey[1]))
fo.close()
# If makeRsaKeys.py is run (instead of imported as a module) call
# the main() function.
if __name__ == '__main__':
main()
Output
The public key and pr ivate keys are generated and saved in the respective
files as shown in the following output.


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67 Key Management 5.4.1.2 Example
An example of generating RSA Key pair is given below. (For ease of
understanding, the primes p & q taken here are small values. Practically,
these values are very high).
 Let two primes be p = 7 and q = 13. Thus, modulus n = pq = 7 x 13
= 91.
 Select e = 5, which is a valid choice since there is no number that is
common factor of 5 and (p − 1)(q − 1) = 6 × 12 = 72, except for 1.
 The pair of numbers (n, e) = (91, 5) forms the public key and can be
made available to anyone whom we wish to be able to send us
encrypted messages.
 Input p = 7, q = 13, and e = 5 to the Extended Euclidean Algorithm.
The output will be d = 29.
 Check that the d calculated is correct by computing −
de = 29 × 5 = 145 = 1 mod 72
 Hence, public key is (91, 5) and private keys i s (91, 29).
5.4.1.3 RSA Encryption
 Suppose the sender wish to send some text message to someone
whose public key is (n, e).
 The sender then represents the plaintext as a series of numbers less
than n.
 To encrypt the first plaintext P, which is a number mod ulo n. The
encryption process is simple mathematical step as −
C = Pe mod n
 In other words, the ciphertext C is equal to the plaintext P multiplied
by itself e times and then reduced modulo n. This means that C is
also a number less than n.
 Returning to o ur Key Generation example with plaintext P = 10, we
get ciphertext C −
C = 105 mod 91
5.4.1.4 RSA Decryption
 The decryption process for RSA is also very straightforward.
Suppose that the receiver of public -key pair (n, e) has received a
ciphertext C.
 Rece iver raises C to the power of his private key d. The result
modulo n will be the plaintext P.
Plaintext = Cd mod n
 Returning again to our numerical example, the ciphertext C = 82
would get decrypted to number 10 using private key 29 −
Plaintext = 8229 mod 91 = 10 munotes.in

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68 Information and Network Security 5.4.1.5 RSA Analysis
The security of RSA depends on the strengths of two separate functions.
The RSA cryptosystem is most popular public -key cryptosystem strength
of which is based on the practical difficulty of factoring the very large
numbers.
 Encryption Function − It is considered as a one -way function of
converting plaintext into ciphertext and it can be reversed only with
the knowledge of private key d.
 Key Generation − The difficulty of determining a private key from
an RSA public key is equivalent to factorin g the modulus n. An
attacker thus cannot use knowledge of an RSA public key to
determine an RSA private key unless he can factor n. It is also a one
way function, going from p & q values to modulus n is easy but
reverse is not possible.
If either of these two functions are proved non one -way, then RSA will be
broken. In fact, if a technique for factoring efficiently is developed then
RSA will no longer be safe.
The strength of RSA encryption drastically goes down against attacks if
the number p and q are no t large primes and/ or chosen public key e is a
small number.
5.4.1.6 RSA security
RSA security relies on the computational difficulty of factoring large
integers. As computing power increases and more efficient factoring
algorithms are discovered, the abi lity to factor larger and larger numbers
also increases.
Encryption strength is directly tied to key size, and doubling key length
can deliver an exponential increase in strength, although it does impair
performance. RSA keys are typically 1024 - or 2048 -bits long, but experts
believe that 1024 -bit keys are no longer fully secure against all attacks.
This is why the government and some industries are moving to a minimum
key length of 2048 -bits.
5.4.2 ElGamal Cryptosystem
The ElGamal encryption system is a pu blic key encryption algorithm by
Taher Elgamal in 1985 that is based on the Diffie -Hellman key exchange.
We give an introduction to the ElGamal Encryption System and an
example in the video
We assume that the message m that Alice encrypts and sends to Bob is an
integer. We describe the three components of ElGamal encryption, namely
key generation, encryption, and decryption.
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69 Key Management Bob: Key Generation
To generate his private key and his public key Bob does the following.
1. Bob chooses a prime p and and a generat or
.
2. Bob chooses a random .b ∈ N.
3. Bob computes

4. Bob publishes his public key ,p,g, B in the key directory.
Alice: Encryption
To encrypt a message
Alice does the following.
1. Alice gets Bob's public key ,p, ,g, B from the key directory.
2. Alice chooses a random .a∈N.
3. Alice computes the shared secret .

4. Alice computes .

5. Alice encrypts m by computing .X=m ⊗ s.
6. Alice sends (A,X) to Bob.
Bob: Decryption
The information available to Bob to decrypt a message are his private
key b and his public key consisting of the prime ,p, the
generator ,g, and .B=gb. To decrypt a message (A,X) Bob does the
following.
1. Bob receives (A,X) from Alice.
2. Bob computes the shared secret .

3. Bob computes the inverse
of s in

4. Bob decrypts the message by computing

We now show that the message M received by Bob is equal to
Alice's plain text message .m. We have

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70 Information and Network Security

We work through a small example.


5.4.2.1 ElGamal Analysis
In ElGamal system, each user has a private key x. and has three
components of public key − prime modulus p, generator g, and public Y =
gx mod p . The strength of the ElGamal is based on the difficulty of
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71 Key Management The secure key size is generally > 1024 bits. Today even 2048 bits long
key are used. On the proce ssing speed front, Elgamal is quite slow, it is
used mainly for key authentication protocols. Due to higher processing
efficiency, Elliptic Curve variants of ElGamal are becoming increasingly
popular.
5.4.3 Elliptic Curve Cryptography (ECC)
Elliptic Curve Cryptography (ECC) is a term used to describe a suite of
cryptographic tools and protocols whose security is based on special
versions of the discrete logarithm problem. It does not use numbers
modulo p.
ECC is based on sets of numbers that are associated with mathematical
objects called elliptic curves. There are rules for adding and computing
multiples of these numbers, just as there are for numbers modulo p.
ECC includes a variants of many cryptographic schemes that were initially
designed for modular n umbers such as ElGamal encryption and Digital
Signature Algorithm.
It is believed that the discrete logarithm problem is much harder when
applied to points on an elliptic curve. This prompts switching from
numbers modulo p to points on an elliptic curve. A lso an equivalent
security level can be obtained with shorter keys if we use elliptic curve -
based variants.
The shorter keys result in two benefits −
 Ease of key management
 Efficient computation
These benefits make elliptic -curve -based variants of encrypti on scheme
highly attractive for application where computing resources are
constrained.
5.4.3.1 RSA and ElGamal Schemes – A Comparison
Let us briefly compare the RSA and ElGamal schemes on the various
aspects. RSA ElGamal It is more efficient for encrypti on. It is more efficient for decryption. It is less efficient for decryption. It is more efficient for decryption. For a particular security level,
lengthy keys are required in RSA. For the same level of security, very
short keys are required. It is wid ely accepted and used. It is new and not very popular in
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72 Information and Network Security 5.5 KEY MANAGEMENT
Cryptographic keys are a vital part of any security system. They do
everything from data encryption and decryption to user authentication.
The compromise of any cryptographic key could lead to the collapse of an
organization’s entire security infrastructu re, allowing the attacker to
decrypt sensitive data, authenticate themselves as privileged users, or give
themselves access to other sources of classified information. Luckily,
proper management of keys and their related components can ensure the
safety of confidential information. Key Management is the process of
putting certain standards in place to ensure the security of cryptographic
keys in an organization. Key Management deal with the creation,
exchange, storage, deletion, and refreshing of keys. They also deal with
the members access of the keys.
5.5.1 Why is Key Management Important
Key management forms the basis of all data security. Data is encrypted
and decrypted via the use of encryption keys, which means the loss or
compromise of any encryption key would invalidate the data security
measures put into place. Keys also ensure the safe transmission of data
across an Internet connection. With authentication methods, like code
signing, attackers could pretend to be a trusted service like Microsoft,
while giving victim’s computers malware, if they steal a poorly protected
key. Keys provide compliance with certain standards and regulations to
ensure companies are using best p ractices when protecting cryptographic
keys. Well protected keys are only accessible by users who need them.
5.5.2 Types of Keys
There are two types of cryptographic keys, symmetric and asymmetric
keys. Symmetric keys deal with data-at-rest, which is data stored in a
static location, such as a database. Symmetri c key encryption uses the
same key for both encryption and decryption. Using data in a database as
an example, while the data is stored in the database, it is encrypted with
the symmetric key. Once an authorized user attempts to access the data,
the inform ation is decrypted with the same symmetric key and made
accessible to the user. The other type of cryptographic key is an
asymmetric key.
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73 Key Management Encryption using asymmetric keys is a little more complicated than
symmetric key encryption. Instead of using the sa me key for both
encryption and decryption, two separate keys called a public and private
key, are used for the encryption and decryption of data. These keys are
created as a pair, so that they relate to each other. The public key of a pair
of asymmetric ke ys is mainly used to encrypt data. This key can be shared
with anyone since it encrypts, not decrypts, data. The private key is used
for the decryption of data encrypted by its public key counterpart, so it
must stay secure.
Asymmetric keys focus on encryp ting data-in-motion . Data -in-motion is
data sent across a network connection, whether it be a public or private
connection. When transporting sensitive data, most encryption processes
use both symmetric and asymmetric keys to encrypt data.
 The data is first encrypted -at-rest by a symmetric encryption key.
 The symmetric key is now encrypted by the public key of the person
who the data is being sent to. That encrypted symmetric key and
the ciphertext are sent to the recipient of the data.
 Once the ciphertext and key reach the recipient, the symmetric key
is decrypte d by that user’s private key, and the ciphertext is
decrypted.
5.5.3 How Key Management Works
Key management follows a lifecycle of operations which are needed to
ensure the key is created, stored, used, and rotated securely. Most
cryptographic keys follow a lifecycle which involves key
 Generation
 Distribution
 Use
 Storage
 Rotation
 Backup/Recovery
 Revocation
 Destruction
The generation of a key is the first step in ensuring that key is secure. If
the key in question is generated with a weak encryption algorit hm, then
any attacker could easily discover the value of the encryption key. Also, if
the key is generated in an insecure location, the key could be
compromised as soon as it is created, resulting in a key that cannot be munotes.in

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74 Information and Network Security safely used for encryption. Key ge nerators, AES encryption algorithms, or
random number generators tend to be used for secure key generation.
The next step of the key lifecycle is ensuring the safe distribution of the
keys. Keys s hould be distributed to the required user via a secure TLS or
SSL connection, to maintain the security of the keys being distributed. If
an insecure connection is used to distribu te the cryptographic keys, then
the security of any data encrypted by these keys is in question, as an
attacker could execute a man -in-the-middle attack and steal the keys.
After distribution of the key, it is used for cryptographic operations. As
previous ly noted, the key should only be used by authorized users, to
make certain the key is not misused, copied, etc. When the key is used to
encrypt data, it must then be stored for later decryption. The most secure
method is via a Hardware Security Module (HSM) or CloudHSM. If an
HSM is not used, then the keys can either be securely stored on the
client’s side, or, if the keys are used on the Cloud, then the Cloud Service
Provider’s Key Management Ser vice can be used.
Once a key’s cryptoperiod, or time period the key is usable, passes, the
key must be rotated. When the key of an encrypted set of data expires, the
key is retired and replaced with a new key. First the data is decrypted by
the old key or key pair and then encrypted by the new key or key pair.
Rotation is necessary because the longer a key is in rotation, the more
chance there is for someone to steal or find out the key. Rotation of keys
can happen before the cryptoperiod expires in cases w here the key is
suspected to be compromised.
Two other ways of dealing with a compromised key are revoking or
destroying the key in question. Revoking a key means the key can no
longer be used to encrypt or decrypt data, even if its cryptoperiod is still
valid. Destroying a key, whether that is due to compromise or due to it no
longer being used, deletes the key permanently from any key manager
database or other storage method. This makes it impossible to recreate the
key, unless a backup image is used. NIST standards require that
deactivated keys be kept in an archive, to allow for reconstruction of the
keys if data encrypted in the past must now be decrypted by that key or
key pair.
5.6 DI FFIE -HELLMAN KEY EXCHANGE
The Diffie -Hellman key exchange was one of the most important
developments in public -key cryptography and it is still frequently
implemented in a range of today’s different security protocols.
It allows two parties who have not pr eviously met to securely establish a
key which they can use to secure their communications. In this article,
we’ll explain what it’s used for, how it works on a step -by-step basis, its
different variations, as well as the security considerations that need to be
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75 Key Management The Diffie -Hellman key exchange was the first widely used method of
safely developing and exchanging keys over an insecure channel .
It may not seem so exciting or groundbreaking in the above terms, so let’s
give an example that explains why the Diffie -Hellman key exchange was
such an important milestone in the world of cryptography, and why it is
still so frequently used today.
Diffie Hellman Key Exchange Algorithm -
Let-
 Private key of the sender = X s
 Public key of the sender = Y s
 Private key of the receiver = X r
 Public key of the receiver = Y r
Using Diffie Hellman Algorithm, the key is exchanged in the following
steps -
Step -01:
One of the parties choose two numbers ‘a’ and ‘n’ and exchange with the
other party.
 ‘a’ is the primitive root of prime number ‘n’.
 After this exchange, both the parties know the value of ‘a’ and ‘n’.
Step -02:
 Both the parties already know their own private key.
 Both the parties calculate the value of their public key and exchange
with each other.
 Sender calculate its public key as -
 Ys = aXs mod n
 Receiver calculate its public key as -
 Yr = aXr mod n
Step-03:
 Both the parties receive public key of each other.
 Now, both the parties calculate the value of secret key.
 Sender calculates secre t key as -
 Secret key = (Y r)Xs mod n munotes.in

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76 Information and Network Security  Receiver calculates secret key as -
 Secret key = (Y s)Xr mod n
Finally, both the parties obtain the same value of secret key.
5.6.1 PRACTICE PROBLEMS BASED ON DIFFIE HELLMAN KEY
EXCHANGE -
Problem -01:
Suppose that two parti es A and B wish to set up a common secret key (D -
H key) between themselves using the Diffie Hellman key exchange
technique. They agree on 7 as the modulus and 3 as the primitive root.
Party A chooses 2 and party B chooses 5 as their respective secrets. The ir
D-H key is -
3
4
5
6
Solution -
Given -
 n = 7
 a = 3
 Private key of A = 2
 Private key of B = 5
Step -01:
Both the parties calculate the value of their public key and exchange with
each other.
Public key of A
= 3private key of A mod 7
= 32 mod 7
= 2
Public key of B
= 3private key of B mod 7
= 35 mod 7
= 5
Step-02: munotes.in

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77 Key Management Both the parties calculate the value of secret key at their respective side.
Secret key obtained by A
= 5private key of A mod 7
= 52 mod 7
= 4
Secret key obtained by B
= 2private key of B mod 7
= 25 mod 7
= 4
Finally, both the parties obtain the same value of secret key.
The value of common secret key = 4.
Thus, Option (B) is correct.
Problem -02:
In a Diffie -Hellman Key Exchange, Alice and Bob have chosen prime
value q = 17 and primitive root = 5. I f Alice’s secret key is 4 and Bob’s
secret key is 6, what is the secret key they exchanged?
16
17
18
19
Solution -
Given -
 n = 17
 a = 5
 Private key of Alice = 4
 Private key of Bob = 6
Step -01:
Both Alice and Bob calculate the value of their public key and ex change
with each other.
Public key of Alice munotes.in

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78 Information and Network Security = 5private key of Alice mod 17
= 54 mod 17
= 13
Public key of Bob
= 5private key of Bob mod 17
= 56 mod 17
= 2
Step -02:
Both the parties calculate the value of secret key at their respective side.
Secret key obta ined by Alice
= 2private key of Alice mod 7
= 24 mod 17
= 16
Secret key obtained by Bob
= 13private key of Bob mod 7
= 136 mod 17
= 16
Finally, both the parties obtain the same value of secret key.
The value of common secret key = 16.
5.6.2 Establishing a shared key between multiple parties
The Diffie -Hellman key exchange can also be used to set up a shared key
with a greater number of participants. It works in the same manner, except
further rounds of the calculations are needed for each party to add in t heir
secret number and end up with the same shared secret.
Just like in the two -party version of the Diffie -Hellman key exchange,
some parts of the information are sent across insecure channels, but not
enough for an attacker to be able to compute the shar ed secret.
5.6.3 Why is the Diffie -Hellman key exchange secure?
On a mathematical level, the Diffie -Hellman key exchange relies on one -
way functions as the basis for its security. These are calculations which
are simple to do one way, but much more difficu lt to calculate in reverse.
More specifically, it relies on the Diffie -Hellman problem, which assumes
that under the right parameters, it is infeasible to calculate gab from the munotes.in

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79 Key Management separate values of g, ga and gb. There is currently no publicly known way
to easily find gab from the other values, which is why the Diffie -Hellman
key exchange is considered secure, despite the fact that attackers can
intercept the values p, g, A, and B.
5.6.4 Authentication & the Diffie -Hellman key exchange
In the real world, the Diffie -Hellman key exchange is rarely used by itself.
The main reason behind this is that it provides no authentication, which
leaves users vulnerable to man -in-the-middle attacks .
These attacks can take place when the Diffie -Hellman key exchange is
implem ented by itself, because it has no means of verifying whether the
other party in a connection is really who they say they are . Without
any form of authentication, users may actually be connecting with
attackers when they think they are communicating with a trusted party.
For this reason, the Diffie -Hellman key exchange is generally
implemented alongside some means of authentication. This often involves
using digital certificates and a public -key algorithm, such as RSA, to
verify the identity of each party.
5.6.5 Variations of the Diffie -Hellman key exchange
The Diffie -Hellman key exchange can be implemented in a number of
different ways, and it has also provided the basis for several other
algorithms. Some of these implementations provide authorization, whil e
others have various cryptographic features such as perfect forward
secrecy.
5.7 SUMMARY
 Cryptography lies at the heart of the modern business - protecting
electronic communications and financial transactions, maintaining
the privacy of sensitive data an d enabling secure authentication and
authorization.
 There are three primary types of keys - Symmetric keys, Private
keys, Hash keys
 Types Public -Key Cryptosystems - RSA algorithm (Rivest -
Shamir -Adleman), ElGamal Cryptosystem, Elliptic Curve
Cryptography (EC C)
 Cryptographic keys are a vital part of any security system. They do
everything from data encryption and decryption to user
authentication.
 Key management forms the basis of all data security. Data is
encrypted and decrypted via the use of encryption keys, which
means the loss or compromise of any encryption key would
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80 Information and Network Security  The Diffie -Hellman key exchange was one of the most important
developments in public -key cryptography and it is still frequently
implemented in a range of today’s different security protocols.
5.8 QUESTIONS
1. Explain Pu blic Key Cryptography.
2. Explain importance of Public Key.
3. Differentiate between Public Key Cryptography and Private Key
Cryptography.
4. Write a short not on Key Management.
5. Explain Types Public -Key Cryptosystems.
6. How RSA algorithm works?
7. How ElGamal algorithm works ?
8. Compare RSA and ElGamal.
9. Why is Key Management Important?
5.9 REFERENCE FOR FU RTHER READING
 https://dis -blog.thalesgroup.com/security/2018/09/26/the -
importance -of-key-management -when -implementing -a-secure -
information -gateway/
 https://www.cryptomathic.com/news -events/blog/cryptographic -
key-management -the-risks -and-mitigations
 https://www.globalsign.com/en/ssl -infor mation -center/what -is-
public -key-cryptography
 https://searchsecurity.techtarget.com/definition/RSA
 https://www.tutorialspoint.com/cryptography/public_key_encryption
.htm
 https://mathstats.uncg.edu/sites/pauli/112/HTML/secelgamal.html
 https://www.encryptionconsulting.com/education -center/what -is-
key-management/
 https://www.comparitech. com/blog/information -security/diffie -
hellman -key-exchange/
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81 6
MESSAGE AUTHENTICATION AND HASH
FUNCTIONS
Unit Structure :
6.0 Objective
6.1 Introduction
6.2 Authentication Requirements
6.3 Authentication Functions
6.3.1 Message Encryption
6.3.2 Message authentication code (MAC)
6.3.3 A Hash function
6.4 Security of Hash Functions and Macs
6.5 Secure Hash Algorithm
6.6 HMAC
6.7 Summary
6.8 Questions
6.9 Reference for further reading
6.0 OBJECTIVE
In this chapter we will discuss about Message Authentication, importance
of Message Authentication. Types of Authentic ation and finally Hash
Function.
In today’s world everywhere there are net of information and now only
data transfer is not important rather it must be “Data transfer with
security!!!”. Hence data must be AUTHENTICATED before it is being
used by user. Her e we will discuss all techniques.
6.1 INTRODUCTION
In most people’s minds, privacy is the goal most strongly associated to
cryptography. But message authentication is arguably even more
important. Indeed you may or may not care if some particular message
you send out stays private, but you almost certainly do want to be sure of
the originator of each message that you act on. Message authentication is
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82 Information and Network Security Message authentication allows one party —the Sender —to send a message
to anoth er party —the Receiver —in such a way that if the message is
modified en route, then the Receiver will almost certainly detect this.
Message authentication is also called “data -origin authentication,” since it
authenticates the point -of-origin for each messa ge. Message authentication
is said to protect the “integrity” of messages, ensuring that each that is
received and deemed acceptable is arriving in the same condition that it
was sent out —with no bits inserted, missing, or modified.
6.2 AUTHENTICATION R EQU IREMENTS
In the context of communications across a network, the following attacks
can be identified:
1. Disclosure : Release of message contents to any person or process
not possessing the appropriate cryptographic key.
2. Traffic analysis : Discovery of t he pattern of traffic between parties.
In a connection oriented application, the frequency and duration of
connections could be determined. In either a connection -oriented or
connectionless environment, the number and length of messages
between parties cou ld be determined.
3. Masquerade : Insertion of messages into the network from a
fraudulent source. This includes the creation of messages by an
opponent that are purported to come from an authorized entity. Also
included are fraudulent acknowledgments of m essage receipt or non -
receipt by someone other than the message recipient.
4. Content Modification : Changes to the contents of a message,
including insertion, deletion, transposition, or modification.
5. Sequence modification : Any modification to a seque nce of
messages between parties, including insertion, deletion, and
reordering.
6. Timing modification : Delay or replay of messages. In a connection -
orientated application, an entire session or sequence of messages
could be a replay of some previous valid session, or individual
messages in the sequence could be delayed or replayed.
7. Repudiation : Denial of receipt of message by destination or denial
of transmission of message by source.
Message authentication is a procedure to verify that received messag es
come from the alleged source and have not been altered. Message
authentication may also verify sequencing and timeliness. A digital
signature is an authentication technique that also includes measures to
counter repudiation by either source or destinati on. Any message
authentication or digital signature mechanism can be viewed as having
fundamentally two levels. At the lower level, there must be some sort of
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83 Key Management a message. This lower level function is then used as primitive in a higher -
level authentication protocol that enables a receiver to verify the
authenticity of a message.
6.3 AUTHENTICATION F UNCTIONS
There are three types of functions that may be used to produce an
authenticator :
1. Message Encryption
2. Message authentication code (MAC)
3. A Hash function
6.3.1 Message Encryption
Encryption is a means of securing digital data using one or more
mathematical techniques, along with a password or "key" used to decrypt
the informati on. The encryption process translates information using
an algorithm that makes the original information unreadable. The process,
for instance, can convert an original text, known as plaintext, into an
alternative form known as ciphertext. When an authoriz ed user needs to
read the data, they may decrypt the data using a binary key. This will
convert ciphertext back to plaintext so that the authorized user can access
the original information. Encryption is an important way for individuals
and companies to pr otect sensitive information from hacking. For
example, websites that transmit credit card and bank account numbers
should always encrypt this information to prevent identity theft and fraud.
The mathematical study and application of encryption is known as
cryptography.

There are two types of encryptions schemes as listed below:
 Symmetric Key encryption
 Public Key encryption
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84 Information and Network Security 1. Symmetric Key encryption
The encryption process where same keys are used for encrypting
and decrypting the information is known a s Symmetric Key
Encryption.
The study of symmetric cryptosystems is referred to as symmetric
cryptography . Symmetric cryptosystems are also sometimes
referred to as secret key cryptosystems .
A few well -known examples of symmetric key encryption methods
are − Digital Encryption Standard (DES), Triple -DES (3DES),
IDEA, and BLOWFISH.

Prior to 1970, all cryptosystems employed symmetric key encryption.
Even today, its relevance is very high and it is being used extensively in
many cryptosystems. It is very unl ikely that this encryption will fade
away, as it has certain advantages over asymmetric key encryption.
The salient features of cryptosystem based on symmetric key encryption
are −
 Persons using symmetric key encryption must share a common key
prior to exc hange of information.
 Keys are recommended to be changed regularly to prevent any
attack on the system.
 A robust mechanism needs to exist to exchange the key between the
communicating parties. As keys are required to be changed
regularly, this mechanism be comes expensive and cumbersome.
 In a group of n people, to enable two -party communication between
any two persons, the number of keys required for group is n × (n –
1)/2.
 Length of Key (number of bits) in this encryption is smaller and
hence, process of en cryption -decryption is faster than asymmetric
key encryption.
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85 Key Management Challenge of Symmetric Key Cryptosystem
There are two restrictive challenges of employing symmetric key
cryptogra phy.
 Key establishment − Before any communication, both the sender
and the receiver need to agree on a secret symmetric key. It requires
a secure key establishment mechanism in place.
 Trust Issue − Since the sender and the receiver use the same
symmetric key, there is an implic it requirement that the sender and
the receiver ‘trust’ each other. For example, it may happen that the
receiver has lost the key to an attacker and the sender is not
informed.
These two challenges are highly restraining for modern day
communication. Today , people need to exchange information with non -
familiar and non -trusted parties. For example, a communication between
online seller and customer. These limitations of symmetric key encryption
gave rise to asymmetric key encryption schemes.
2. Public Key en cryption / Asymmetric Key Encryption
The encryption process where different keys are used for
encrypting and decrypting the information is known as
Asymmetric Key Encryption. Though the keys are different, they are
mathematically related and hence, retriev ing the plaintext by
decrypting ciphertext is feasible. The process is depicted in the
following illustration −

Asymmetric Key Encryption was invented in the 20th century to
come over the necessity of pre -shared secret key between
communicating persons. The salient features of this encryption
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86 Information and Network Security  Every user in this system needs to have a pair of dissimilar
keys, private key and public key . These keys are mathematically
related − when one key is used for encryption, the other can decryp t
the ciphertext back to the original plaintext.
 It requires to put the public key in public repository and the private
key as a well -guarded secret. Hence, this scheme of encryption is
also called Public Key Encryption .
 Though public and private keys of t he user are related, it is
computationally not feasible to find one from another. This is a
strength of this scheme.
 When Host1 needs to send data to Host2, he obtains the public key
of Host2 from repository, encrypts the data, and transmits.
 Host2 uses hi s private key to extract the plaintext.
 Length of Keys (number of bits) in this encryption is large and
hence, the process of encryption -decryption is slower than
symmetric key encryption.
 Processing power of computer system required to run asymmetric
algorithm is higher.
Symmetric cryptosystems are a natural concept. In contrast, public -
key cryptosystems are quite difficult to comprehend.
You may think, how can the encryption key and the decryption key
are ‘related’, and yet it is impossible to determine t he decryption key
from the encryption key? The answer lies in the mathematical
concepts. It is possible to design a cryptosystem whose keys have
this property. The concept of public -key cryptography is relatively
new. There are fewer public -key algorithms known than symmetric
algorithms.
Challenge of Public Key Cryptosystem
Public -key cryptosystems have one significant challenge − the user needs
to trust that the public key that he is using in communications with a
person really is the public key of that pe rson and has not been spoofed by
a malicious third party.
This is usua lly accomplished through a Public Key Infrastructure (PKI)
consisting a trusted third party. The third party securely manages and
attests to the authenticity of public keys. When the third party is requested
to provide the public key for any communicating person X, they are
trusted to provide the correct public key.
The third party satisfies itself about user identity by the process of
attestation, notarization, or some other process − that X is the one and
only, or globally unique, X. The most common metho d of making the munotes.in

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87 Key Management verified public keys available is to embed the m in a certificate which is
digitally signed by the trusted third party.
6.3.2 Message authentication code (MAC)
MAC algorithm is a symmetric key cryptographic technique to provide
message authentication. For establishing MAC process, the sender and
receiv er share a symmetric key K.
Essentially, a MAC is an encrypted checksum generated on the underlying
message that is sent along with a message to ensure message
authentication.
The process of using MAC for authentication is depicted in the following
illustr ation −

Let us now try to understand the entire process in detail −
 The sender uses some publicly known MAC algorithm, inputs the
message and the secret key K and produces a MAC value.
 Similar to hash, MAC function also compresses an arbitrary long
input into a fixed length output. The major difference between hash
and MAC is that MAC uses secret key during the compression.
 The sender forwards the message along with the MAC. Here, we
assume that the message is sent in the clear, as we are concerned of
providing message origin authentication, not confidentiality. If
confidentiality is required then the message needs encryption.
 On receipt of the message and the MAC, the receiver feeds the
received message and the shared secret key K into the MAC
algorithm a nd re -computes the MAC value.
 The receiver now checks equality of freshly computed MAC with
the MAC received from the sender. If they match, then the receiver
accepts the message and assures himself that the message has been
sent by the intended sender.
 If the computed MAC does not match the MAC sent by the sender,
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88 Information and Network Security altered or it is the origin that has been falsified. As a bottom -line, a
receiver safely assumes that the message is not the genuine.
Limitations of MAC
There are two major limitations of MAC, both due to its symmetric nature
of operation −
 Establishment of Shared Secret.
 It can provide message authentication among pre -decided
legitimate users who have shared key.
 This requires establishment of shared secret prior to use of
MAC.
 Inability to Provide Non -Repudiation
 Non-repudiation is the assurance that a message originator
cannot deny any previously sent messages and commitments
or actions.
 MAC technique does not provide a non -repudiation service. If
the sender and receiver get involved in a dispute over message
origination, MACs cannot provide a proof that a message was
indeed sent by the sender.
 Though no third party can compute the MAC, still sender
could deny having sent the m essage and claim that the receiver
forged it, as it is impossible to determine which of the two
parties computed the MAC.
6.3.3 A Hash function
A hash function is a serious mathematical process that holds a critical role
in public key cryptography. Why? Be cause it’s what helps you to:
 Securely store passwords in a database
 Ensure data integrity (in a lot of different applications) by indicating
when data has been altered,
 Make secure authentication possible, andOrganize content and files
in a way that incre ases efficiency.
A hash function is a unique identifier for any given piece of content.
It’s also a process that takes plaintext data of any size and converts it
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A basic illustration of how the hashing proce ss works. A simple
illustration of what a hash function does by taking a plaintext data input
and using a mathematical algorithm to generate an unreadable output.
Properties of a Strong Hash Algorithm
So, what makes for a strong hashing algorithm? There ar e a few key traits
that all good ones share:
 Determinism — A hash algorithm should be deterministic ,
meaning that it always gives you an output of identical size
regardless of the size of the input you started with. This means that
if you’re hashing a sing le sentence, the resulting output should be
the same size as one you’d get when hashing an entire book.
 Pre-Image Resistance — The idea here is that a strong hash
algorithm is one that’s preimage resistance, meaning that it’s
infeasible to reverse a hash value to recover the original input
plaintext message. Hence, the concept of hashes being irreversible,
one-way functions.
 Collision Resistance — A collision occurs when two objects
collide. Well, this concept carries over in cryptography with hash
values. If two unique samples of input data result in identical
outputs, it’s known as a collision. This is bad news and means that
the algorithm you’re using to hash the data is broken and, t herefore,
insecure. Basically, the concern here is that someone could create a
malicious file with an artificial hash value that matches a genuine
(safe) file and pass it off as the real thing because the signature
would match. So, a good and trustworthy h ashing algorithm is one
that is resistant to these collisions.
 Avalanche Effect — What this means is that any change made to an
input, no matter how small, will result in a massive change in the
output. Essentially, a small change (such as adding a comma)
snowballs into something much larger, hence the term “ avalanche
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90 Information and Network Security  Hash Speed — Hash algorithms should operate at a reasonable
speed. In many situations, hashing algorithms should compute hash
values quickly; this is considered an ideal property of a
cryptographic hash function. However, this property is a little more
subjective. You see, faster isn’t always better because the speed
should depend on how the hashing algorithm is going to be used.
Sometimes, you want a faster hashing algorithm, and other times it’s
better to use a slower one that takes more time to run through. The
former is better for website connections and the latter is better for
password hashing.
What Does a Hash Functio n Do?
One purpose of a hash function in cryptography is to take a plaintext input
and generate a hashed value output of a specific size in a way that can’t be
reversed. But they do more than that from a 10,000 -foot perspective. You
see, hash functions tend to wear a few hats in the world of cryptography.
In a nutshell, strong hash functions:
 Ensure data integrity,
 Secure against unauthorized modifications,
 Protect stored passwords, and
 Operate at different speeds to suit different purposes.
1. Ensure Data I ntegrity
Hash functions are a way to ensure data integrity in public key
cryptography . What I mean by that is that hash functions serve as a
check -sum, or a way for someone to identify whether data has been
tampered with after it’s been signed. It also serves as a means of
identity verification.
For example, let’s say you’ve logged on to public Wi -Fi to send me
an email. (Don’t do that, by the way. It’s very insecure.) So, you
write out the message, sign it using your digital certificate, and send
it on its way across the internet. This is what you might call
prime man-in-the-middle attack territory — meaning that someone
could easily intercept your message (again, because public wireless
networks are notoriously insecure) and modify it to suit their evil
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91 Key Management

The example above is of a digitally signed email that’s been
manipulated in transit via a MitM attack. The hash digest changes
completely when any of the email content gets modified after being
digitally signed, signaling that it can’t be trusted.
2. Secure Against Unauthorized Modifications
One of the best aspects of a cryptographic hash function is that it
helps you to ensure data integrity. But if you apply a hash to data,
does it mean that the message can’t be altered? No. But what it does
is inform the message recipient that the message has been changed.
That’s because even the smallest of changes to a message will result
in the creation of an entirely new hash value.
Think of hashing kind of like you would a smoke alarm. While a
smoke alarm doesn’t stop a fire from starting, it does let you know
that there’s da nger before it’s too late.
3. Enable You to Verify and Securely Store Passwords
Nowadays, many websites allow you to store your passwords so you
don’t have to remember them every time you want to log in. But
storing plaintext passwords like that in a publi c-facing server would
be dangerous because it leaves that information vulnerable to
cybercriminals. So, what websites typically do is hash passwords to
generate hash values, which is what they store instead.
But password hashes on their own isn’t enough to protect you
against certain types of attacks, including brute force attacks. This is
why you first need to add a salt. A salt is a unique, random number
that’s applied to plaintext passwords before they’re hashed. This
provides an additional layer of security and can protect passwords
from password cracking methods like rainbow table attacks.
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92 Information and Network Security It’s also important to note that hash functions a ren’t one-size-fits-all
tools. As we mentioned earlier, different hash functions serve
different purposes depending on their design and hash speeds. They
work at different operational speeds — some are faster while others
are much slower. These speeds can aid or impede the security of a
hashing algorithm depending on how you’re using it. So, some fall
under the umbrella of secure hashing algorithms while others do not.
How Does Hashing Work?
When you hash a message, you take a string of data of any size as
your input, run it through a mathematical algorithm that results in
the generation of an output of a fixed length.
In some methods of hashing, that original data input is broken up
into smaller blocks of equal size. If there isn’t enough data in any of
the blocks for it to be the same size, then padding (1s and 0s) can be
used to fill it out. Then those individual blocks of data are run
through a hashing algorithm and result in an output of a hash value.
The process looks something like this:

6.4 SECURITY OF HASH FUNCTIONS AN D MACS
We can group attacks on the basis of hash functions and MACs: brute -
force attacks and cryptanalysis.
Brute -Force Attacks -The nature of brute -force attacks differs somewhat
for hash functions and MACs.
Hash Functions :-The strengt h of a hash function against brute -force
attacks depends on the length of the hash code produced by the algorithm.
There are three desirable properties:
 One-way: For any given code h, it is computationally infeasible to
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93 Key Management  Weak coll ision resistance: For any given block x, it is
computationally infeasible to find y ≠x with H(y) = H(x).
 Strong collision resistance: It is computationally infeasible to find
any pair (x, y) such that H(x) = H(y).
For a hash code of length n, the level of effort required, as we have seen is
proportional to the following:
If strong collision resistance is required, then the value 2n/2 determines the
strength of the hash code against brute -force attacks. Thus a 128 -bit code
may be viewed as inadequate. The n ext step up, if a hash code is treated as
a sequence of 32 bits, is a 160 -bit hash length. With a hash length of 160
bits, the same search machine would require over four thousand years to
find a collision. However, even 160 bits is now considered weak.
Message Authentication Codes: -A brute -force attack on a MAC is a more
difficult undertaking because it requires known message -MAC pairs. To
attack a hash code, we can proceed in the following way. Given a fixed
message x with n -bit hash code h = H(x), a brut e-force method of finding
a collision is to pick a random bit string y and check if H(y) = H(x). The
attacker can do this repeatedly off line. Whether an off -line attack can be
used on a MAC algorithm depends on the relative size of the key and the
MAC.
To proceed, we need to state the desired security property of a MAC
algorithm, whi ch can be expressed as follows: Computation resistance:
Given one or more text -MAC pairs [x i, C(K, x i)], it is computationally
infeasible to compute any text -MAC pair [x, C(K, x)] for any new input x
≠ xi.In other words, the attacker would like to come up with the valid
MAC code for a given message x. There are two lines of attack possible:
Attack the key space and attack the MAC value.
General Structure of Secure Hash Co de

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94 Information and Network Security Cryptanalysis -The way to measure the resistance of a hash or MAC
algorithm to cryptanalysis is to compare its strength to the effort required
for a brute -force attack. That is, an ideal hash or MAC algorithm will
require a cryptanalytic effort greater than or equal to the brute -force effort.
Hash Functions: -In past few years there has been considerable effort, and
some successes, in developing cryptanalytic attacks on hash functions. To
understand these, we need to look at the overall structure of a ty pical
secure hash function, This structure, referred to as an iterated hash
function and is the structure of most hash functions in use today, including
SHA and Whirlpool. The hash function takes an input message and
partitions it into L fixed -sized blocks of b bits each. If necessary, the final
block is padded to b bits. The final block also includes the value of the
total length of the input to the hash function. The inclusion of the length
makes the job of the opponent more difficult.
6.5 SECURE HASH ALG ORITHM
The Secure Hash Algorithm (SHA) was developed by the National
Institute of Standards and Technology (NIST) and published as a federal
information processing standard (FIPS 180) in 1993; a revised version was
issued as FIPS 180 -1 in 1995 and is gener ally referred to as SHA -1. The
actual standards document is entitled Secure Hash Standard. SHA is based
on the MD4 algorithm which is a message digest algorithm that was
developed by Ron Rivest at MIT (the “R” in the RSA (Rivest -
ShamirAdelman) public key e ncryption algorithm). MD4 was later
replaced with the popular MD5 algorithm also by Ron Rivest however
advances in cryptanalysis and computing power have led to their decline
in popularity. Both MD4 and MD5 produce a 128 bit message digest
whereas SHA -1 produces a 160 bit as will be seen.
In 2002, NIST produced a revised version of the standard, FIPS180 -2, that
defined three new versions of SHA, with hash value lengths of 256, 384,
and 512 bits, known as SHA -256, SHA -384, and SHA -512 respectively.
These ne w versions have the same underlying structure and use the same
types of modular arithmetic and logical binary operations as SHA -1. In
2005, NIST announced the intention to phase out approval of SHA -1 and
move to a reliance on the other SHA versions by 2010 . Shortly thereafter,
a research team described an attack in which two separate messages could
be found that deliver the same SHA -1 hash using 2 69 operations, far
fewer that the 2 80 operations previously thought needed to find a
collision with an SHA -1 hash. This result should hasten the transition to
the other versions of SHA however we will still concentrate on SHA -1 as
the underlying structures of the others is the same. SHA -1 takes as input a
message with a maximum length of less than 2 64 bits and pr oduces as
output a 160 bit message digest.
The input is processed in 512 -bit blocks. Figure 10.4 depicts the overall
processing of a message to produce a digest. Although this diagram has
MD5 as the hash function the structure is exactly the same for SHA -1 with
the exception that the message length is limited in size (its isn’t for MD5)
and the hash value (and intermediate values CVi) are 160 bits and not 128
as shown (which is the case for MD5). The processing consists of the
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95 Key Management 1. Append padding bi ts: The message is padded so that its length is
congruent to 448 modulo 512 (length ≡ 448 (mod 512)). That is, the
length of the padded message is 64 bits less than an integer multiple
of 512 bits. Padding is always added, even if the message is already
of the desired length. Thus, the number of padding bits is in the
range of 1 to 512 bits. The padding consists of a single 1 -bit
followed by the necessary number of 0 -bits.
2. Append length: A block of 64 bits is appended to the message. This
block is treat ed as an unsigned 64 -bit integer (most significant byte
first) and contains the length of the original message (before
padding).
3. Initialize MD buffer: A 160 bit buffer is used to hold intermediate
values and final results of the Hash function represent ed as 5, 32 bit
registers (A, B, C, D, E) initialized as follows: A = 67452301 B =
EF CDAB89 C = 98BADCF E D = 10325476 E = C3D2E1F0
4. Process message in 512 bit (16 word) blocks: The heart of the
algorithm is a module which consists of four “rounds” of processing
of 20 steps each (see figure). Each round has similar structure but
uses a different primitive logical function (f1, f2, f3 and f4). Each
round takes as input the current 512 -bit block being processed (Yq)
and the 160 -bit buffer value ABCDE and updates the contents of the
buffer. Each round also makes use of an additive constant Kt where
0 ≤ t ≤ 79 indicates one of the 80 steps across four rounds. In fact,
only four distinct constants are used (one for 0 ≤ t ≤ 19, 20 ≤ t ≤ 39,
40 ≤ t ≤ 59 and 60 ≤ t ≤ 79). The output of the fourth round is added
(modulo 2 32) to the input to the first round (CVq) to produce
CVq+1.
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96 Information and Network Security Fig. Message Digest Generation Using MD5 (equally applicable to SHA -1
with 160 bits instead of 128 etc.).
5. Output after all L 512 b it blocks have been processed the output
from the Lth stage is the 160 bit digest.

We can summarise the behavior SHA -1 as follows:
CV 0 = IV
CV q+1 = SUM32(CV q, ABCDE q)
MD = CV L
where
IV = initial value of the ABCDE buffer, defined in step 3.
ABCDE q = the output of the last round of processing of the qth message
block.
L = the number of blocks in the message (including padding and length
fields).
SUM 32 = Addition modulo 2 32 performed separately on each word of the
pair of inputs. MD = final message d igest value.
6.6 HMAC
HMAC algorithm stands for Hashed or Hash based Message
Authentication Code. It is a result of work done on developing a MAC
derived from cryptographic hash functions. HMAC is a great resistant
towards cryptanalysis attacks as it uses the Hashing concept twice. HMAC
consists of twin benefits of Hashing and MAC, and thus is more secure
than any other authentication codes. RFC 2104 has issued HMAC, and
HMAC has been made compulsory to implement in IP security. The FIPS
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97 Key Management Objectives –
As the Hash Function, HMAC is also aimed to be one way, i.e, easy to
generate output from input but complex the other way round.
It aims at being less effected by collisions than the hash functions.
HMAC reuses the algorit hms like MD5 and SHA -1 and checks to replace
the embedded hash functions with more secure hash functions, in case
found.
HMAC tries to handle the Keys in more simple manner.
HMAC algorithm –
The working of HMAC starts with taking a message M containing blo cks
of length b bits. An input signature is padded to the left of the message and
the whole is given as input to a hash function which gives us a temporary
message digest MD’. MD’ again is appended to an output signature and
the whole is applied a hash fun ction again, the result is our final message
digest MD.
Here is a simple structure of HMAC:
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98 Information and Network Security Here, H stands for Hashing function,
M is original message
Si and So are input and output signatures respectively,
Yi is the ith block in original message M, wher e i ranges from [1, L)
L = the count of blocks in M
K is the secret key used for hashing
IV is an initial vector (some constant)
The generation of input signature and output signature Si and So
respectively .

6.7 SUMMARY
 Message authentication allows one party —the Sender —to send a
message to another party —the Receiver —in such a way that if the
message is modified en route, then the Receiver will almost certainly
detect this.
 Message authentication is also called “data -origin authentication,”
since it auth enticates the point -of-origin for each message.
 In the context of communications across a network, the following
attacks can be identified:
 Disclosure, Traffic analysis, Masquerade, Content
Modification, Sequence modification, Timing modification,
Repudia tion
 There are three types of functions that may be used to produce an
authenticator:
i. Message Encryption
ii. Message authentication code (MAC)
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99 Key Management  Encryption is a means of securing digital data using one or more
mathematical techniques , along with a password or "key" used to
decrypt the information.
 The encryption process where same keys are used for encrypting
and decrypting the information is known as Symmetric Key
Encryption.
 The encryption process where different keys are used for
encrypting and decrypting the information is known as
Asymmetric Key Encryption.
 MAC algorithm is a symmetric key cryptographic technique to
provide message authentication. For establishing MAC process, the
sender and receiver share a symmetric key K.
 A hash function is a serious mathematical process that holds a
critical role in public key cryptography.
 HMAC algorithm stands for Hashed or Hash based Message
Authentication Code. It is a result of work done on developing a
MAC derived from cryptographic hash functions. HMAC is a great
resistant towards cryptanalysis attacks as it uses the Hashing concept
twice.
6.8 QUESTIONS
1. In the context of communications across a network, what are the
attacks can be identified?
2. What are the types of Authentication Function?
3. Write a short n ote on Message Encryption.
4. Explain Message authentication code (MAC).
5. Write a short note on A Hash function.
6. Write a short note on Symmetric Key encryption
7. Write a short note on Asymmetric Key encryption
8. Differentiate between Symmetric Key encryption and Asymmetric
Key encryption
9. Write Properties of a Strong Hash Algorithm.
10. How Does Hash ing Work?
11. Write a short note on HMAC. munotes.in

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100 Information and Network Security 6.9 REFERENCE FOR FU RTHER READING
 https://www.cc.gatech.edu/~aboldyre/teaching/Fall05cs6260/m -
mac.pdf
 http://www.facweb.iitkgp.ac.in/~sourav/AuthenticationRequirement
s.pdf
 https://www.slideshare.net/RajasekarVr/message -authentication -
7533308 4-
 https://www.thesslstore.com/blog/what -is-a-hash-function -in-
cryptography -a-beginners -guide/
 http://www.faadooengineers.com/online -

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101 7
DIGITAL SIGNATURES AND
AUTHENTICATION AND
AUTHENTICATION APPLICATIONS
Unit Structure :
7.0 Objective
7.1 Introduction
7.2 Digital Signatures
7.3 Authentication Protocols
7.3.1 Authentication protocols developed for PPP Point -to-Point
Protocol
7.4 Digital Signature Standard
7.5 Authentication Applications
7.5.1 Kerberos
7.5.2 X.509 Authentication
7.5.3 Public -Key Infrastructure
7.6 Summary
7.7 Question s
7.8 Reference for further reading
7.0 OBJECTIVE
In this chapter we mainly discuss about Authentication Process and
Protocol which are responsible for implementing Authentication.
Authentication is important because it enables organizations to keep their
networks secure by permitting only authenticated users (or processes) to
access its protected resources , which may include computer systems,
networks, databases, websites and other network -based applications or
services.
7.1 INTRODUCTION
Producing a secure authentication process that keeps users happy is easier
said than done, but it's necessary in order to keep them safe online.
Controlling access is the basis of all security. The right people should be
allowed in, and the wrong people kept out. This is do ne by confirming – or
authenticating – the identity of the person seeking access, and then
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102 Information and Network Security Authentication is normally achieved by the presentation of a User ID
(usually the user’s email address) to identify the person, and a secret
password known only to that person to confirm the identity.
But there are huge problems with this process. Fundamentally, it does not
authenticate the person; if a criminal acquires and uses the person’s User
ID and password, the c riminal is automatically authorized to gain access.
So, strictly speaking, a password does not authenticate the user, it simply
authorizes a device regardless of who is using it.
7.2 DIGITAL SIGNATUR ES
The Digital Signature is a technique which is used to validate the
authenticity and integrity of the message. We know that there are four
aspects of security: privacy, authentication, integrity, and non -repudiation.
We have already discussed the first aspect of security and other three
aspects can be achieved by using a digital signature.
The basic idea behind the Digital Signature is to sign a document. When
we send a document electronically, we can also sign it. We can sign a
document in two ways: to sign a whole document and to sign a digest.
Message authen tication protects two parties who exchange messages from
any third party. However, it does not protect the two parties against each
other. Several forms of dispute between the two are possible. For example,
suppose that John sends an authenticated message to Mary using one of
the schemes described earlier. Consider the following disputes that could
arise:
 Mary may forge a different message and claim that it came from
John. Mary would simply have to create a message and append an
authentication code using t he key that John and Mary share.
 John can deny sending the message. Because it is possible for Mary
to forge a message, there is no way to prove that John did in fact
send the message.
Both scenarios are of legitimate concern. In situations where there i s
not complete trust between sender and receiver, something more
than authentication is needed. The most attractive solution to this
problem is the digital signature. The digital signature is analogous to
the handwritten signature. It must have the followi ng properties:
 It must verify the author and the date and time of the signature.
 It must authenticate the contents at the time of the signature.
 It must be verifiable by third parties, to resolve disputes. Thus, the
digital signature function includes t he authentication function.
On the basis of these properties, we can formulate the following
requirements for a digital signature: munotes.in

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103 Digital Signatures and Authentication and Authentication Applications  The signature must be a bit pattern that depends on the message
being signed.
 The signature must use some information uniq ue to the sender, to
prevent both forgery and denial.
 It must be relatively easy to produce the digital signature.
 It must be relatively easy to recognise and verify the digital
signature.
 It must be computationally infeasible to forge a digital signatu re,
either by constructing a new message for an existing digital
signature or by constructing a fraudulent digital signature for a given
message.
 It must be practical to retain a copy of the digital signature in
storage.

Digital Signature is used to ach ieve the following three aspects:
 Integrity: The Digital Signature preserves the integrity of a message
because, if any malicious attack intercepts a message and partially or
totally changes it, then the decrypted message would be impossible.
 Authenticatio n: We can use the following reasoning to show how
the message is authenticated. If an intruder (user X) sends a message
pretending that it is coming from someone else (user A), user X uses
her own private key to encrypt the message. The message is
decrypte d by using the public key of user A. Therefore this makes
the message unreadable. Encryption with X's private key and
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104 Information and Network Security  Non-Repudiation: Digital Signature also provides non -repudiation.
If the sender d enies sending the message, then her private key
corresponding to her public key is tested on the plaintext. If the
decrypted message is the same as the original message, then we
know that the sender has sent the message.
Note: Digital Signature does not pr ovide privacy. If there is a need
for privacy, then another layer of encryption/decryption is applied.
Signing the Digest
 Public key encryption is efficient if the message is short. If the
message is long, a public key encryption is inefficient to use. The
solution to this problem is to let the sender sign a digest of the
document instead of the whole document.
 The sender creates a miniature version (digest) of the document and
then signs it, the receiver checks the signature of the miniature
version.
 The h ash function is used to create a digest of the message. The hash
function creates a fixed -size digest from the variable -length
message.
 The two most common hash functions used: MD5 (Message Digest
5) and SHA -1 (Secure Hash Algorithm 1). The first one produ ces
120-bit digest while the second one produces a 160 -bit digest.
 A hash function must have two properties to ensure the success:
 First, the digest must be one way, i.e., the digest can only be created
from the message but not vice versa.
 Second, hashing is a one -to-one function, i.e., two messages should
not create the same digest.
 Following are the steps taken to ensure security:
 The miniature version (digest) of the message is created by using a
hash function.
 The digest is encrypted by using the sender 's private key.
 After the digest is encrypted, then the encrypted digest is attached to
the original message and sent to the receiver.
 The receiver receives the original message and encrypted digest and
separates the two. The receiver implements the hash f unction on the
original message to create the second digest, and it also decrypts the
received digest by using the public key of the sender. If both the
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105 Digital Signatures and Authentication and Authentication Applications At the Sender site

At the Receiver site

7.3 AUTHENTICATION PROTO COLS
7.3.1 Authentication protocols developed for PPP Point -to-Point
Protocol
1. PAP - Password Authentication Protocol
Password Authentication Protocol is one of the oldest authentication
protocols. Authentication is initialized by the client sending a packet
with credentials (username and password) at the beginning of the
connection, with the client repeating the authentication request until
acknowledgement is received. It is highly insecu re because
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106 Information and Network Security vulnerable even to the most simple attacks
like eavesdropping and man-in-the-middle based attacks. Although
widely supported, it is specified that if an implementation offers a
stronger authentication m ethod, that method must be offered before
PAP. Mixed authentication (e.g. the same client alternately using
both PAP and CHAP) is also not expected, as the CHAP
authentication would be compromised by PAP sending the password
in plain -text.

2. CHAP - Challenge -handshake authentication protocol
The Challenge -Handshake Authentication Protocol (CHAP) is used
to periodicall y verify the identity of the peer using a 3 -way
handshake. This is done upon initial link establishment, and MAY
be repeated anytime after the link has been established.
1. After the Link Establishment phase is complete, the
Authenticator sends a "challen ge" message to the peer.
2. The peer responds with a value calculated using a "one -way
hash" function.
3. The authenticator checks the response against its own
calculation of the expected hash value. If the values match,
the authentication is acknowl edged; otherwise the connection
SHOULD be terminated.
4. At random intervals, the authenticator sends a new challenge
to the peer, and repeats steps 1 to 3.
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107 Digital Signatures and Authentication and Authentication Applications
3. EAP - Extensible Authentication Protocol
Extensible Authentication Protocol (EAP) is an authe ntication
framework, not a specific authentication mechanism, frequently used
in wireless networks and point -to-point connections. It provides
some common functions and negotiation of authentication methods
called EAP methods.
The EAP protocol can support multiple authentication mechanisms
without having to pre -negotiate a particular one. There are currently
about 40 different methods defined.
EAP authentication is initiated by the server (authenticator), whereas
many other authentication protocols are init iated by the client (peer).
The EAP authentication exchange proceeds as follows:
1) The authenticator (the server) sends a Request to authenticate
the peer (the client).
2) The peer sends a Response packet in reply to a valid Request.
3) The authenticator sends an additional Request packet, and the
peer replies with a Response. The sequence of Requests and
Responses continues as long as needed. EAP is a ‘lock step’
protocol, so that other than the initial Request, a new Request
cannot be sent prior to recei ving a valid Response.
4) The conversation continues until the authenticator cannot
authenticate the peer (unacceptable Responses to one or more
Requests), in which case the authenticator implementation
MUST transmit an EAP Failure (Code 4). Alternatively, the
authentication conversation can continue until the
authenticator determines that successful authentication has
occurred, in which case the authenticator MUST transmit an
EAP Success (Code 3).
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108 Information and Network Security 7.4 DIGITAL SIGNATUR E STANDARD
The National Institute of Standards and Technology (NIST) has published
Federal Information Processing Standard FIPS 186, known as the Digital
Signature Standard (DSS). The DSS was originally proposed in 1991 and
revised in 1993 in response to public feedback concerning the securit y of
the scheme. There was a further minor revision in 1996. In 2000, an
expanded version of the standard was issued as FIPS 186 -2, subsequently
updated to FIPS 186 -3 in 2009. This latest version also incorporates digital
signature algorithms based on RSA and on elliptic curve cryptography. In
this section, we discuss the original DSS algorithm.
THE DSS APPROACH
The DSS uses an algorithm that is designed to provide only the digital
signature function. Unlike RSA, it cannot be used for encryption or key
exch ange. Nevertheless, it is a public -key technique.
Figure contrasts the DSS approach for generating digital signatures to that
used with RSA. In the RSA approach, the message to be signed is input to
a hash function that produces a secure hash code of fixed length. This hash
code is then encrypted using the sender’s private key to form the
signature. Both the mes- sage and the signature are then transmitted. The
recipient takes the message and produces a hash code. The recipient also
decrypts the signature u sing the sender’s public key. If the calculated hash
code matches the decrypted signature, the signa - ture is accepted as valid.
Because only the sender knows the private key, only the
sender could have produced a valid signature.
The DSS approach also mak es use of a hash function. The hash code is
pro- vided as input to a signature function along with a random number k
generated for this particular signature. The signature function also depends
on the sender’s private key (PRa) and a set of parameters know n to a
group of communicating principals.
We can consider this set to constitute a global public key (PUG).1 The
result is a signature consisting of two components, labeled s and r.
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109 Digital Signatures and Authentication and Authentication Applications At the receiving end, the hash code of the incoming message is generated .
This plus the signature is input to a verification function. The verification
function also depends on the global public key as well as the sender’s
public key (PUa), which is paired with the sender’s private key. The
output of the verification function is a value that is equal to the signature
component r if the signature is valid. The signa - ture function is such that
only the sender, with knowledge of the private key, could have produced
the valid signature.
We turn now to the details of the algorithm.
The Digital Signature Algorithm
The DSA is based on the difficulty of computing discrete logarithms and
is based on schemes originally presented by ElGamal [ELGA85] and
Schnorr [SCHN91].
Figure summarizes the algorithm. There are three parameters that are pub-
lic and can be common to a group of users. A 160 -bit prime number q is
chosen. Next, a prime number p is selected with a length between 512 and
1024 bits such that q divides (p - 1). Finally, g is chosen to be of the form
h(p - 1)/q mod p, where h is an integer between 1 and (p - 1) with the
restriction that g must be greater than 1.2 Thus, the global public -key
components of DSA have the same for as in the Schnorr signature scheme.
With these numbers in hand, each user selects a private key and gener ates
a public key. The private key x must be a number from 1 to (q - 1) and
should be cho - sen randomly or pseudorandomly. The public key is
calculated from the private key as y = gx mod p. The calculation of y
given x is relatively straightforward. Howeve r, given the public key y, it is
believed to be computationally infeasible to determine x, which is the
discrete logarithm of y to the base g, modp
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110 Information and Network Security To create a signature, a user calculates two quantities, r and s, that are
func- tions of the public key c omponents (p, q, g), the user’s private key
(x), the hash code of the message H(M), and an additional integer k that
should be generated randomly or pseudorandomly and be unique for each
signing. At the receiving end, verification is performed using the fo rmulas
shown in Figure.The receiver generates a quantity v that is a function of
the public key com - ponents, the sender’s public key, and the hash code of
the incoming message. If this quantity matches the r component of the
signature, then the signature is validated.
Figure depicts the functions of signing and verifying.
The structure of the algorithm, as revealed in Figure, is quite interesting.
Note that the test at the end is on the value r, which does not depend on the
message at all. Instead, r is a function of k and the three global public -key
components. The multiplicative inverse of k (mod q) is passed to a
function that also has as inputs the message hash code and the user’s
private key. The structure of this function is such that the receiver can
recover r using the incoming message and signature, the public key of the
user, and the global public key. It is certainly not obvious from Figure that
such a scheme would work. A proof is provided in Appendix K. Given the
difficulty of taking discrete lo garithms, it is infeasible for an oppo -nent to
recover k from r or to recover x from s.
Another point worth noting is that the only computationally demanding
task in signature generation is the exponential calculation gk mod p.
Because this value does not depend on the message to be signed, it can be
computed ahead of time.

Indeed, a user could precalculate a number of values of r to be used to sign
documents as needed. The only other somewhat demanding task is the
determina - tion of a multiplicative inve rse, k- 1. Again, a number of these
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111 Digital Signatures and Authentication and Authentication Applications 7.5 AUTHENTICATION A PPLICATIONS
7.5.1 Kerberos
Kerberos authentication is currently the default authorization technology
used by Microsoft Windows, and implementations of Kerberos exist in
Apple OS, FreeBSD, UNIX, and Linux.
Microsoft introduced their version of Kerberos in Windows2000. It has
also become a standard for websites and Single -Sign-On implementations
across platforms. The Kerberos Consortium maintains Kerberos as
an open -source project .
Kerberos is a vast improvement on previous authorization technologies.
The strong cryptography and third -party ticket authorization make it much
more difficult for cybercriminals to infiltrate your network. It is not totally
without flaws, and in order to defend against those flaws, you need to first
understand them.
Kerberos has made the internet and its denizens more secure, and enables
users to do more work on the Internet and in the office without
compromising safety.
Kerberos Protocol Flow Overview
Let's take a more detailed look at what Kerberos authentication is and how
it works by breaking it down into its core components.
Here are the princ ipal entities involved in the typical Kerberos workflow:
 Client. The client acts on behalf of the user and initiates
communication for a service request
 Server. The server hosts the service the user wants to access
 Authentication Server (AS). The AS perfor ms the desired client
authentication. If the authentication happens successfully, the AS
issues the client a ticket called TGT (Ticket Granting Ticket). This
ticket assures the other servers that the client is authenticated
 Key Distribution Center (KDC). In a Kerberos environment, the
authentication server logically separated into three parts: A database
(db), the Authentication Server (AS), and the Ticket Granting Server
(TGS). These three parts, in turn, exist in a single server called the
Key Distributio n Center
 Ticket Granting Server (TGS). The TGS is an application server
that issues service tickets as a service
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112 Information and Network Security First, there are three crucial secret keys involved in the Kerberos flow.
There are unique secret keys for the client/user, the TGS, and the server
shared with the AS.
 Client/user. Hash derived from the user's password
 TGS secret key. Hash of the password employed in determining the
TGS
 Server secret key. Hash of the password used to determine the
server pr oviding the service.
The protocol flow consists of the following steps:

Here are the most basic steps taken to authenticate in a Kerberized
environment.
1. Client requests an authentication ticket (TGT) from the Key
Distribution Center (KDC)
2. The KDC v erifies the credentials and sends back an encrypted TGT
and session key
3. The TGT is encrypted using the Ticket Granting Service (TGS)
secret key
4. The client stores the TGT and when it expires the local session
manager will request another TGT (this pro cess is transparent to the
user)
If the Client is requesting access to a service or other resource on the
network, this is the process:
5. The client sends the current TGT to the TGS with the Service
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113 Digital Signatures and Authentication and Authentication Applications 6. The KDC verifies the TGT of the user and that the user has access to
the service
7. TGS sends a valid session key for the service to the client
8. Client forwards the session key to the service to prove the user has
access, and the service gran ts access.
7.5.2 X.509 Authentication
X.509 is a standard format for public key certificates , digital documents
that securely associate cryptographic key pairs with identities such as
websites, individuals, or organizations.
First introduced in 1988 alongs ide the X.500 standards for electronic
directory services, X.509 has been adapted for internet use by the IETF’s
Public -Key Infrastructure (X.509) (PKIX) working group. RFC
5280 profiles the X .509 v3 certificate, the X.509 v2 certificate revocation
list (CRL), and describes an algorithm for X.509 certificate path
validation.
Common applications of X.509 certificates include:
 SSL/TLS and HTTPS for authenticated and encrypted web browsing
 Signed and encrypted email via the S/MIME protocol
 Code signing
 Document signing
 Client authentication
 Government -issued electronic ID
Key Pairs and Signatures
No matter its int ended application(s), each X.509 certificate includes
a public key, digital signature , and information about both the identity
associated with the certificate and its issuing certificate authority (CA) :
 The public key is part of a key pair that also includ es a private
key. The private key is kept secure, and the public key is included in
the certificate. This public/private key pair:
 Allows the owner of the private key to digitally sign documents;
these signatures can be verified by anyone with the correspo nding
public key.
 Allows third parties to send messages encrypted with the public key
that only the owner of the private key can decrypt.
 A digital signature is an encoded hash (fixed -length digest) of a
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114 Information and Network Security X.509 certificate is signed by a publicly trusted CA, such as
SSL.com, the certificate can be used by a third party to verify the
identity of the entity presenting it.
Note: Not all applications of X.509 certificates require public trust.
For exam ple, a company can issue its own privately trusted
certificates for internal use. For more information, please read our
article on Private vs. Public PKI.
 Each X.509 certificate includes fields specifying
the subject , issuing CA, and other required informa tion such as the
certificate’s version and validity period . In addition, v3 certificates
contain a set of extensions that define properties such as acce ptable
key usages and additional identities to bind a key pair to.
Certificate Fields and Extensions
To review the contents of a typical X.509 certificate in the wild, we will
examine www.ssl.com’s SSL/TLS certificate, as shown in Google
Chrome. (You can c heck all of this in your own browser for any HTTPS
website by clicking the lock on the left side of the address bar.)
 The first group of details includes information about the Subject ,
including the name and address of the company and the Common
Name (or F ully Qualified Domain Name) of the website that the
certificate is intended to protect. ( Note: the Seria l Number shown
in this subject field is a Nevada business identification number, not
the serial number of the certificate itself.)

 Scrolling down, we encounter information about the Issuer . Not
coincidentally, in this case, the Organization is “SSL Corp” for
both subject and issuer, but the issuer’s Common Name is the name
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115 Digital Signatures and Authentication and Authentication Applications

 Below the Issuer, we see the c ertificate’s Serial Number (a positive
integer uniquely identifying the certificate), X.509 Version (3),
the Signature Algorithm , and dates specifying the
certificate’s Validity Period .

 Next, we arrive at the Public Key, Signature , and associated
informa tion

 In addition to the fields above, X.509 v3 certificates include a group
of Extensions that offer a dditional flexibility in certificate use. For
example, the Subject Alternative Name extension allows the
certificate to be bound to multiple identities. (For this reason,
multiple -domain certificates are sometimes referred to as SAN
certificates ). In the example below, we can see that the certificate
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116 Information and Network Security

 The Fingerprints shown below the certificate information in
Chrome are not part of the certificate itself, but are independently
calculated hashes that can be used to uniquely identify a certificate.

Certificate Chains
For bo th administrative and security -related reasons, X.509 certificates are
typically combined into chains for validation. As shown in the screenshot
from Google Chrome below, the SSL/TLS certificate for www.ssl.com is
signed by one of SSL.com’s intermediate ce rtificates, SSL.com EV SSL Intermediate CA RSA R3. In turn, the intermediate certificate is signed by SS L.com’s EV RSA root:

For publicly trusted websites, the web server will provide its own end-
entity certificate, plus any intermediates required for va lidation. The root
CA certificate with its public key will be included in the end user’s
operating syste m and/or browser application, resulting in a complete chain
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117 Digital Signatures and Authentication and Authentication Applications Revocation
X.509 certificates that must be invalidated before their Not Valid
After date may be revoked . As mentioned above, RFC 5280 profiles
certificate revocation lists (CRLs), time -stamped lists of revo ked
certificates that can be queried by browsers and other client software.
7.5.3 Public -Key Infrastruct ure
The most distinct feature of Public Key Infrastructure (PKI) is that it uses
a pair of keys to achieve the underlying security service. The key pair
comprises of private key and public key.
Since the public keys are in open domain, they are likely to b e abused. It
is, thus, necessary to establish and maintain some kind of trusted
infrastructure to manage these keys.
Key Management
It goes without sayi ng that the security of any cryptosystem depends upon
how securely its keys are managed. Without secure procedures for the
handling of cryptographic keys, the benefits of the use of strong
cryptographic schemes are potentially lost.
It is observed that cry ptographic schemes are rarely compromised through
weaknesses in their design. However, they are often co mpromised through
poor key management.
There are some important aspects of key management which are as
follows −
 Cryptographic keys are nothing but spec ial pieces of data. Key
management refers to the secure administration of cryptographic
keys.
 Key manage ment deals with entire key lifecycle as depicted in the
following illustration −
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118 Information and Network Security  There are two specific requirements of key management for public
key cryptography.
 Secrecy of private keys. Throughout the key lifecycle, secret
keys must remain secret from all parties except those who are
owner and are authorized to use them.
 Assurance of public keys. In public key cryptography, the
public keys are in ope n domain and seen as public pieces of
data. By default there are no assurances of whether a public
key i s correct, with whom it can be associated, or what it can
be used for. Thus key management of public keys needs to
focus much more explicitly on assuran ce of purpose of public
keys.
The most crucial requirement of ‘assurance of public key’ can be achieved
through the public -key infrastructure (PKI), a key management systems
for supporting public -key cryptography.
Public Key Infrastructure (PKI)
PKI provid es assurance of public key. It provides the identification of
public keys and their distribution. An ana tomy of PKI comprises of the
following components.
 Public Key Certificate, commonly referred to as ‘digital certificate’.
 Private Key tokens.
 Certificat ion Authority.
 Registration Authority.
 Certificate Management System.
Digital Certificate
For analogy, a certificate can be considered as the ID card issued to the
person. People use ID cards such as a driver's license, passport to prove
their identity. A digital certificate does the same basic thing in the
electronic world, but with one difference.
Digital Certificates are not only issued to people but they can be issued to
computers, software packages or anything else that need to prove the
identity in th e electronic world.
 Digital certificates are based on the ITU standard X.509 which
defines a standard ce rtificate format for public key certificates and
certification validation. Hence digital certificates are sometimes also
referred to as X.509 certificat es.
Public key pertaining to the user client is stored in digital certificates
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119 Digital Signatures and Authentication and Authentication Applications information such as client information, expiration date, usage, issuer
etc.
 CA digitally signs this entire informat ion and includes digital
signature in the certificate.
 Anyone who needs the assurance about the public k ey and
associated information of client, he carries out the signature
validation process using CA’s public key. Successful validation
assures that the p ublic key given in the certificate belongs to the
person whose details are given in the certificate.
The process of obtaining Digital Certificate by a person/entity is
depicted in the following illustration.

As shown in the illustration, the CA accepts t he application from a client
to certify his public key. The CA, after duly verifying identity of client,
issues a digital certificate to that client.
Certifying Authority (CA)
As discussed above, the CA issues certificate to a client and assist other
users to verify the certificate. The CA takes responsibility for identifying
correctly the identity of the cl ient asking for a certificate to be issued, and
ensures that the information contained within the certificate is correct and
digitally signs it.
Key Fun ctions of CA
The key functions of a CA are as follows −
 Generating key pairs − The CA may generate a key pair
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120 Information and Network Security  Issuing digital certificates − The CA could be thought of as the
PKI equivalent of a passport agency − t he CA issues a certificate
after client provides the credentials to confirm his identity. The CA
then signs the certificate to prevent modification of the details
contained in the certificate.
 Publishing Certificates − The CA need to publish certificates s o
that users can find them. There are two ways of achieving this. One
is to publish certificates in the equivalent of an electronic telephone
directory. The other is to send your certificate out to those people
you think might need it by one means or anoth er.
 Verifying Certificates − The CA makes its public key available in
environment to assist verification of his signature on clients’ digital
certificate.
 Revocation of Certificates − At times, CA revokes the certificate
issued due to some reason such as compromise of private key b y
user or loss of trust in the client. After revocation, CA maintains the
list of all revoked certificate that is available to the environment.
Classes of Certificates
There are four typical classes of certificate −
 Class 1 − These certificates can be easi ly acquired by supplying an
email address.
 Class 2 − These certificates require additional personal information
to be supplied.
 Class 3 − These certificates can only be purchased after checks have
been made about the requestor’s identity.
 Class 4 − They ma y be used by governments and financial
organizations needing very high levels of trust.
Registration Authority (RA)
CA may use a third -party Registration Authority (RA) to perform the
necessary checks on the person or company requesting the certificate to
confirm their identity. The RA may appear to the client as a CA, but they
do not actually sign the certificate that is issued.
Certificate Management System (CMS)
It is the management system through which certificates are published,
temporarily or permanen tly suspended, renewed, or revoked. Certificate
management systems do not normally delete certificates because it may be
necessary to prove their status at a point in time, perhaps for legal reasons.
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121 Digital Signatures and Authentication and Authentication Applications Private Key Tokens
While the public key of a client is stored on the certificate, the associated
secret private key can be stored on the key owner’s computer. This
method is generally no t adopted. If an attacker gains access to the
computer, he can easily gain access to private key. For this reason, a
private key is stored on secure removable storage token access to which is
protected through a password.
Different vendors often use differ ent and sometimes proprietary storage
formats for storing keys. For example, Entrust uses the proprietary .epf
format, while Verisign, GlobalSign, and Baltimore use the standard .p12
format.
Hierarchy of CA
With vast networks and requirements of global com munications, it is
practically not feasible to have only one trusted CA from whom all users
obtain their certificates. Secondly, availability of only one CA may lead to
difficulties if CA is compromised.
In such case, the hierarchical certification model i s of interest since it
allows public key certificates to be used in environments where two
communicating parties do not have trust relationships with the same CA.
 The root CA is at the top of the CA hierarchy and the root CA's
certificate is a self -signed certificate.
 The CAs, which are directly subordinate to the root CA (For
example, CA1 and CA2) have CA certificates that are signed by the
root CA.
 The CAs under the subordinate CAs in the hierarchy (For example,
CA5 and CA6) have their CA certificates sig ned by the higher -level
subordinate CAs.
Certificate authority (CA) hierarchies are reflected in certificate chains. A
certificate chain traces a path of certificates from a branch in the hierarchy
to the root of the hierarchy.
The following illustration s hows a CA hierarchy with a certificate chain
leading from an entity certificate through two subordinate CA certificates
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122 Information and Network Security

Verifying a certificate chain is the process of ensuring that a specific
certific ate chain is valid, correctly signed, and trustworthy. The following
procedure verifies a certificate chain, beginning with the certificate that is
presented for authentication −
 A client whose authenticity is being verified supplies his certificate,
gener ally along with the chain of certificates up to Root CA.
 Verifier takes the certificate and validates by using public key of
issuer. The issuer’s public key is found in the issuer’s certificate
which is in the chain next to client’s certificate.
 Now if the higher CA who has signed the issuer’s certificate, is
trusted by the verifier, verification is successful and stops here.
 Else, the issuer's certificate is verified in a similar manner as done
for client in above steps. This process continues till either trusted
CA is found in between or else it continues till Root CA.

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123 Digital Signatures and Authentication and Authentication Applications 7.6 SUMMARY
 The Digital Signature is a technique which is used to validate the
authenticity and integrity of the message.
 Message authentication protects two parties who exchange messages
from any third party. However, it does not protect the two parties
against each other. Several forms of dispute between the two are
possible.
 Authentication Protocols are PAP - Password Authentication
Protocol, CHAP - Challenge -handshake authentication protocol ,
EAP - Extensible Authentication Protocol.
 The DSS uses an algorithm that is designed to provide only the
digital signature function. Unlike RSA, it cannot be used for
encryption or key exchange. Nevertheless, it is a public -key
technique
 Kerberos has made the internet and its denizens more secure, and
enables users to do more work on the Internet and in the offi ce
without compromising safety.
 X.509 is a standard format for public key certificates , digital
documents that securely associate cryptographic key pairs with
identities such as websites, individuals, or organizations.
 The most distinct feature of Public K ey Infrastructure (PKI) is that it
uses a pair of keys to achieve the underlying security service. The
key pair comprises of private key and public key.
 PKI provides assurance of public key. It provides the identification
of public keys and their distribut ion
 For analogy, a certificate can be considered as the ID card issued to
the person. People use ID cards such as a driver's license, passport to
prove their identity. A digital certificate does the same basic thing in
the electronic world, but with one di fference.
 Digital Certificates are not only issued to people but they can be
issued to computers, software packages or anything else that need to
prove the identity in the electronic world.
7.7 QUESTIONS
1. Write a short note on Digital Signature.
2. What are the aspects to achieve Digital Signature?
3. What are the Authentication Protocols?
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124 Information and Network Security 5. How does CHAP - Challe nge-handshake authentication protocol
works?
6. Write a short note on EAP - Extensible Authentication Protocol.
7. Elaborate Digital Signature Standard
8. Explain The Digital Signature Algorithm.
9. Write a short note on Kerberos
10. How X.509 Authentication works?
11. What is Public Key Infrastructure (PKI)?
12. Write a short note on Digital Certificate.
7.8 REFERENCE FOR FU RTHER READING
 http://www.facweb.iitkgp.ac.in/~sourav/AuthenticationRequirement
s.pdf
 https://doubleoctopus.com/security -wiki/protocol/extensible -
authentication -protocol/
 https://www.brainkart.com/article/Digital -Signature -Standard_8465/
 https://www.varonis.com/blo g/kerberos -authentication -explained/
 https://www.simplilearn.com/what -is-kerberos -article
 https://www.ssl.com/faqs /what -is-an-x-509-certificate/
 https://blog.avast.com/the -importance -of-authentication -avast
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125 8
ELECTRONIC MAIL SECURITY AND IP
SECURITY
Unit Structure :
8.1 Objectives
8.2 Introduction
8.3 Pretty Good Privacy
8.3.1 What is PGP?
8.3.2 How does PGP work?
8.3.3 Disadvantages of PGP
8.4 S/MIME
8.4.1 What is S/MIME?
8.4.2 How does S/MIME work?
8.5 Differentiate between PGP and S/MIME
8.6 IP Security
8.6.1 Overview
8.6.2 Benefits of IP Security
8.6.3 Components of IP Security
8.6.4 Working of IP Security
8.7 IP Security Architecture
8.8 Authentication Header (AH)
8.9 Encapsulating Securit y Payload
8.10 Combining Security Association
8.10.1 Implementing SA
8.10.2 Modes of operations
8.11 Key Management
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126 Information and Network Security 8.1 OBJECTIVES
This chapter would make you understand the following concepts
 Why is Email Security needed?
 How does Pretty Good Privacy ensure Email Security
 S/MIME provides Security
 How to secure transmission of packets at network layer using IP
protocol
 How to ensure confidentiality and authentication
8.2 INTRODUCTION
Electronic mail is commonly known as E -mail or Email is a technique of
exchanging information(mail) between people using any electronic
devices. In 1971, Ray Tomlinson invented networked email. He developed
the first system which made it possible to send mail between
people/organizations using diff erent devices across the Internet, using the
@ symbol which links the username with the destination device.
By the mid -1970s, it gained its popularity as email.
Nowadays, Emails are being used by everyone, maybe for professional or
personal use which may even carry confidential information.
Hence, it becomes very essential to provide security to emails as well.
Email security ensures us that there is control over the content of any
email account.
An email service provider enables efficient email security to protect any
email accounts and its data from attackers or hackers and deliver the mail
to a valid recipient.
8.3 PRETTY GOOD PRIVACY (PGP)
Prior to the Pretty Good Privacy technique was introduced, the email
provider, Internet Provider, hackers, or ev en the government could view
and read your messages.
But later, PGP was developed in the 1990s by Phil Zimmermann, allowing
email and any other types of messages to be exchanged securely.
In today's time, PGP has already been standardized into OpenPGP, w hich
enables anyone to write Pretty Good Privacy (PGP) software which is
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127 Electronic Mail Security and IP Security 8.3.1 What is PGP?
PGP is a cryptographic technique that allows people to communicate
securely online. PGP is an open source a nd free of cost software package
for providing email security.
When you send any mail using PGP technique, the mail content is
converted into ciphertext which is in an unreadable format before it passes
over the Internet. Its only the recipient who has the key can convert the
unreadable text back into the readable format.
PGP also helps in authenticating the identity of the sender and verifies that
the message was not modified during the transmission of the message.
OpenPGP.js is one of the world’s most wi dely used OpenPGP libraries
and it has been closely audited by the security experts.
It was designed to provide all the four aspects of security, i.e.,
confidentiality, authentication, integrity, and non -repudiation while
sending an email.
It uses a combin ation of public key encryption and secret key encryption
to provide confidentiality.
It uses the technique of digital signature (i.e., combining public key
encryption and hashing) to provide authentication, integrity, and non -
repudiation.
Hence, we can ea sily say that the digital signature technique uses one
secret key, one hash function, and two private -public key pairs.
It also provides compression by using EMAIL compatibility using the
radix -64 encoding scheme and the ZIP algorithm.
8.3.2 How does PGP w ork?
Initially, PGP was very difficult to be used as it required additional
software applications with an email provider or a client. We also had to
manually create encryption keys and exchange them with our contacts.
Using ProtonMail which is a library, P GP is built and runs automatically
and privately to the users. When you compose any mail to another
ProtonMail user and click the send button, the message encryption and
signature techniques are applied automatically without any human
interference. You don ’t have to do anything at all, nor do we need any
specialized technical knowledge or technical person. ProtonMail has made
PGP encryption easy, accessible, and convenient to everyone.
The steps taken by PGP to create secure e -mail are as follows:
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128 Information and Network Security

 A hashing function is used to hash an email message to create a
digest.
 Using the sender's private key, the digest is encrypted to form a
signed digest, and then this signed digest is added to the original
message.
 The s igned digest and the original message are together encrypted
by using a one -time secret key which is created by the sender.
 Using the receiver's public key, the secret key is encrypted for
security reasons.
 The encrypted combination of digest and the messa ge along with
encrypted secret key is sent together.
The following steps shows how PGP generates the original message using
a combination of three keys and hashing:
ii) PGP implementation at the Receiver’s side
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129 Electronic Mail Security and IP Security  At the receiver’s end, the recipient receiv es the combination of
message, digest and the encrypted secret key and message digest.
 By using the receiver's private key, the encrypted secret key is
decrypted to get the one -time secret key.
 The secret key is however then used to decrypt the combination of
the message and digest.
 By using the sender's public key, the digest is decrypted
 To create a digest, the original message is hashed by using a hashing
function.
 Now, both the digests are compared. If both of the digests are equal
then it means that al l the aspects of security are maintained.
8.3.3 Disadvantages of PGP Encryption
 Complicated: Different versions of PGP makes the understanding
difficult for the administration.
 Compatibility issues: Both the recipient and the sender must have
compatible ve rsions of PGP. For example, if a sender encrypts an
email message by using PGP with one encryption technique, and the
receiver has a different version of PGP then the data cannot be read.
 Complexity: Usually, other security schemes use symmetric
encryption which uses one key or asymmetric encryption which uses
two different keys. But PGP uses a hybrid approach which
implements symmetric encryption with two keys. PGP is more
complex, and hence, it is less familiar than any other symmetric or
asymmetric metho ds.
 No Recovery: Computer administrators face a common problem of
losing their passwords. In such a scenario, an administrator must use
a special program to restore the passwords. For example, a
technician has access to a PC which can be used to restore a
password. However, PGP does not provide any such facility for
password recovery as the encryption methods used are very strong.
8.4 S/MIME
Isn`t it enough that our Email Server Is Already Encrypted?
Keeping in mind the security concerns, encrypting your em ail servers with
Digital Certificates is a good initiative towards security. This has also
prevented outsiders from getting access to your email and mail servers and
prevented from viewing confidential data. Still, your emails will be
protected to and from the encrypted server but the hackers can still invade
into your emailing system and view your messages or have access to them
while they are passing through other servers.
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130 Information and Network Security 8.4.1 What is S/MIME?
S/MIME, or Secure/Multipurpose Internet Mail Extensions, is a type of
“end-to-end” encryption technology that allows the sender to encrypt the
emails messages.
S/MIME implements asymmetric cryptography to protect our emails from
unwanted access by unauthorized persons.
It allows us to digitally sign our emails to verify the sender as an
authorised sender of the email message, making it effective against
attacks.
8.4.2 How does S/MIME work?
S/MIME uses a pair of mathematically associated keys (private key and a
public key) to operate.
It is operationally impossible to find out the private key based on the
public key.
Emails are basically encrypted using the recipient’s public key.
The email can only be decrypted with the recipient’s private key, which is
only known to the recipient.
You can be assured that only t he intended recipient is able to access the
confidential data of your emails unless the private key is compromised.
An S/MIME certificate must be installed on both the email clients, the
recipient and the sender.
S/MIME also attaches a digital signature t o an email message which
verifies that the sender is authorized to send emails.
People usually think, is S/MIME Certificate needed even If we Have an
Encrypted Email Server?
There’s a difference in the way email server certificates work and
S/MIME certific ates work.
Rather than the email messages, Email server certificates basically encrypt
the email communication channel; meaning it encrypts emails during
transmission.
While S/MIME certificates encrypt the email messages.
Hence, hackers can get inside the server and have access to your emails
even if you have a digital certificate installed on your email server,
Here’s where S/MIME certificates come into picture.
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131 Electronic Mail Security and IP Security How Does an S/MIME Certificate Work in Email Security?

Figure: Public Key encryption (Asym metric)
Verification of a message is necessary and to do so, an email which is
signed with an S/MIME certificate generates a hash which in turn helps to
create the digital signature in encrypting the email. The recipient then
confirms the digital signatur e, and the email gets decrypted by the
authorised recipient..
Hence, when a person sends an email message, it is encrypted with the
public key. This encrypted email is decrypted by the private key which is
related to the public key.
In this way, it prote cts email from unauthorized access and also
authenticates the identity of the sender/receiver
8.4.3 Benefits of S/MIME Certificates
The benefits of S/MIME Certificates are as follows:
 Non repudiation: At any point, the sender cannot deny the email and
its contents were sent by him/her as a digital signature as a proof that
the email message has been received from the signer’s email client.
 Protection from email corruption: No attacker can modify or insert
any malicious software such as viruses, computer worms, trojan
horses, spyware, rootkit, etc. while the email is in transmission.
 Protection against email spoofing: Digital signature prevents the
email recipients from email spoofing. Nobody can forge the digital
signature of the organization’s official staff members.
 Warns the recipients: If anybody tampers the email or digital
signature, it immediately sends alerts to the recipients about the
security risk. So that the recipients can take preventive measures to
protect themselves from becoming victims of any cyberattack.
 Protects business’s reputation: If the customers/clients receives an
email containing viruses or malicious software from the
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132 Information and Network Security business’s reputation. With the implementatio n of S/MIME
certificates, no email can be tampered to an extent.
 Prevents data leakage: An S/MIME certificate prevents data from
data leakage and eavesdropping. The organization’s confidential
communications with external and intern al stakeholders remain
secured.
8.5 DIFFERENCE BETWEEN P GP AND S/MIME: S.NO PGP S/MIME 1. It is basically designed
for processing the plain
readable text email
messages It is designed to protect email
messages as well as
multimedia files. 2. PGP is less expensive. S/MIME is c omparatively
more expensive than PGP. 3. PGP is good for office as
well as personal use. S/MIME is best for industrial
use. 4. PGP is comparatively
less efficient S/MIME is more efficient
than PGP. 5. It is dependent on user
key exchange. S/MIME relies on a valid
certificate for key exchange. 6. PGP is less convenient
than S/MIME. S/MIME is more convenient
because of the secure
transformation of all the
applications. 7. PGP possibly contains
4096 public keys. S/MIME contains only 1024
public keys. 8. PGP is the standard for
strong encryption. S/MIME is also a standard for strong encryption but has few drawbacks. 9. PGP can also be used in
VPNs. S/MIME cannot be used in
VPNs, It is only used in email
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133 Electronic Mail Security and IP Security 8.6 IPSECURITY - OVERVIEW
The IP security (IPSec) is a secure, standard network protocol suite
which helps in communication between two points across the IP network.
It provides data confidentia lity, integrity, and authentication. It also
supports the encryption, decryption and authentication of packets.

Figure: IP Security Scenario
8.6.1 Benefits of IP Security –
IPsec can be used achieving the following:
 It provides encryption of application layer data.
 It provides security for switches/routers sending data across the
internet.
 It also provides authentication without encryption, such as to
authenticate that the data originated from a known or a valid sender.
 It protects network data by settin g circuits using IPsec tunneling
through which data can be encrypted and sent between the two
endpoints, as it's done in Virtual Private Network(VPN) connection.
8.6.2 Components of IP Security –
The components of IP Security is as follows:
1. Authenticati on Header (AH) – It provides data authentication,
integrity, and anti replay but it does not provide encryption. The anti
replay technique protects against unauthorized transmission of data
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134 Information and Network Security

Figure : Authentication Header (AH)
2. Encapsulating Security Payload (ESP) – It provides encryption,
authentication, data integrity, and anti replay. It also provides
authentication and encryption for packet payload.

Figure: Encapsulating Security Payload (ESP )
3. Internet Key Exchange (IKE) – It's a network layer security
protocol designed to dynamically exchange encryption keys and
support Security Association (SA) between 2 endpoints. The
ISAKMP (Internet Security Association and Key Management
Protocol) not only supports key exchange but also authentication of
packets.
Internet Key Exchange (IKE) provides a framework for
implementing algorithms such as MD5, SHA and also protects
message content.
8.6.3 Working of IP Security –
1. The host (sender) checks if the packet has to be transmitted using
IPsec or not depending on the contents in the packet. These packet
traffic sets the security policy for themselves by applying
appropriate encryption. The incoming packets are also examined by
the host whether they are properly encrypted or not.
2. The IKE(Internet Key Exchange) Phase 1 begins in which the 2
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135 Electronic Mail Security and IP Security 3. The channel which was created is now being used to sec urely deal
with the method that the IP Security provides confidentiality of data.
4. The IKE(Internet Key Exchange) Phase 2 is managed over the
secure channel in which the two hosts will agree upon the type of
cryptography algorithms to be used and also th e secret key to be
used by those algorithms.
5. The data is then exchanged across the created IPsec encrypted
tunnel. These packets are further decrypted and encrypted by the
hosts using IPsec SAs.
6. Once the communication between the hosts is done or whe n the
session times out then the IPsec tunnel is terminated.
8.7 IPSEC (IP SECUR ITY) ARCHITECTURE
IPSec uses two protocols to secure the data or traffic. These protocols are
AH (Authentication Header) and ESP (Encapsulation Security Payload).
IPSec Arch itecture also includes algorithms, protocols, DOI( Domain of
Interpretation) , and Key Management. All the above stated components
are essential to provide the three main services:
 Confidentiality
 Integrity
 Authentication

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136 Information and Network Security 1. Architecture:
IP Security Architecture includes the concepts, protocols,
definitions, algorithms and security requirements of IP Security
technology.
2. ESP Protocol:
ESP(Encapsulation Security Payload) provides confidentiality or
authentication or bot h the services.
3. Encryption algorithm:
Encryption algorithm includes documents that specify the encryption
algorithms used for Encapsulation Security Payload(ESP).
4. AH Protocol:
AH (Authentication Header) Protocol provides both Integrity and
Authent ication service.
5. Authentication Algorithm:
The Authentication Algorithm includes documents that specify the
authentication algorithm which is used for both AH and also for the
authentication of ESP.
6. DOI (Domain of Interpretation):
DOI is an identi fier (unique) which supports both AH and ESP
protocols. It contains values required for documentation related to
each.
7. Key Management:
Key Management contains a document that defines how the keys
must be exchanged between sender and receiver.
8.8 AUT HENTICATION HEADER (A H)
Authentication Header (AH) authenticates the origin of data and also
verifies whether the data has been modified (integrity) during the
transmission between source and destination.
Authentication Header is implemented only in one wa y: Authentication
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137 Electronic Mail Security and IP Security

Figure: Authentication Header Packet Format
Next Header – The 8 -bit field that defines the type of header which is
attached after the Authentication Header. For example, 4 indicates IPv4, 6
indicates TCP.

Payload Length – Payload length is basically the length of the
Authentication header and we will use a scaling factor of 4. Whatever may
be the size of the header, it has to be divided by 4 and then subtract by 2.
We will subtract by 2 because we are not counting the first 8 bytes of the
Authentication header.
Say that the payload length is given as X. Then (X+2)*4 will be the
original Authentication header length.
Reserved – This 16 -bit field is set to “zero” by the sender as this fi eld is
reserved for future use .
Security Parameter Index (SPI) – It is a 32 -bit field which helps to
identify all packets which belong to the present connection. If we are
sending data from Source A to Destination B. Both A and B will know the
algorithm and key they will use. For Authe ntication, key and hashing
functions will be needed which only the source and destination will know.
Secret key between A and B is usually exchanged by using the Diffie
Hellman algorithm. So the Hashing algorithm and secret key for Security
parameter index of connection has to be finalized.
Sequence Number – This unsigned 32 -bit field consists of a counter
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138 Information and Network Security be assigned a sequence number. It will start from 0 till 232 – 1. sequence
numbers are used to prevent replay attack.
In Replay attack, the same message is transmitted twice or more without
the receiver knowing whether both messages are sent from the same
source or not.
Authentication Data Authentication data is a variable length field that
contains Integrity Check Value (ICV) for packets during transmission
through the internet. Using a secret key and a hashing function, the sender
will generate a message digest which will be sent to the receiver. On the
other hand, Receive will use the same hashing function and secret key. If
both message digest is the same then the receiver will accept data. If the
message digest is not the same, then the receiver will be discarded
concluding that the message has been modified during transmissio n.
8.9 ENCAPSULATION SECURITY PAYLOAD (ES P)
Encapsulating Security Payload (ESP) is a set of protocols that enables
encrypting and authenticating the packets of data to and fro during
transmission using a Virtual Private Network (VPN) to function secure ly.
Encapsulation Security Payload is implemented in any one of the
following ways:
 ESP with only encryption keeping Authentication as optional.
 ESP with Authentication.

Figure: ESP Packet Format
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139 Electronic Mail Security and IP Security ● Security Parameter Index(SPI): This parameter is defin ed in the
Security Association. It gives a unique number to the connection that
is established between a Client and a Server.
● Sequence Number: A Unique Sequence number is assigned to
every packet so that at the receiver end the packets are arranged
seque ntially to maintain the order and read the message meaningful..
● Payload Data: Payload data is the actual message that is to be
encrypted using encryption algorithm to achieve confidentiality
during transmission of packet.
● Padding: These are the extra b its added to the original message in
order to maintain confidentiality.
● Next Header: Next header defines the type of header which is
attached after the Encapsulating Security Payload Header
● Authentication Data This field is optional in ESP packet form at
which is used to verify the identity of the sender.
8.10 COMBINING SECU RITY ASSOCIATIONS
8.10.1 Implementing SA
The Security Association (SA) agrees upon a shared security protocol
between 2 network endpoints systems to communicate securely.
In the Se curity Association (SA), both the parties (the sender and the
receiver) need to communicate before the actual data exchange. Security
association informs what security parameter index, secret key and hashing
algorithm are to be used.

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140 Information and Network Security The IPsec Architecture document contains four scenarios of combinations
of Security Associations that are supposed to be supported by security
gateways (e.g. router, firewall) and IPsec hosts (e.g. server or a
workstation). These are illustrated in Figure above.
The lower diagram of the above figure indicates the physical connectivity
between the elements whereas the upper diagram indicates the logical
connectivity via one or more SAs. Each SA can either be ESP or AH.
Case 1. All security is contributed between endpoint systems that
implement IPsec. For any two endpoint systems to communicate through
an SA, they need to share the secret keys before actual transmission.
Among the possible combinations are
A. Authentication Header (AH) in transport mode
B. Encapsulating Security Payload (ESP) in transport mode
C. ESP followed by AH in transport mode (an ESP Security
Association inside an AH Security Association)
D. Any one of A, B, or C inside an ESP or AH in t unnel mode
Case 2. Security is provided between gateways (routers, firewalls, etc.)
only and IPSec is not implemented to any host. This case demonstrates a
simple virtual private network (VPN) support. The security architecture
document states that only a single tunnel SA is required in this case. The
tunnel can support ESP, ESP with authentication or AH. Since the IPsec
services are applied to the entire inner packet, nested tunnels are not
needed..
Case 3. This case is built on case 2 by also adding end -to-end security.
The gateway -to-gateway tunnelling provides either confidentiality,
authentication, or both for all traffic moving between the endpoints. If the
gateway -to-gateway tunnelling is ESP, it also provides traffic
confidentiality. Individual hosts can apply additional IPsec services
whenever required for any given applications or users by way of end to
end SAs.
Case 4. It supports a remote host who uses the Internet to reach an
organization’s firewall and gains access to some private network entit ies
(eg. workstation or server) working behind the firewall. Only tunnel mode
is needed to be implemented between the remote host and the firewall. We
can use one or two SAs between the local host and the remote host.
8.10.2 Modes of Operation in IPSecurit y
The encapsulation mode which is used defines the way in which the
original IP data packet is modified. There are basically two encapsulation
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141 Electronic Mail Security and IP Security 1. Transport Mode
The Transport mode encapsulation k eeps the original IP header.
Hence, when the transport mode is used, the IP header retains the
original source and destination of the packet.
Transport Mode is mostly used in a host -to-host scenario, where the
security endpoints and the data endpoints ar e the same.
Once the datagram is encapsulated in transport mode, the datagram
is transported in the same way as the original packet.

Figure: Transport Mode encapsulation of IP Packet
Transport Mode encapsulation can be implemented using AH or ESP

Figu re: IP packet encapsulation using AH in transport mode

Figure: IP packet encapsulation using ESP in transport mode
2. Tunnel Mode:
Tunnel mode encapsulates the original IP header and payload and
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142 Information and Network Security When tunnel mode is being used, the outer IP header (new IP
header) specifies the source and destination of the endpoints.
Tunnel Mode encapsulation can be implemented using AH or ESP

Figure: IP packet encapsulation using AH in tunnel mode

Figure: IP packet encapsulation using ESP in tunnel mode
The choice of using either transport mode or tunnel mode depends on the
design of the network and majorly on the logical connections between the
end to end systems. Possibly, tunnel mode is needed if any of the IKE
(Internet Key Exchange) end is a security gateway that is applying
IPSecurity on behalf of another host.
A datagram that is encapsulated using the tunnel mode is transmitted,
through the security gateways, assuming that t he secure IPSec packet will
not transmit through the same network path as that of the original
datagram.
8.11 KEY MANAGEMEN T
Security associations (SAs) support key generation for authentication of
the peer connections and for confidentiality of informat ion thus providing
internet security. This is known as key management. The key management
of IPSecurity includes determining and distributing secret keys.
IPSec supports two types of key management:
1. Manual
2. Automated
1. Manual Key Management:
A syste m administrator manually configures the system with its own
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143 Electronic Mail Security and IP Security 2. Automated Key Management:
This is a more practical approach which helps in creating keys for
security associations (SAs) and the keys used to be d istributed for
secure configuration and communication.
The default automated key management protocol of IPSec include:
 Oakley Key Determination Protocol -
It implements the popular key exchange algorithm ie Diffie Hellman
algorithm but with added security functionality.
Oakley can be used for major 3 authentication methodologies which are as
follows:
 Digital Signature
 Symmetric Key encryption
 Public Key encryption
 Internet Security Association and Key Management Protocol
(ISAKMP)
It basically provides a f ramework for key management and contains many
message types which can be used to implement key exchange algorithm
It is used for creating, modifying, negotiating, and deleting SAs and its
parameters. It also specifies various techniques for key generation and
methodologies for authenticating end to end systems.
8.12 UNIT END QUESTI ONS
1. Why is Email security needed?
2. How can Pretty Good Privacy ensure Email security?
3. State and explain the disadvantages of PGP
4. How can S/MIME promote Email security?
5. State and explain the benefits of S/MIME Certificate
6. Differentiate between PGP and S/MIME
7. What is IPSecurity? What is the need of IPSecurity?
8. Explain Authentication Header and its packet format?
9. Explain ESP and its packet format?
10. State a nd explain the components of IPSecurity architecture
11. How does the Security association provide IPSecurity?
12. Explain the two modes of operations implemented by IPSecurity

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144 9
WEB SECURITY AND INTRUSION
Unit Structure :
9.1 Objectives
9.2 Web Security Considerations
9.2.1 Introduction
9.2.2 Web Security threats
9.2.3 Web traffic security approaches
9.3 Secure Socket Layer & Transport Layer Security
9.3.1 SSL - Introduction
9.3.2 SSL Protocol Stack
9.3.3 TLS - Introduction
9.3.4 How does TLS work?
9.4 Secure Electronic Transaction
9.5 Intruders
9.5.1 Introduction
9.5.2 Types of intruders
9.6 Intrusion Techniques
9.7 Intrusion Detection
9.7.1 What is an Intrusion Detect ion System?
9.7.2 Types of IDS
9.8 Unit End Questions
9.1 OBJECTIVE
This chapter would make you understand the following concepts
 Threats in the network
 How to establish a secure connection between devices?
 Who are intruders?
 Intrusion techniques used by the intruders/hacker
 Techniques used for detecting any unauthorised access to private
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145 Web Security and Intrusion 9.2 WEB SECURITY CONSIDE RATIONS
9.2.1 Introduction:
Web security is a client/server programme that runs via the Internet and
intranets using TCP. Web security, som etimes referred to as "cyber
security," is safeguarding data by avoiding, detecting, and reacting to
assaults.
The World Wide Web is highly apparent. Many security vulnerabilities
are hidden in complex software. Web servers are simple to set up and
maintai n.
9.2.2 Web Security Threats -
Passive and active assaults are two methods to categorize these risks.
 Passive assault : Passive Eavesdropping on network communication
between the browser and the server, as well as getting access to
material on a Web site that is meant to be limited, are examples of
passive attacks.
 Active assaul t: Impersonating another user, changing
communications in transit between client and server, and
manipulating content on a Web site are all examples of active
assaults.
Another app roach to categories Web security concerns is by the threat's
location: the Web server, the Web browser, and the network traffic
between the browser and the server. Computer system security includes
issues such as server and browser security
9.2.3 Web Traff ic Security Approaches -
There are several ways to establish Web security.
The different techniques discussed are comparable in terms of the services
they provide and, to some degree, the mechanisms they employ, but they
differ in terms of their scope of application and position within the TCP/IP
protocol stack.
 IP security is one technique to provide Web security. IPsec has the
benefit of being transparent to end users and applications while also
providing a general -purpose solution. IPsec also contains a filtering
feature, allowing only chosen traffic to be subjected to IPsec
processing.
 SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are
two Internet standards (TLS). There are two options for
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146 Information and Network Security Although Web browsers are simple to use, Web servers are simple to set
up and administer, and Web content is becoming more complicated, the
underlying software is extremely complex.
Web -based services are frequently use d by casual and unskilled users.
Such users are often unaware of the security risks they face and lack the
tools and knowledge necessary to take effective countermeasures.
Some of the countermeasures to be adopted are as follows:
 Updated Software -
Keeping your software up to date is essential. It is crucial in
maintaining the security of your website.
 SQL Injection -
It's a hacker's effort to alter your database. It's simple to include
malicious code in your query that may be used to modify your
database, such as changing tables, retrieving information, or deleting
data.
 Cross Site Scripting (XSS) -
It gives attackers the ability to insert client -side script into web
pages. As a result, it is important to ensure that when building a
form, you examine the da ta being submitted and encode or strip
away any HTML.
 Error Messages -
You must be cautious in how much information you provide in error
messages. If a user is unable to log in, for example, the error
message should not indicate which field is incorrect: u sername or
password.
 Validation of Data -
Validation should be done both on the server and on the client.
 Passwords -
It's a good idea to impose password restrictions such as a minimum
of eight characters, including upper - and lower -case letters,
numbers, and special characters. In the long run, it will aid in the
protection of user information.
 Upload files -
The user's file might contain a script that, when run on the server,
launches your website.
 SSL -
When sending personal information between a website and a web
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147 Web Security and Intrusion 9.3 SECURE SOCKET LAYER AND TRANSPORT
LAYER SECURITY
9.3.1 SSL - Introduction
SSL stands for Secure Socket layer. SSL is a technique for establishing a
secure connection between two dev ices linked over the Internet or a
private network. SSL is a part of transport layer security.
It was developed by Netscape in 1995 for the purpose of sending data
from browser and server.
You can check that the website uses SSL/TLS by checking the URL
contains HTTPS rather than HTTP.

Figure: HTTP vs HTTPS
 SSL encrypts the data before it is sent to the internet i.e., anyone
who tries to steal the data will see a ciphertext that is impossible to
decode.
 SSL authenticates the devices and then communicates b etween them
by which data integrity is achieved. So, there will be no interference
between the user and server.
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148 Information and Network Security 9.3.2 SSL Protocol Stack

Figure: SSL Protocol Stack
In the SSL protocol, the application da ta is split into fragments. The
fragment is encrypted and then appended by SSL header.
The SSL Protocol Stack three main protocols implemented that are stated
as follows:
1. Handshake protocol:
This allows client and server to connect securely before sendi ng the
data.
2. Change cipher protocol:
This protocol will not be triggered unless the handshake protocol is
complete.
3. Alert Protocol:
Sends alert messages to the receiving party
After the handshake protocol is completed, the change cipher
protocol is executed.
Basically, it changes the pending state to the current state by sending
1 byte of message.
Then the Alert protocol is continued and so on.
9.3.3 TLS - Introduction
Transport Layer Securities (TLS) are designed to provide safety on the
transport l ayer. TLS is derived from SSL. Transport Layer Security (TLS)
is an internet Engineering task pressure (IETF) general protocol that gives
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149 Web Security and Intrusion
There are 3 primary comp onents to what the TLS protocol accomplishes:
Encryption, Authentication, and Integrity.
 Encryption
hides the information being transferred from third events.
 Authentication
guarantees that the events replacing information are who they
declare to be.
 Integrity
verifies that the statistics have no longer been cast or tampered with.
9.3.4 How does transport Layer security work?
TLS implements customer server handshake mechanism to create an
encrypted, authenticated and secure connection
Here are the followin g steps:
 Communicating devices exchange encryption abilities.
 An authentication method happens by using a digital certificate to
assist show the server is the entity it claims to be.
 A consultation key change happens. For the duration of this
procedure, cu stomers and servers need to agree on a key to set up
the truth that the secure session is certainly between the customer
and server -- and no longer something inside the middle trying to
hijack the information.
TLS makes use of a public key trade method to set up a shared secret
among the communicating devices.
The two handshake methods used by TLS are:
1. Rivest -Shamir -Adleman (RSA) handshake
2. Diffie -Hellman handshake.
Each method results in the same goal of setting up a shared secret between
communica ting devices so the communication cannot be hijacked. Once
the keys are exchanged, statistics transmissions between devices on the
encrypted session can start.
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150 Information and Network Security

Figure: Working of TLS
TLS is composed of two layers namely
1) TLS record Protocol
2) TLS H andshake Protocol.
TLS Handshake Protocol:
The TLS Handshake Protocol lets the server and purchaser to authenticate
each other and to negotiate an encryption algorithm and cryptographic
keys earlier than records are exchanged. The main responsibility of t he
TLS Handshake Protocol is to certify the authentication of the source and
key exchange which is required to establish secure connection. whilst
establishing a connection, the Handshake Protocol manages the following:
 Cipher suite negotiation.
 Authentica tion of the server and optionally, the consumer.
 Consultation key data exchange.
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151 Web Security and Intrusion The consumer and server make touch and choose the cipher suite with a
view to be used in the course of their message change. (Authentication &
Encr yption mixture)
 Authentication:
In TLS, a server proves its identification to the patron. The patron
may additionally want to prove its identification to the server. PKI,
the use of public/personal key pairs, is the premise of this
authentication. The corr ect method used for authentication depends
on the cipher suite negotiated.
 Key change:
The client and server change random numbers and a special variety
referred to as the Pre -grasp secret. Those numbers are mixed with
additional facts allowing patrons and servers to create their shared
secret, called the grasp secret. The grasp keys used by consumer and
server to generate the write MAC mystery, that is the session key
used for hashing, and the write key, that's the consultation key used
for encryption.
The TLS Handshake Protocol entails the subsequent steps:
 The customer sends a "hey customer" message to the server, along
with the customer's random value and supported cipher suites.
 The server responds through sending a "Hey Server" message to the
customer , in conjunction with the server's random value.
 For authentication purposes, the server sends it’s certificate to the
customer and may request a certificate from the customer as well.
 The server in response sends the "Hey Server finished" message to
the customer.
 If the server has asked for a certificate from the customer, the
customer sends it.
 The customer generates any random Premaster secret and also
encrypts it with the general public key from the server's certificate to
achieve confidentiality, thus sending the encrypted Premaster
mystery to the server.
9.4 SET ELECTRONIC TRANS ACTION
Earlier, before the implementation of Secure Electronic Transaction,
Transaction via credit card was done. Here the customer used to fill the
transaction details via for m or phone call, these details were sent to the
bank for verification. After a successful verification, the actual transaction
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152 Information and Network Security Service providers who used to accept credit card payments had to pay card
charge, authoriz ation fee, etc.
Also, there were security and confidentiality issues as merchants were able
to view card details of customers and customers sometimes filled wrong
card details.
On 1st February 1996, MasterCard and Visa in collaboration with some
renowned c ompanies like Microsoft, IBM, Netscape, etc made a single
safeguarding technique for online transactions through open networks.
This transaction standard is known as Secure Electronic Transaction
(SET).
SET was created to ensure security and integrity of online/electronic
transactions. It also ensured that all e -commerce systems must meet 3
integral requirements of security namely, authentication, integrity and
non-repudiation.
SET is not a transaction system or payment system. It is a security
transaction protocol that is used to create a trust between service provider
and service seeker. The trust was created by providing digital signatures
like barcodes and one time passwords (OTP), authorization certificates,
etc.
1996 was not the 1st time when SET pro tocol was used. It was introduced
way back, but due to disadvantages like installing complex software for
SET and strong encryption technology that made the system complex to
process, it was not very popular.
Later on modifications were made in SET protoco l by overcoming the
disadvantages like making the verification process simple, providing
authentication to merchants and customers, strong encryption techniques
to maintain confidentiality of data, encrypting sensitive data for
merchants as well, storing passwords in hashed forms, etc.
All these modifications made SET protocol easy to use, safe and authentic.
SET is successful by providing features mentioned below:
 Provide confidentiality – achieved by encryption of data
 Ensures integrity – achieved by us ing digital signatures
 Merchant authentication – achieved by use of merchant certificate
and digital signatures
 Protection of legitimate parties – by using best security standards
 Interoperability between network and software – achieved by use of
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153 Web Security and Intrusion Requirements in SET:
Mutual authentication (As mentioned above it provides a mutual trust
between customer and seller).
 Payment information (PI) and order information (OI) confidentiality.
 Resistive against message modification.
 Provid es interoperability.
Entities in SET:

Figure: shows entities and relation between different entities of SET.
 cardholder – customer
 merchant – supplier
 certificate authority – authority that issue certificates like X.509V3
 issue r – institute that provider payment card
 acquirer – provides relation with merchant
 payment gateway – 3rd party
Functionalities of SET:
1. Authentication:
 Merchant authentication: Earlier merchants also used to
perform some illegal activities as they used to receive the
credit card details of the customers. But after SET these thefts
were prevented by encrypting the data on the merchant side
and only showing it to financial institutes. SET also allows
cardholders to check merchants’ relationship with financ ial
institutes.
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154 Information and Network Security Both of these authentications are done using standard
X.509V3 certificates.
2. Integrity:
modification of messages with the help of signatures in S ET is not
supported as a result instead digital signatures are used to protect
any unauthorized access.
3. Confidentiality:
SET ensures that messages of merchants and customers are not
made publically available. Hence, maintains confidentiality of both.
Messages are encrypted using traditional DES.
How does SET protocol work?

Figure: Working of the SET protocol
The process starts where the merchant customizes a form as a wakeup
message for payment initiation. After that payment initiation is done,
where a message is received by the customer's browser, the customer must
fill in the credit card details there.
After filling the card details, the authentication process of the customer
proceeds. Where a verification message is sent on card holders mobile
numb er. Unless and until the customer will not enter the correct OTP the
order will not be placed.
After successful verification of the customer and receiving credit card
details payment gateway is applied. These credit card details are encrypted
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155 Web Security and Intrusion Later these credit card details are checked if they are valid or not. If the
card is valid it will check if the card has enough to make a purchase or not.
The merchant will now fulfill the order and after shipping the order he
requests for payment. The payment gateway receives the request and
transfers the fund from cardholders issuing institute to the merchant's
account.
The merchant then sends the confirmation to the cardholder.
The process is compl eted by updating the amount in cardholders issuing
bank after depositing payment to merchants bank.
9.5 INTRUDERS
9.5.1 Introduction:
An intruder is someone who enters an area without a permit to enter. The
legal definition of intruder is, Intruders are t he attackers who try to breach
the protection of a network. They attack the network so as to gain
unauthorized access to confidential data.

In the above example, Lily and John are communicating over a web
connection. Bob being the intruder is trying to gain access to their
communication and modifying the messages before they are sent to the
receiver. Thus, we can see there’s loss of confidentiality and integrity. We
conclude that the intruder can result in loss of three main security goals
(CIA triads) i.e., confidentiality, integrity and availability. Intrusions
usually involve stealing of confidential information or a network's
valuable resource, endangering the network and it’s data.
To detect and respond to network intrusions, organizations need to know
how intrusions work and study the response systems that are designed
with various attack techniques for destruction. Considering the current
scenario where moving on digital networks, it can be very difficult to
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156 Information and Network Security 9.5.2 Types of Intruders
Based on the type of attack Intruders are classified into three types they
are:
1. Masquerader
An unauthorized person penetrates a system to gain access to a
legitimate account. it's an attack during which the attacker pretends
to be some other person. For e.g. The attacker could send the
updated sales figure to Bob and sign the email as Alice.

Figure: Masquerade attack
Again, the e -mail will result in a series of packets over the net that
are received by Bob. Changing sender's email address is
comparatively simple, although protection mechanisms exist like
sender policy fr amework technically it's also easy to vary the source
of IP message. Such a technique is known as IP spoofing.
A masquerader is typically an outsider and the above example is a
masquerade attack.
2. Misfeasor
Misfeasor is someone who has access to data b ut misuses it for
his/her own privileges. Misfeasors (basically insiders) are
considered among the foremost difficult intruders to detect because
of their knowledge and authorization within the organization. There
are forms of insider threats,
 Malicious in sider
 Careless insider
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157 Web Security and Intrusion Traditional security measures tend to target external threats and
aren't always capable of finding an interior threat emanating from
inside the organization. So, misfeasors may be a dangerous attack.
3. Clandestine user
Cland estine user is a person who takes administrative control of the
system having confidential information and uses this control to
escape audit and gain access controls.
Clandestine can be either an outsider or insider of an organisation.
An intruder attack r anges from benign to dangerous. At the benign
end, there are many people who only explore the internet and see
what is out there.
At the other end, people who try to read confidential data, perform
unauthorized modifications to the data, or hamper the inf ormation.
One of the most common attacks in intrusion is password guessing
Examples of intrusions:
 Defacing an online server
 Guessing and cracking passwords
 Copying a database holding Master card numbers
 Viewing confidential data, such as medical informati on or
payroll records without authorization
 Using unauthorised application for capturing passwords and
usernames
 Posing as an executive, calling the assistance desk, resetting
the executive’s email password, and learning the new
password.
 Using an unattend ed, logged -in workstation without
permission
Intruder's behavioral patterns are changing to evade detection and
countermeasures. Intrusions violate the security policy of a system.
9.6 INTRUSION TECHN IQUES
 Any unauthorised action on the network is stated as a Network
Intrusion. Network breaches include the stealing of valuable network
resources and virtually risking network security and/or data security.
 Organizations and their respective cyber security teams need to have
a complete hold of what network i ntrusion is and how intrusions
techniques work so as to detect and take collective action against
network intrusions.
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158 Information and Network Security 1. Buffer overflow attacks:
i. Buffer is basically a memory storage space that stor es data as
it is transmitted from one location to another. A buffer
overflows when the amount of data exceeds the actual storage
capacity of the memory buffer. Due to which, the application
that wants to write data to the buffer overwrites memory in the
buffer.
ii. Attackers take advantage of buffer overflow errors by
overwriting an application's memory. This alters the program's
execution path, leading to damage to files or the disclosure of
personal information. This is popularly known as Buffer
Overflow Attack.
iii. To obtain access to IT systems, an attacker might insert extra
code and send new instructions to the software.
iv. If boundary -checking logic is applied and executable code or
malicious strings are recognized before they can be put to the
buffer, this attack technique becomes much more difficult to
execute.
2. Asymmetric routing:
i. When a packet travels one route to its destination and then
returns to the source from a different route, this is known as
Asymmetric Routing.
ii. If a network supp orts asymmetric routing, hackers will
frequently use alternative routes to get access to the target
system or a network.
iii. This attack approach is ineffective against networks that are
not enabled for asymmetric routing.
iv. Because of their low cost, e asily available equipment, and
considerable damaging potential, asymmetric cyber -attacks are
becoming increasingly widespread.
3. CGI Scripts:
i. The Common Gateway Interface (CGI) is a standard network
protocol for providing Web interaction between server s and
clients.
ii. A CGI software invokes other server applications to construct
a user -specific response in accordance to the requirements
received. When the CGI software has finished tasks, it sends
the result to the web server, which then delivers a res ponse to
the client (user).
iii. The CGI program is intelligent enough to detect and provide
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159 Web Security and Intrusion user. Mostly, Dropbox, Google Drive use CGI to provide the
user-specific data.
iv. By confirming a use r's authenticity, the CGI application is
sophisticated enough to detect and display user -specific
information. Dropbox most likely uses CGI to offer user -
specific data.
v. The software's Intrusion Detection and Prevention System
(IDPS) provides fully autom ated monitoring services to
combat CGI attacks
4. Trojans:
i. The word "Trojan" comes from an old Greek legend about a
deceiving Trojan horse that led to the destruction of the city of
Troy. A Trojan virus is similar to a computer virus as it hides
behind seemingly harmless applications or tries to manipulate
you into downloading it.
ii. It is a computer program intended to gain access to your
confidential data or seems to be a game and destroys data on a
hard disk.
iii. Trojan horse seems to be functional software but it damages
data or resources once it is executed or installed on the system.
Backdoors are created by using Trojans on the systems.
iv. Once the Trojans programs are executed, then it can allow any
hackers to steal your confidential data, and gain access to your
system.
5. Worms:
i. Worms are pieces of code (software) that replicate themselves by
exploiting security and network flaws.
ii. Spreading of worms does not need any human intervention. - The
capacity of worms’ replication is more. It c an transmit thousands of
copies of itself thus can result in destructive effects.
iii. Human involvement is not required in the spread of worms. Worms
have a higher replication capacity. It has the ability to generate
thousands of copies of itself, which c an have a catastrophic effect.
iv. Worms propagate without the assistance of any external software or
files. A worm is typically transmitted to a computer or network via a
link or file sent over the email, chat, or other forms of digital
communication.
v. Worms are capable of deleting and altering files. They can also
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160 Information and Network Security 6. Traffic Flooding :
i. Traffic Flood is one of the types of DoS (Denial of Service)
attack which aim at web servers.
ii. It simply creates traffic loads that are too large for network
intrusion detection systems to fully examine.
iii. The attack takes advantage of a feature of the HTTP protocol
by simultaneously initiating many connections to attend to a
single demand.
iv. Attackers can sometimes carry out undetected attacks and even
induce an undetected "fail -open" state in the resultant
congested and chaotic network environment.
9.7 INTRUSION DETEC TION
9.7.1 What is an Intrusion Detection System?
In general intrusion detectio n means finding out an Intruder who causes an
intrusion in the system. In the IT field there are Intrusion Detection
Systems or Softwares that monitors a system for Malicious activities.
These Malicious activities are detected and collected by a Central ev ent
management and security information system. There are some IDS’s
(Intrusion Detection Systems) which can prior detect the intrusion and
take necessary actions against them, these types of systems are called
Intrusion Prevention System (IPS).
9.7.2 Type s of Intrusion Detection techniques:
There are wide variety of IDS which ranging from tiered monitoring
system to antivirus but most common types are:
1. Network intrusion detection system (NIDS):
This is a system which Analyzes incoming network traffic.
2. Host based intrusion detection system (HIDS):
This is a system that Monitors important system files.
Some subset of IDS types are as follows:
1. Signature –based:
These types of IDS detect threats by looking for specific patterns in
a network or syst em. For example: There are a communication two
system and network protocol has defined that the message length
should be only of 8 bit while sending, if a malicious threat interferes
in that message then that message length will increase, So, because
of th is IDS will come to know there is an intruder in the system and
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161 Web Security and Intrusion 2. Anomaly – based:
This technology is also known as adaptive IDS, in this type of
system it uses machine learning and AI model to find the behavior
of that malware and detect it.
9.8 UNIT END QUESTIO NS
1. What are the web security threats?
2. Explain SSL
3. Explain SSL Protocol Stack
4. How does TLS provide security?
5. How can we adopt secure transactions?
6. What are intruders?
7. State and explain the types of intruders
8. Explain the intrusion techniques
9. Explain the ways of detecting intrusions
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162 10
MALICIOUS SOFTWARE AND FIREWALL
Unit Structure :
10.1 Objectives
10.2 What is Malicious Software?
10.2.1 Types of malicious software
10.3 Virus and its threats
10.3.1 What is a virus?
10.3.2 Types of viruses
10.3.3 Life Cycle of virus
10.3.4 Virus countermeasures
10.4 DDOS
10.4.1 DOS
10.4.2 DDOS
10.5 Firewall Design Principles
10.5.4 Introduction
10.5.2 Characteristics of Firewall
10.6 Types of firewall
10.7 Unit End Questions
10.1 OBJECTIVE
This chapter would make you understand the following concepts
 What are malwares and in detail about virus and its countermeasures
 What are firewalls
 Need of firewall to prevent from security attack
 Which types of firewall suites best in various situation
10.2 WHAT IS MALICIOUS SOFTWARE?
Malware means malic ious software. This software helps hackers to gather
sensitive information, interrupt users computers operations, and
unauthorized access to computers.
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163 Malicious Software and Firewall Viruses, spyware, Trojan Horse, Worms and other malicious programs are
examples of malware.
Sometimes, malware can be genuine software which can be created by the
company's official website.
10.2.1 Types of malicious software
The malicious software are mainly categorised into two types:
1. Indepen dent of host program:
 Worms
 Zombies
2. Need host program:
 Trapdoors
 Logic Bombs
 Trojan horses
 Viruses
10.3 VIRUS AND ITS THRE ATS
10.3.1 What is a virus?
A small piece of software attached to programs is known as virus
It spreads from one machine to another by leaving its infection on route.
Some virus effects are mild which can be ignored while others can damage
software, data, files or hardware.
Virus needs human intervention to get spread.
It cannot infect the system unless we execute or open the file or program
containing it.
10.3.2 Types of viruses:
 Record infectors
These most often connect to program documents but can infect any
file with executable code, consisting of script files or software
configuration documents. When the program, script, or
confi guration is carried out, the virus is executed nicely.
 Machine or boot -report infectors
Machine or boot -document infectors do not always infect a file.
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164 Information and Network Security device processes. These regions encompass the boot document,
which is a phase of the disk committed to booting the operating
system. On a diskette, these infectors attach themselves to the DOS
boot zone; on a tough disk, they attach themselves to the master
Boot file. Having infected a master Boot document, the virus
spreads to the boot sectors of the inserted media.
 Multipartite viruses
Multipartite viruses infect boot information as well as files. With its
hybrid nature, a multipartite virus inherits the worst features of each
of its p arents and consequently is way more contagious and adverse
than both.
 Macro viruses
Macro viruses infect macro -enabled files, especially in the Microsoft
office suite of programs -extra particularly, Microsoft Word and
Excel. Whilst opened, an inflamed docu ment executes a macro
robotically, or the consumer does so by accident. The macro inflicts
damage after which infects other files on the disk. A macro is hard
and fast of executable instructions designed to run in place of a
repetitive task. Although macro s aren't particular to Microsoft
merchandise, it is through Microsoft merchandise that many macro
viruses unfold. Macro viruses are the most commonplace kind of
viruses, but they do pretty little damage.
 Stealth viruses
Stealth viruses use many strategies to thwart detection. One
approach is to redirect the addresses inside an application that point
to different programs or gadget facts and have them a factor to the
virus file as a substitute. Whilst this system calls for that
supplementary software or syst em data, it truly runs the virus code.
This infects the record without absolutely injecting additional code,
which could show up as a symptom of virus scanning software
programs. Every other, not unusual stealth approach modifies a
record but displays its size as it turned into before contamination.
Therefore, it nullifies the ability to use the reporting period as an
indicator of contamination.
 Encrypted viruses
Encrypted viruses enjoy the advantages of other encrypted fabrics.
Initially, encrypted viruses seem no longer as viruses, but as
nondescript without sense. However, whilst an inflamed program is
accomplished, a small piece of undeniable, unencrypted code
decrypts the relaxation of the virus, which then proceeds to do its
damage. When, and if, an en crypted virus is detected, it is very
tough to research because it is not difficult to reverse engineer just
like the unencrypted viruses. This makes it hard to decide the
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165 Malicious Software and Firewall Encryption is most usef ul while coupled with a polymorphic
approach.
 Polymorphic viruses
Polymorphic viruses try to avoid detection by altering their shape or
the encryption strategies. Each time contamination happens, a
polymorphic virus modifications its shape, puzzling virus (detection)
scanning software. Due to the fact virus scanners use specific
“signature” characteristics to pick out viruses, any virus that adjusts
its shape affords a formidable new assignment.
10.3.3 Life Cycle of Virus:
There are four stages in a life cy cle of virus:
1. Dormant Phase
2. Propagation Phase
3. Triggering Phase
4. Execution Phase
1. Dormant Phase:
 Basically, in this phase the virus is inactive
 The virus gets activated by executing the event
 Examples:
 Date/timestamp events, presence of another file or disk usage
reaching some capacity.
 All viruses may not have this phase.
2. Propagation Phase:
 Virus replicates itself and attaches itself to some program.
 The replica may have slight changes as viruses change their
forms to avoid detection.
 Every virus infected program contains a copy of the virus
which enters itself into a propagation phase.
3. Triggering Phase:
 Virus performs the function it is intended for.
 Many system events cause the triggering phase such as
counting how many times the virus r eplicates itself.
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166 Information and Network Security 4. Execution Phase:
 Virus executes its function which can be harmless like just a
message on the screen or damaging like deletion of
information or programs.
10.3.4 Virus countermeasures
1. Antivirus:
Rather than detecting and removing vi ruses, it is always better to
prevent it. It does not allow viruses to enter into the system and also
blocks it to make any modifications.
Tries to locate the virus by the type of its infection.
Once detected, it finds out that specific virus which has in fected the
program.
Once identified, try to remove almost all virus traces from infected
areas so that it cannot spread anymore.
A good antivirus should be installed and a \updated regularly
Generation of antivirus
There are mainly four generations of ant ivirus as stated below:
A. 1st Generation
a. st generation antiviruses are also known as simple scanners.
b. Virus signature needed for identification of virus.
c. But these signature specific scanners can scan only known
viruses
d. Length of programs and alteration is observed to check for
virus attacks.
B. 2nd Generation
a. 2nd generation antivirus are also known as Heuristic scanners,
b. It does not depend on simple virus signatures.
c. Basic concept behind this is to search those blocks of codes
which are associated with viruses.
d. Example, such programs can find out the encryption key used
for a virus, then decrypt it and eliminate the virus and cleans
code.
e. It checks for integrity means if a virus affects the program
without altering chec ksum then integrity check traps that
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167 Malicious Software and Firewall C. 3rd Generation
a. 3rd generation antiviruses are also known as activity traps.
b. They reside in memory.
c. Rather than viruses' structure, it searches for viruses based on
their action.
d. But main taining a huge virus signatures database is not
needed.
e. But it is needed to find out a small set of actions which
identifies the attempted infection and then intervenes it.
D. 4th Generation
a. 4th generation antivirus is also known as full featured
protection.
b. It consists of many antivirus techniques combined with other
packages.
c. It includes components for activity and scanning traps.
d. It also controls access capability that means restricting virus
ability to enter into a system and restri cting virus capabilities
to alter files in order to pass infection.
e. A wide range of defence strategy is implemented with this
generation of antivirus extending the scope of defence to
tackle more general purpose measures for computer security.
2. Gene ric Decryption (GD)
GD facilitates the antivirus program to speedily scan and easily
detect most complicated polymorphic viruses.
3. Digital Immune System
 IBM developed the Digital Immune system for virus
protection.
 The main aim behind this is growing threats of internet based
virus spreads.
 When a new virus enters a network, the digital immune system
automatically detects it, analyse it. capture it, protect against it,
deletes it and also spreads information about it to other
systems running IBM Digit al Immune system so that it can
easily be detected before its execution.

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168 Information and Network Security 4. Behaviour Blocking Software
 Behaviour blocking software along with OS observes
behaviour of a program in real time for harmful actions.
 Then this software blocks potentially ha rmful actions before it
harms the system.
Observed behaviour includes:
 Attempts to view, modify or delete files.
 Attempt to perform operations which are unrecoverable or
formatting of disk drives.
 Modifying the contents or logic of the executable files.
 Alterations in crucial system settings. -Interruption in network
communication
5. Updating vaccine softwares like antivirus
Antivirus software should be installed and updated periodically to
protect against viruses.
6. Scanning of email attachment file s While downloading email
attachment files we should
 Be very cautious with email attachments of unidentified
sources.
 Not be misled by the appearance of attachment files.
 Be cautious of doubtful files attached to emails even though
you received them fro m known resources.
7. Symptoms of Virus Infection
Must not be ignored Virus symptoms like disappearing files,
appearance of unknown icons on screen or taskbar, appearance of
unknown codes in programs, unknown emails, etc. are observed on
the systems then do not ignore these symptoms and scan computers
for viruses and take necessary actions.
8. Downloaded Files should be scanned
9. For Applications utilize Security Functions
10. Security Patches Should be applied
11. Back up the data on a regular basis .

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169 Malicious Software and Firewall 10.4 DDOS
10.4.1 DOS Attack

Figure: Denial of Service (DoS) attack
 It stands for Denial of Service.
 The cyber attack in which the attacker makes a machine or network
resource unavailable to the intended users by temporarily or
deliberately stopping services of a host machine connected to the
internet.
 A single machine is used to launch an attack.
 Comparatively less complicated.
 There is no malware involvement.
10.4.2 DDOS

Figure: Distributed Denial of Service(DDoS) attack
 It stands for Distributed Denial of Service.
 A cyber attack in which the incoming traffic floods the victim
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170 Information and Network Security  The attacker gains access to number of machines and use these
machines to attack the victim
 More complicated and diff icult to prevent.
 Uses malware to affect multiple machines.
10.5 FIREWALL DESIGN PRIN CIPLE
10.5.1 Introduction
A firewall is basically a single point which keeps unauthorized users
blocked from the protected network, thus prohibiting potential vulnerable
services from leaving or entering the private network, and provides
protection from various kinds of cyber attacks, IP spoofing and routing
attacks.
A firewall monitors security related events. Audits and alarms are usually
implemented on the firewall sys tem.
A firewall is a protection technology that controls outgoing and incoming
packets totally based on predefined security rules. A firewall is generally
like a barrier between a reliable and an untrusted community, which
includes the Internet, Intrane t. The devices which provide firewalls for
people’s safety can be routers which are used for routing. A firewall
analyses the packets entering and leaving and filters them so that
suspicious and unsecured actions don’t cause harm to the system.

Figure. Firewall protects private network from public network
While internet access provides benefits to the institute/organization, it
allows the outside world to reach and interact with private network assets.
Thus creating a threat to the organization’s assets .
The firewall is basically like a barrier that is inserted between the private
network and internet(public network) to establish a monitored link and to
build a security wall. The aim of this security wall is to protect the private
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171 Malicious Software and Firewall where audit and security can be applied. The firewall may be implemented
using a single device or a set of devices that need to cooperate collectively
to perform the firewall functionality.
10.5.2 Character istics of Firewall:
 All traffic passing from outside to inside, and vice versa, has to pass
through the firewall. This is possible by blocking all access to the
private network except via the firewall.
 Various firewall configurations are possible and need to be done.
 As specified by the local security policy, only authorized traffic will
be allowed to pass through the firewall.
 There are various types of firewalls that are used which implement
different types of security policies.
10.6 TYPES OF FIREWAL L
The various types of firewall are as follows:
1. Packet filtering router

Figure: Packet Filtering Firewall
In packet filtering router technique, the router applies a set of predefined
rules to every incoming IP packet and then based on the rules forwar ds or
discards the packet. The router is configured to filter out packets going in
and out the private network. Filtering rules defined are based on the
information within a network packet:
 Source IP address – It’s the IP address of the sender of the IP pa cket.
 Destination IP address – It’s the IP address of the receiver.
 Source and destination transport level address – They are transport
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172 Information and Network Security  IP protocol field – It defines the transport protocol through which
the packet is received.
The packet filter has a list of rules, based on the matches in the TCP or IP
header fields, appropriate action will be taken regarding the packets. If
there’s any match found to one of the predefined rules, then that rule is
invoked to decide whether to discard or forward the packet. If there’s no
match to any rule, then a predefined default action will be taken.
Two default policies are possible:
 discard: The one which is not permitted is prohibited.
 forward: The one which is not prohibited is permi tted.
The discard policy as default is more conservative. Initially everything is
blocked, and then services need to be added.
Advantages of packet filter router
 It is very Simple to configure and maintain
 It's very Transparent to users and quick in action
Disadvantages of packet filter firewalls
 Packet filter firewalls cannot prevent attacks from the application
layer as they don’t examine upper layer data.
 It does not provide advanced level user authentication schemes.
 They are vulnerable to atta cks like IP spoofing.
2. Application level gateway

Figure: Application level Gateway Firewall
An Application level gateway is also known as a proxy server which acts
as a relay of application level traffic. The user will contact the gateway
using a TCP/I P application, such as FTP or Telnet or HTTP, and the
gateway in turn will ask the user for the name(identity) of the remote host
which it has to access. When the user responds back and provides it with
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173 Malicious Software and Firewall contact the application on the remote host and then will relay the TCP
segments which contain the application data between the two endpoints.
Advantages of Application level gateway
 It is far more secure than packet filter firewall
 It is very easy to log and audit all the incoming traffic at the
application level
Disadvantages of Application level gateway
 There’s additional processing overhead for each connection.
3. Circuit level gateway

Figure: Circuit Level Gateway Firewall
Circuit level gateway can work as a standalone system or can be a specific
function executed by an application level gateway for specific applications
Unlike other firewalls, a Circuit level gateway does not allow an end -to-
end TCP connection; rather, the gateway creates two TCP connections,
one between it’s own self and a TCP user on an inner host and another
between its ownself and a TCP user on an outer host. As these two
connections are established then the gateway will transmit the TCP
segments from one conne ction to the other connection as shown in figure
above without inspecting the contents inside the segments. The security
function consists of determining which connections will be allowed and
which is not to be allowed.
A Circuit level gateways is best app lied where the system administrator
trusts the internal users fully. The gateway is configured to provide proxy
level or application level services on both inbound connections as well as
circuit level functionality for outbound connections
Bastion host
It is basically a system identified by the firewall system administrator
which serves as a platform for both circuit level gateway and an
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174 Information and Network Security

Figure:Bastion Host Firewall
Some characteristics of a Bastion host are listed below:
 The Bastion host hardware platform contributes to the execution of a
secure operating system (OS) thus, making it a trusted system.
 For security reasons, only the services that are considered essential
are installed on the Bastion host by the network administrator.
 It requires additional authentication to be applied before a user is
allowed any access to the proxy services.
 Every proxy supports only a subset of the standard application’s
command set.
 Every proxy is configured to allow access only to specific host
syste ms.
 Every proxy maintains audit information about all traffic logs, every
connection established and the duration of every connection.
 On the Bastion host, each and every proxy works independently.
 A proxy only reads its initial configuration file.
 On the Bastion host, every proxy runs on a non privileged user in a
secured and private directory.


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175 Malicious Software and Firewall 4. Stateless Firewall Filter


Figure: Stateless Firewall
This type of firewall monitors the network traffic. It does not have any
information about th e traf fic patterns and restricts the pattern based on
either the source or the destination. It is also known as the Access control
list ( ACL). This firewall basically does not inspect the traffic. It simply
allows or denies packets into the network based on the set of filtering
rules.
File Transfer Protocol (FTP) is a type of Stateless firewall which is
implemented for sending and receiving files between two computer
systems.
Stateless firewall advantages -
 It is comparatively less complex and much cheaper.
 It is very simple to implement and configure.
 It supports fast delivery Performance.
 It works efficiently under heavy traffic and pressure.
Stateless firewall disadvantages -
 It does not check the entire packet but only checks if the packet
satisfies the r edefin ed set of security rules.
 It does not inspect or monitor the traffic.
 Some additional configurations are required to increase the level of
protection for security concerns.
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176 Information and Network Security 5. Stateful Firewall
Unlike Stateless firewalls, Stateful firewalls monitor the ent ire state of
network connections. It tracks the state of network connections when the
data packets are being filtered.
As the Stateful firewalls keeps track of the state, it is aware of all the
communication paths and has the capability to implemen t many IP
security functions such as encryptions, authentication and so on.
This Firewall inspects the packets for a match with the rule in the firewall.
If a match is not found, it is simply discarded but if the match is found
then it is allowed into the networ k and can travel freely within the
network.
Common example is the Transport Control Protocol(TCP). It simply saves
the connection record by storing its port number, source and destination IP
address, etc.
Stateful firewall advantages -
 This firewall is com paratively faster in detecting forgery or any
unauthorized communication taking place.
 It has an ability for making future filtering suggestions based on the
cumulative of present and past findings.
 For effective communication, many ports are not r equired to be kept
open.
 It has a Powerful memory and extensive logging capabilities, and it
is robust in attack prevention.
Stateful firewall disadvantages -
 The data transfer rate is quite slow.
 The firewall has to be updated periodically with the latest availa ble
technologies or else the system will be compromised by the hackers.
 As this firewall maintains tables of the access list, it requires a high
processing and memory power.
10.7 UNIT END QUESTIONS
1. What is malicious software? How are they classif ied?
2. What a re virus es?
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177 Malicious Software and Firewall 5. What measures can be taken to protect the system from virus attack?
6. What is DDOS attack
7. Differentiate between DoS and DDoS attack
8. What is Firewall? What is the purpose of a firewall?
9. Explain the design principle of a firewall
10. Explain Packet Filter firewall in brief
11. Explain Appli cation laye r firewall in brief
12. Explain Circuit level firewall in brief
13. Explain Stateless firewall
14. Explain Stateful firewall
15. Distinguish between Stateless and Stateful Firewall
REFERENCE
1. Cryptography and Network, Behrouz A Fourouzan, Debdeep
Mukhopadhyay, 2nd Edition,TMH,2011
2. https://www.researchgate.net /publication/3 20758708_E -
COMMERCE_SECURITY_WITH_SECURE_ELECTRONIC_TR
ANSACTION_PROTOCOL_A_SURVEY_AND_IMPLEMENTAT
ION
3. https://docs.microsoft.com/en -us/windows/win32/secauthn/tls -
handshake -protocol
4. https://www.brainkart.com/article/Web -Security -
Considera tions_8479/
5. https://www.sciencedirect.com/topics/computer -
science/authentication -header
6. https://www.techopedia.com/definition/1504/encapsulating -security -
payload -esp
7. https://www.rese archgate.net/p ublication/3728380_
A_reference_model_for_firewall_technology
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