Mumbai University Solved Question Papers
Information and Network Security
Previous Year Question Paper with Solution
BSc CS · Sem 5
Nov 2024 Examination
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BSc CS Sem 5 Information and Network Security Nov 2024 Question Paper with Solutions
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First published on munotes.in on 17 October 2026.
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Model answers written and edited by the munotes.in editorial desk.
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The University does not publish an official answer key for this paper. The answers in this volume are model answers, written to show how a full-mark answer is built. They are a study aid, not an authority on what an examiner marked.
The question paper reproduced here is the paper as set by the University of Mumbai at the Nov 2024 examination.
Contents
The questions below are the paper as the University of Mumbai set it at the Nov 2024 examination, in the order it was set.
MarksPage
MarksPage
The Paper as Set
The questions in this volume are the questions asked at the Nov 2024 examination, reproduced as the University of Mumbai set them, in the order it set them. Nothing has been reworded, added or left out. Only the answers are ours. See the original question paper.
Duration 2½ hours · Total marks 75 · 24 questions answered
Instructions printed on the paper
- N.B. (1) All questions are compulsory. (2) Figures to the right indicate marks. (3) Illustrations, in-depth answers and diagrams will be appreciated. (4) Mixing of sub-questions is not allowed.
How to use this volume
Solve the paper first, under exam conditions and against the clock. Then read the answers here and mark your own. Reading a solution before attempting the question feels productive and teaches very little, because recognising an answer is not the same as being able to write one.
SECTION I
Q.1
Attempt ANY FOUR from the following · 20 Marks
(a)Differentiate between substitution technique and transposition technique in encryption.[5]
Answer
For full marks, cover: a definition of each with examples of ciphers, a worked illustration on the same word, a difference table, and how modern ciphers combine the two.
Substitution techniques
A substitution technique replaces the letters (or bits) of the plaintext with other letters, numbers or symbols. The positions stay the same; the symbols change.
Ciphers: Caesar, monoalphabetic, Playfair, Hill, Vigenere (polyalphabetic), one-time pad.
Illustration: EXAM with a Caesar shift of 3 becomes HADP: every letter has changed, but each sits where it was.
Transposition techniques
A transposition technique performs a permutation of the plaintext letters: the letters stay the same but their order is rearranged.
Ciphers: rail fence, simple columnar (row) transposition, double transposition.
Illustration: EXAM rearranged by a fixed pattern might become MXAE: the same four letters in a different order.
Differences
| Basis | Substitution | Transposition |
|---|---|---|
| What changes | the identity of each symbol | the position of each symbol |
| Letters in the ciphertext | different from the plaintext's | exactly the plaintext's letters |
| Letter frequencies | changed (monoalphabetic keeps the pattern, polyalphabetic flattens it) | identical to the plaintext's, which reveals that a transposition was used |
| Key | the mapping (shift, keyword alphabet, matrix) | the permutation (rails, column order) |
| Typical attack | frequency analysis | anagramming, trying column orders |
| Examples | Caesar, Playfair, Vigenere | rail fence, columnar transposition |
| In modern ciphers | gives confusion (S-boxes) | gives diffusion (permutations) |
Combining them
Either alone is weak. Applying several stages of substitution and transposition produces a far stronger cipher; Shannon called the two effects confusion and diffusion. This is the idea behind product ciphers such as DES (S-boxes and permutations) and AES (SubBytes and ShiftRows/MixColumns).
Conclusion. Substitution replaces each letter with another symbol while keeping positions (Caesar turns EXAM into HADP), while transposition keeps the letters but changes their order (EXAM into MXAE), leaving letter frequencies unchanged. Each alone is easily broken, but repeated rounds of both give the confusion and diffusion on which DES and AES are built.
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