CS-703-A Cryptography & Information Security - Dec 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the concept of modular arithmetic with suitable examples. Derive Euler's theorem and state its significance in cryptography.
   How to answer

   • Definition of modular arithmetic with congruence notation
   • Examples of addition, multiplication mod n
   • Statement and derivation of Euler's theorem using Euler's totient function
   • Significance in RSA, key generation and modular inverses
2. 7 Marks
   bFind the multiplicative inverse of $11 \bmod 26$ using the Extended Euclidean Algorithm and verify your answer.
   How to answer

   • Apply Extended Euclidean Algorithm to 11 and 26
   • Compute gcd and Bezout coefficients step-wise
   • Obtain inverse value and verify by $11 \cdot x \equiv 1 \pmod{26}$
3. 7 Marks
   aCompare Classical Cryptosystems such as Caesar Cipher and Playfair cipher. Perform frequency analysis on a simple ciphertext example.
   How to answer

   • Principle of Caesar Cipher with shift example
   • Principle of Playfair cipher with matrix rules
   • Comparison on key space, type and security
   • Frequency analysis steps on simple ciphertext example
4. 7 Marks
   bExplain the Advanced Encryption Standard (AES) process. Write down its key expansion and round transformation steps.
   How to answer

   • AES overall structure and parameters (128/192/256)
   • Key expansion procedure
   • Round transformations: SubBytes, ShiftRows, MixColumns, AddRoundKey
   • Number of rounds and final round difference
5. 7 Marks
   aUsing the RSA algorithm, encrypt the plaintext $M=9$ using $p=3, q=11, e=7$. Compute $n, \phi(n), d,$ and ciphertext $C$.
   How to answer

   • Compute $n = p \cdot q$ and $\phi(n)$
   • Verify $e$ is coprime and find $d$ as inverse of $e$ mod $\phi(n)$
   • Encrypt $C = M^e \bmod n$ calculation
6. 7 Marks
   bDiscuss Diffie-Hellman Key Exchange algorithm. What are Computational and Decisional Diffie-Hellman Problems?
   How to answer

   • Diffie-Hellman key exchange steps with discrete logarithm
   • Shared secret derivation on both sides
   • Computational Diffie-Hellman Problem definition
   • Decisional Diffie-Hellman Problem definition
7. 7 Marks
   aDefine Hash Function. Differentiate between MD5 and SHA algorithms with suitable examples.
   How to answer

   • Definition and properties of hash function
   • MD5 working, digest size and vulnerabilities with example
   • SHA family working and security features with example
   • Comparison table of MD5 vs SHA
8. 7 Marks
   bWhat is Digital Signature? Explain the Digital Signature Standard (DSS) with block diagram.
   How to answer

   • Definition of digital signature and its services
   • DSS signing and verification steps
   • Block diagram of DSS operation
   • Role of hash and keys in DSS
9. 7 Marks
   aExplain Kerberos Authentication System with suitable diagram.
   How to answer

   • Need for Kerberos authentication
   • Components: AS, TGS, client, server
   • Ticket exchange steps with diagram
   • Use of timestamps and session keys
10. 7 Marks
    bDefine Firewalls. Explain types of Firewalls and their role in Network Security Architecture.
    How to answer

    • Definition of firewall
    • Types: packet filter, stateful, proxy, NGFW
    • Comparison of types with examples
    • Role in Network Security Architecture
11. 7 Marks
    aDiscuss Email Security services and explain the working of Pretty Good Privacy (PGP) and S/MIME.
    How to answer

    • Email security services: confidentiality, integrity, authentication
    • PGP working steps for encryption and signing
    • S/MIME working and certificate use
    • Comparison of PGP vs S/MIME
12. 7 Marks
    bExplain IP Security (IPSec) protocols- AH and ESP. How are these used for secure data transmission?
    How to answer

    • IPSec overview and modes
    • AH header, services and operation
    • ESP header, services and operation
    • Use of AH and ESP for secure data transmission
13. 7 Marks
    aWhat is Steganography? Explain any two tools used for hiding information in digital media.
    How to answer

    • Definition of steganography and difference from cryptography
    • Tool 1 working for hiding data in digital media
    • Tool 2 working with example
    • Cover media and detection considerations
14. 7 Marks
    bWrite short notes on Vulnerability Scanning Tools and DoS Attack Understanding Tools with examples.
    How to answer

    • Purpose of vulnerability scanning with examples of tools
    • DoS attack understanding tools with examples
    • Key features and outputs of each category
    • Use in security assessment
15. 14 Marks
    Write short notes on any **two** of the following:

    1. a)Steganalysis Tools
    2. b)SSL/TLS Protocols
    3. c)Differential Cryptanalysis
    4. d)Elliptic Curve Cryptography
    How to answer

    • Select any two topics from (a)-(d)
    • Key definitions and working principles for each
    • Relevant diagrams, protocols or examples
    • Applications and security significance