CY-503-B Network Security - Jun 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 working of the polyalphabetic substitution cipher with examples.
   How to answer

   • Define polyalphabetic substitution vs monoalphabetic
   • Explain Vigenere table / key repetition mechanism
   • Worked encryption-decryption example
   • Why frequency analysis is defeated
2. 7 Marks
   bWrite short notes on Perfect Secrecy and its significance.
   How to answer

   • Define perfect secrecy Shannon definition
   • Condition P(M|C)=P(M) and key length requirement
   • One-time pad as example
   • Significance and practical limitations
3. 7 Marks
   aDescribe the evolution of cryptography from classical to modern techniques.
   How to answer

   • Classical ciphers: Caesar, mono/poly alphabetic, transposition
   • Mechanical era: Enigma, Vernam
   • Modern era: DES, AES, RSA, hash
   • Transition from secrecy by obscurity to Kerckhoffs and computational security
4. 7 Marks
   bAnalyze the role of one-way functions in securing cryptographic systems.
   How to answer

   • Define one-way and trapdoor one-way function
   • Easy forward, infeasible inverse property
   • Uses: hashing, key exchange, digital signatures, password storage
   • Examples: modular exponentiation, hash functions
5. 7 Marks
   aDiscuss the computational hardness assumptions critical to public-key cryptography.
   How to answer

   • Need for hard problems in public-key crypto
   • Integer factorization assumption
   • Discrete logarithm and elliptic curve assumptions
   • Impact if assumptions broken
6. 7 Marks
   bHow are discrete logarithms used in cryptographic primitives? Provide an example.
   How to answer

   • Define discrete logarithm problem
   • Use in Diffie-Hellman, ElGamal, DSA
   • Worked small-number example
   • Why hardness provides security
7. 7 Marks
   aWrite short notes on Quantum Computing and its Implications for Cryptography.
   How to answer

   • Basics of qubits, superposition, entanglement
   • Shor algorithm threat to RSA/ECC
   • Grover algorithm effect on symmetric keys
   • Post-quantum cryptography need
8. 7 Marks
   bDiscuss the significance of pseudo-random generators in secure encryption schemes.
   How to answer

   • Define PRG and expansion of short seed
   • Role in stream ciphers and key generation
   • Security requirement: indistinguishability
   • Example: use in OTP-like secure encryption
9. 7 Marks
   aHow does large integer arithmetic enhance the implementation of public-key cryptosystems?
   How to answer

   • Need for big integers in RSA, DH, ECC
   • Modular exponentiation with large primes
   • Security scales with key size
   • Implementation challenges and efficient arithmetic
10. 7 Marks
    bExplain the working principle of the Feistel network with a block diagram.
    How to answer

    • Split plaintext into L0,R0 and iterative rounds
    • Round function F and XOR structure
    • Decryption same structure with reverse keys
    • Block diagram of one round and n rounds
11. 7 Marks
    aWrite short notes on Collision-Resistant Hash Functions.
    How to answer

    • Define collision resistance property
    • Difference from preimage/second-preimage resistance
    • Birthday attack relevance
    • Applications in signatures and integrity
12. 7 Marks
    bCompare and contrast the security notions of ECB and CBC modes of encryption.
    How to answer

    • ECB: deterministic, leaks patterns, no IV
    • CBC: chaining with IV, probabilistic, needs padding
    • Security notions: IND-CPA comparison
    • Error propagation and parallelization contrast
13. 7 Marks
    aHow does the GCM mode provide simultaneous message confidentiality and integrity?
    How to answer

    • GCM as authenticated encryption with CTR + GHASH
    • Confidentiality via counter encryption
    • Integrity via authentication tag
    • Single pass efficiency and use in TLS
14. 7 Marks
    bExplain the construction and use of the RSA encryption algorithm.
    How to answer

    • Key generation: primes p,q, n, phi, e,d
    • Encryption C=M^e mod n and decryption formula
    • Example with small primes
    • Uses: key exchange, signatures, certificates
15. 7 Marks
    aWrite short notes on Hybrid Encryption and its role in securing communication.
    How to answer

    • Combine asymmetric key encapsulation + symmetric data encryption
    • Steps: generate session key, encrypt key, encrypt data
    • Advantages: speed + secure key distribution
    • Example: PGP, TLS hybrid use
16. 7 Marks
    bCompare symmetric encryption with asymmetric encryption, focusing on key management.
    How to answer

    • Symmetric: single shared key, fast; Asymmetric: public/private pair
    • Key distribution, storage and scalability comparison
    • Table of key management challenges
    • Hybrid solution conclusion