CY-302 Fundamentals of Cryptography - 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
   aDefine cryptography and explain symmetric-key encryption.
   How to answer

   • Define cryptography: goals of confidentiality, integrity, authentication
   • Define symmetric-key encryption: single shared secret key for encryption/decryption
   • Explain working: Enc_k, Dec_k, correctness and key distribution
   • Give examples e.g. stream ciphers, block ciphers
2. 7 Marks
   bDiscuss historical ciphers and the concept of computational security.
   How to answer

   • List historical ciphers: Caesar, monoalphabetic, Vigenere, etc.
   • Explain classical attacks on them
   • Define computational security vs perfect secrecy
   • Explain efficient adversaries and negligible advantage
3. 7 Marks
   aExplain perfect secrecy with reference to one-time pad encryption.
   How to answer

   • Define perfect secrecy: Pr[M=m|C=c] = Pr[M=m]
   • Describe one-time pad construction: C = M xor K
   • Show OTP achieves perfect secrecy with uniform random key
   • State key-length requirement and single use
4. 7 Marks
   bDiscuss characterizations and limitations of perfect secrecy.
   How to answer

   • Give characterizations: Shannon theorem, |K| >= |M|
   • Explain indistinguishability equivalence
   • Discuss limitations: key length, key management, reuse insecurity
   • Conclude impracticality for large messages
5. 7 Marks
   aExplain CPA-secure ciphers constructed from PRFs.
   How to answer

   • Define PRF and CPA security game
   • Describe randomized encryption: r, F_k(r) xor m
   • Explain why randomization prevents CPA advantage
   • State security reduction to PRF security
6. 7 Marks
   bDiscuss modes of operation of block ciphers with examples.
   How to answer

   • Define block cipher modes of operation
   • Explain ECB, CBC, CTR, OFB with diagrams flow
   • Compare error propagation and parallelism with examples
   • Mention IV/nonce requirements
7. 7 Marks
   aExplain DES and AES algorithms highlighting their features.
   How to answer

   • Outline DES: Feistel, 56-bit key, 16 rounds, S-boxes
   • Outline AES: SPN, 128-bit block, 128/192/256 key, rounds
   • Compare security, key size, performance
   • Highlight features: avalanche, resistance to attacks
8. 7 Marks
   bDiscuss Message Authentication Codes and cryptographic hash functions.
   How to answer

   • Define MAC: authenticity with secret key
   • Define cryptographic hash: preimage, collision resistance
   • Explain constructions e.g. HMAC, SHA
   • Differentiate MAC vs hash vs digital signature
9. 7 Marks
   aExplain Brute force and cryptanalysis in cryptography with the help of related example.
   How to answer

   • Define brute force: exhaustive key search
   • Define cryptanalysis types: ciphertext-only, known-plaintext etc.
   • Give example e.g. brute forcing DES, frequency analysis of Caesar
   • Discuss complexity and countermeasures
10. 7 Marks
    bDiscuss birthday attacks and applications of cryptographic hash functions.
    How to answer

    • Explain birthday paradox and sqrt bound for collisions
    • Describe birthday attack on hash functions
    • List applications of hash: password storage, integrity, signatures
    • Mention SHA collision resistance requirements
11. 7 Marks
    aExplain the random oracle model and authenticated encryption.
    How to answer

    • Define random oracle as ideal public random function
    • Explain use in security proofs
    • Define authenticated encryption: confidentiality + integrity
    • Give examples e.g. GCM, CCM
12. 7 Marks
    bDiscuss generic constructions of authenticated encryption schemes.
    How to answer

    • Define generic composition: Encrypt-then-MAC, etc.
    • Explain security of Encrypt-then-MAC
    • Discuss weaknesses of MAC-then-Encrypt, Encrypt-and-MAC
    • Mention standards for AE construction
13. 7 Marks
    aExplain Diffie Hellman key exchange and cyclic groups.
    How to answer

    • Define cyclic group and generator
    • Describe DH protocol steps: g^a, g^b exchange
    • Explain shared secret computation g^{ab}
    • Mention man-in-the-middle vulnerability
14. 7 Marks
    bDiscuss discrete logarithm problem and elliptic curve cryptography.
    How to answer

    • Define discrete logarithm problem and hardness
    • Define elliptic curve group operation
    • Explain ECC advantage: smaller keys, ECDH/ECDSA
    • Compare RSA/DLP vs ECC security levels
15. 7 Marks
    aExplain CCA-secure public-key hybrid encryption schemes.
    How to answer

    • Define CCA security and hybrid encryption (KEM+DEM)
    • Explain KEM encapsulation and DEM data encryption
    • Show how CCA-secure KEM gives CCA-secure PKE
    • Give example e.g. RSA-KEM + AES-GCM
16. 7 Marks
    bDiscuss Digital signatures, TLS/SSL and interactive protocols.
    How to answer

    • Define digital signatures: Sign, Verify, unforgeability
    • Explain TLS/SSL handshake for secure channel
    • Define interactive protocols e.g. identification, zero-knowledge
    • Summarize applications in authentication