CS-703-A Cryptography and Information 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
   aEncrypt the following using play fair cipher using the keyword MONARCHY. Use X for blank spaces "SWARAJ IS MY BIRTH RIGHT".
   How to answer

   • Construct 5x5 Playfair matrix from keyword MONARCHY merging I/J
   • Prepare plaintext digraphs: SW AR AJ IS MY BI RT HR IG HT with X for spaces/repeats
   • Apply Playfair rules for same-row, same-column and rectangle cases
   • Write resulting ciphertext digraphs
2. 7 Marks
   bExplain the Key Generation, Encryption and Decryption of DES algorithm in detail.
   How to answer

   • Initial permutation and 16-round Feistel structure of DES
   • Key generation: PC-1, shifts, PC-2 to get 16 x 48-bit subkeys
   • Encryption round: expansion, S-box, permutation, XOR and swap
   • Decryption as reverse order of subkeys
3. 7 Marks
   aPerform encryption and decryption using RSA algorithm for the following. $p = 7, q = 11, e = 17, m = 8$
   How to answer

   • Compute n = p*q = 77 and phi(n) = 60
   • Verify e = 17 with d such that e*d mod phi = 1, find d = 53
   • Encrypt c = m^e mod n for m = 8
   • Decrypt m = c^d mod n to recover 8
4. 7 Marks
   bTo find :

   1. i)85 mod 120
   2. ii)89 mod 119
   3. iii)Primitive root of 13
   4. iv)Primitive root of 7
   How to answer

   • Evaluate 85 mod 120 and 89 mod 119 directly
   • Recall definition of primitive root modulo prime
   • Test candidates for mod 13 and mod 7 using powers
   • List primitive roots found
5. 7 Marks
   aDescribe the differences between steganography and cryptography with example in details.
   How to answer

   • Define steganography as hiding existence and cryptography as hiding content
   • Compare on goal, visibility, security principle, robustness
   • Give example of LSB image hiding vs Caesar/AES encryption
   • Summarize differences in table
6. 7 Marks
   bGive the five modes of operation of Block cipher. Explain any two.
   How to answer

   • List ECB, CBC, CFB, OFB, CTR modes
   • Explain working of any two with diagrams and equations
   • Discuss error propagation, IV and parallelism
   • Mention uses and limitations
7. 14 Marks
   Find the secret key shared between user A and user B using Diffe-Hellman algorithm for the following $q = 353$; $\alpha$ (primitive root) $= 3, XA = 45$ and $XB = 50$.
   How to answer

   • Compute public values YA = alpha^XA mod q and YB = alpha^XB mod q
   • Compute shared secret K = YB^XA mod q = YA^XB mod q
   • Show both sides yield same key value
   • State final secret key
8. 7 Marks
   aDesign the role of Ticket Granting Server in inters realm operations of Kerberos.
   How to answer

   • Recall Kerberos realms, KDC, AS and TGS components
   • Explain inter-realm key sharing between TGSs
   • Describe ticket request flow across realms
   • Show referral ticket mechanism with diagram
9. 7 Marks
   bIllustrate the steps involved in signature generation and verification functions of DSS.
   How to answer

   • List DSS global parameters p, q, g and keys
   • Steps for signature generation: k, r, s calculation
   • Steps for verification: w, u1, u2, v calculation
   • Condition v = r for valid signature
10. 7 Marks
    aDiscuss transport mode and tunnel mode authentication in IP? Describe how ESP is applied to both these modes.
    How to answer

    • Define transport mode vs tunnel mode in IPSec
    • Explain AH/ESP header placement in each mode
    • Show how ESP provides confidentiality and authentication in both
    • Compare with diagrams
11. 7 Marks
    bDraw the IP security authentication header and describe the function of each field.
    How to answer

    • Draw IP Authentication Header format with all fields
    • Explain Next Header, Payload Length, SPI, Sequence Number
    • Explain Authentication Data / ICV function
    • Mention anti-replay and integrity purpose
12. 7 Marks
    aWith a neat diagram, explain the steps involved in SHA algorithm for encrypting a message with maximum length of less than 2128 bits and produces as output a 512-bit message digest.
    How to answer

    • State SHA-512 message padding and parsing for <2128 bits
    • Explain 80-round compression with message schedule
    • Show buffer update with constants and functions
    • Draw block diagram producing 512-bit digest
13. 7 Marks
    bExplain the technical details of firewall and describe any three types of firewall with neat diagram.
    How to answer

    • Define firewall purpose and filtering principle
    • Explain packet filter, stateful, application proxy types
    • Draw neat diagrams for three types
    • Compare advantages and limitations
14. 14 Marks
    Write a short notes (any three) :

    1. i)Spoofing tools
    2. ii)ECC
    3. iii)Firewalls
    4. iv)Cryptanalysis
    How to answer

    • Define each chosen term with principle and purpose
    • Explain working with example or diagram
    • Mention applications and attacks/countermeasures
    • Conclude with significance in information security