(SEM VII) THEORY EXAMINATION 2022-23 CRYPTOGRAPHY & NETWORK SECURITY

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CRYPTOGRAPHY & NETWORK SECURITY (KCS074)

B.Tech SEM VII – Complete Solved Question Paper (2022–23)
⏱ Time: 3 Hours | 📊 Marks: 100

SECTION A

Attempt all questions in brief (2 × 10 = 20 marks)

(a) Explain Shannon Confusion and Diffusion

Confusion hides the relationship between the key and ciphertext, making it complex.
Diffusion spreads the influence of one plaintext bit over many ciphertext bits, reducing statistical patterns.

(b) Apply the Caesar Cipher (p = D(3, C)) and decrypt the ciphertext “PHHW PH”

Caesar cipher with shift 3: P → M, H → E, H → E, W → T
Decrypted text: MEET ME

(c) Calculate Φ(35)

35 = 5 × 7 Φ(35) = 35 × (1 − 1/5) × (1 − 1/7)
Φ(35) = 35 × 4/5 × 6/7 = 24

(d) Find gcd(1970, 1066) using Euclid’s algorithm 1970 = 1066 × 1 + 904
1066 = 904 × 1 + 162 904 = 162 × 5 + 94
162 = 94 × 1 + 68 94 = 68 × 1 + 26
68 = 26 × 2 + 16 26 = 16 × 1 + 10
16 = 10 × 1 + 6 10 = 6 × 1 + 4
6 = 4 × 1 + 2 4 = 2 × 2 + 0

gcd = 2

(e) What is Birthday attack?

A birthday attack exploits hash collisions based on probability theory, showing that collisions can occur faster than brute-force due to the birthday paradox.

(f) Role of compression function in hash function

The compression function reduces variable-length input into fixed-length output, ensuring diffusion, collision resistance, and security in hash algorithms.

(g) What is realm?

A realm is a logical network domain in Kerberos authentication where users and services share a common authentication database.

(h) Services provided by PGP

PGP provides: Authentication

Confidentiality Compression

Email compatibility Digital signatures

(i) Why does ESP include a padding field?

ESP padding ensures proper block alignment, hides plaintext length, and improves security.

(j) Mention four SSL protocols SSL Handshake Protocol

SSL Change Cipher Spec SSL Alert Protocol

SSL Record Protocol

SECTION B

Attempt any THREE (10 × 3 = 30 marks)

(a) Block cipher vs Stream cipher & modes of operation

Block Cipher: Encrypts data in fixed-size blocks (e.g., AES, DES).
Stream Cipher: Encrypts data bit-by-bit (e.g., RC4).

Modes of operation:
ECB, CBC, CFB, OFB, CTR — used to improve security and handle large data securely.

(b) Chinese Remainder Theorem (CRT)

Given: X ≡ 2 (mod 3)
X ≡ 3 (mod 5) X ≡ 2 (mod 7)

Solution using CRT gives: X ≡ 23 (mod 105)

(c) Why Message Authentication is required? Explain MAC

Message authentication ensures data integrity and authenticity.
MAC (Message Authentication Code) uses a secret key and message to generate a fixed-length tag verified by the receiver.
( Draw MAC block diagram in exam)

(d) Digital Certificate & X.509 format

A digital certificate binds a public key with identity.

X.509 fields: Version

Serial number Issuer

Subject Validity period

Public key Signature

Revocation: Using CRL or OCSP.

(e) Dual signature in SET

Dual signature links payment information and order information while keeping them private.
SET transaction involves customer, merchant, payment gateway, and bank in a secure sequence.

SECTION C

Q3 (Attempt any one)

(a) Playfair cipher & encryption Key = PLAYFAIR
Message = HIDE THE GOLD IN THE TREESTUMP

After applying Playfair rules:
Ciphertext: BM OD ZB XD NA BE KU DM UI XM MO UV IF

(b) One round of DES & complementation property

DES round consists of expansion, XOR with key, substitution (S-box), permutation, and XOR with left half.
Property: If plaintext and key are complemented, the ciphertext is also complemented.

Q4 (Attempt any one)

(a) AES algorithm & difference with DES

AES uses block size 128 bits and key sizes 128/192/256 bits, while DES uses 56-bit key.
AES is faster, more secure, and resistant to brute-force attacks.