(SEM VII) THEORY EXAMINATION 2023-24 CRYPTOGRAPHY AND NETWORK SECURITY

SECTION A (2 × 10 = 20 Marks)

Attempt all questions (Short Answers)

a) Steganography

Steganography is the technique of hiding secret information inside another file (image, audio, video, text) so that the existence of the message is concealed.
Example: Hiding a secret text message inside an image file using LSB (Least Significant Bit) method.

b) Cryptanalysis

Cryptanalysis is the process of breaking cryptographic systems to obtain the plaintext or secret key without knowing the key.
It is used to evaluate the strength of encryption algorithms.

c) GCD using Euclid’s Algorithm

Compute GCD (24120, 1640)                        24120 = 1640 × 14 + 1160
1640 = 1160 × 1 + 480                                   1160 = 480 × 2 + 200
480 = 200 × 2 + 80                                         200 = 80 × 2 + 40
80 = 40 × 2 + 0

GCD = 40

d) Primitive Roots of 13                                   Since 13 is prime, φ(13) = 12
Primitive roots modulo 13 are:

2, 6, 7, 11

e) Birthday Attack

A birthday attack exploits the probability of hash collisions.
It states that collisions are likely when about √n values are tried for an n-bit hash.

f) Services of Digital Signature                            Authentication

Integrity                                                               Non-repudiation

g) X.509 Certificate

An X.509 certificate is a digital certificate used to verify the identity of a user, device, or server using a public key.

h) Services of Pretty Good Privacy (PGP)

Any four:                                                            Authentication

Confidentiality                                                   Compression

Email compatibility                                            Digital signature

i) Intrusion Detection

Intrusion Detection is the process of monitoring network/system activities to detect unauthorized access or attacks.

j) Firewall

A firewall is a network security system that monitors and controls incoming and outgoing traffic based on security rules.
Usage: Prevents unauthorized access to networks.

 SECTION B (Attempt Any Three – 10 Marks Each)

a) DES (Data Encryption Standard)

DES is a symmetric block cipher that encrypts 64-bit plaintext using a 56-bit key.

Working Steps:

Initial Permutation (IP)                                       16 rounds of Feistel structure

Expansion                                                           XOR with subkey

Substitution (S-box)                                           Permutation

Final Permutation (FP)

Diagram:
Plaintext → IP → 16 Rounds → FP → Ciphertext

b) RSA Algorithm (Numerical)

Given:                                                                   p = 17, q = 11
n = p × q = 187                                                    φ(n) = (16)(10) = 160
e = 7, d = 23

Public Key: (e, n) = (7, 187)                                  Private Key: (d, n) = (23, 187)

Plaintext M = 77

Encryption:                                                         C = M⁷ mod 187 = 11

Decryption:                                                         M = C²³ mod 187 = 77

c) Digital Signature & DSA

A digital signature ensures authentication, integrity, and non-repudiation.

Signing Process:                                                 Hash the message

Encrypt hash using private key                             Attach signature with message

Verification Process:

Decrypt signature using public key                      Compare hashes

DSA uses modular arithmetic and SHA for signing and verification.

d) Diffie–Hellman Key Exchange

Used to securely exchange keys over an insecure channel.

Steps:                                                                   Public values: p, g

Private keys: a, b                                                   Shared key: gᵃᵇ mod p

Advantage: No prior shared key required.

e) Secure Electronic Transaction (SET)

SET is used to secure online credit card transactions.

Entities:
Cardholder, Merchant, Bank

Provides confidentiality and authentication using digital certificates.

SECTION C (Attempt Any One – 10 Marks)

a) Cryptography & Its Types

Cryptography is the science of secure communication.

Types:

Symmetric Key Cryptography (DES, AES)                     Asymmetric Key Cryptography (RSA)

Hash Functions (SHA)

b) Block Cipher & Modes

A block cipher encrypts data in fixed-size blocks.

Modes:

ECB – Simple but insecure                                         CBC – More secure, uses IV

SECTION D (Attempt Any One – 10 Marks)

a) Fermat’s Little Theorem

If p is prime and gcd(a,p)=1, then
aᵖ⁻¹ ≡ 1 (mod p)

Given:                                                                         Find a ≡ 97⁹⁴ mod 73
Using Fermat theorem → a = 24

b) Chinese Remainder Theorem

X ≡ 2 (mod 3)                                                             X ≡ 3 (mod 5)