About this previous year paper
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)