THEORY EXAMINATION (SEM–VI) 2016-17 CRYPTOGRAPHY & NETWORK SECURITY

CRYPTOGRAPHY & NETWORK SECURITY – EEE024

B.Tech (SEM VI) | Section-wise Solved Answers

SECTION – A

(Explain the following – 2 marks each)

(a) Symmetric vs Asymmetric Cryptography

Symmetric cryptography uses the same secret key for encryption and decryption, making it fast but difficult for key distribution. Asymmetric cryptography uses a public–private key pair, simplifying key distribution but requiring more computation.

(b) Problems with One-Time Pad

The one-time pad requires a truly random key equal in length to the message and strict one-time use. Key generation, storage, and distribution make it impractical.

(c) Decryption key of Hill Cipher

The decryption key is the inverse of the encryption matrix modulo 26. It is calculated using the determinant, its modular inverse, and adjoint matrix.

(d) Why some block cipher modes use only encryption

Modes like OFB and CTR convert block ciphers into stream ciphers. Since encryption and decryption processes are identical, only encryption is required.

(e) Row Transposition Cipher

Using key 2 5 4 1 3, plaintext is written row-wise and read column-wise based on key order to generate ciphertext.

(f) Primitive roots of 19

Two primitive roots of 19 are 2 and 10, since their powers generate all non-zero residues modulo 19.

(g) Secret key distribution methods

Secret keys can be distributed manually, via a trusted third party, using public-key encryption, or through key exchange protocols.

(h) Authenticated Diffie–Hellman

Authenticated Diffie–Hellman combines key exchange with authentication to prevent man-in-the-middle attacks.

(i) Requirements of digital signature

A digital signature must ensure authenticity, integrity, non-repudiation, and be verifiable by third parties.

(j) Why PGP signs before compression

Signing before compression ensures message integrity and reduces data size, improving efficiency and security.

SECTION – B

(Attempt any five – explained properly)

(a) Feistel Cipher & DES Complementation Property

Feistel structure divides plaintext into halves and processes them iteratively.
DES complementation property states that complementing plaintext and key complements the ciphertext.

(b) Euler’s Theorem & Totient Function

Euler’s theorem states that if gcd(a,n)=1, then

aϕ(n)≡1 (mod n)a^{\phi(n)} \equiv 1 \ (\text{mod } n)aϕ(n)≡1 (mod n)

For 300 = 2² × 3 × 5²,

ϕ(300)=80\phi(300) = 80ϕ(300)=80 

(c) Chinese Remainder Theorem

CRT finds a number satisfying multiple congruences.
For given equations, the solution is x ≡ 11 (mod 60).

(d) MAC vs Hash & Kerberos Communication

MAC uses a secret key and ensures authentication and integrity, while hash functions ensure integrity only.
Kerberos uses tickets and symmetric keys to authenticate client–server communication.

(e) Digital vs Ink Signatures & ElGamal

Digital signatures provide authentication and integrity.
ElGamal signatures differ for the same message due to random values used during signing.

(f) S/MIME Algorithms & Header Fields

S/MIME uses RSA, AES, SHA, and certificates. Headers include MIME-Version, Content-Type, and Content-Transfer-Encoding.

(g) X.509 Authentication

X.509 provides scalable authentication using certificates. Certificates can be revoked using CRLs or OCSP.

(h) Secure Electronic Transaction (SET)

Participants include cardholder, merchant, issuer, and acquirer. Dual signatures protect order and payment information separately.

SECTION – C

(Attempt any two – long answers)

(3) Diffie–Hellman & MITM Attack

Diffie–Hellman is vulnerable to man-in-the-middle attacks without authentication.
Given p=71, g=7, private keys 5 and 12, the shared secret key is 29.

(4) Firewall & Intrusion Detection

Firewalls protect networks by filtering traffic. Configurations include packet filtering, proxy, and stateful firewalls.
Intrusion detection identifies malicious activity using signature-based and anomaly-based methods.

(5) Short Notes

Linear & Differential Cryptanalysis – Techniques to break block ciphers by analyzing patterns.

IPSec – Provides security at IP layer using AH and ESP.

Viruses & Threats – Malware that spreads and damages systems.