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

CRYPTOGRAPHY & NETWORK SECURITY – ECS029

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

SECTION – A

(Explain the following – 2 marks each)

(a) Weakness of DES

DES uses a 56-bit key, which is now too small and vulnerable to brute-force attacks. Advances in computing power have made DES insecure for modern applications.

(b) Cryptanalysis

Cryptanalysis is the study of techniques used to break cryptographic systems by analyzing weaknesses in algorithms, keys, or implementations.

(c) Difference between RSA and Diffie–Hellman

RSA is used for encryption and digital signatures, while Diffie–Hellman is used only for secure key exchange. RSA is based on factorization, whereas Diffie–Hellman relies on discrete logarithms.

(d) Euler’s Totient Function

Euler’s Totient Function φ(n) gives the number of positive integers less than n that are relatively prime to n. It is widely used in RSA algorithm.

(e) Benefits provided by MAC

Message Authentication Code ensures data integrity and authentication by verifying that the message has not been altered and comes from a legitimate sender.

(f) Firewall

A firewall is a security device that monitors and controls incoming and outgoing network traffic based on predefined security rules.

(g) Requirements of security

Security requires confidentiality, integrity, authentication, authorization, and non-repudiation to protect data and communication.

(h) Shannon’s confusion and diffusion

Confusion hides the relationship between key and ciphertext, while diffusion spreads plaintext structure over ciphertext to resist statistical attacks.

(i) Monoalphabetic cipher

A monoalphabetic cipher replaces each plaintext letter with a fixed ciphertext letter throughout the message.

(j) Euclidean theorem

The Euclidean theorem provides a method to compute the greatest common divisor (GCD) of two integers using repeated division.

SECTION – B

(Attempt any five – explained properly)

(a) Differences between Kerberos Version 4 and 5

Kerberos V5 supports stronger encryption, flexible authentication, better error handling, and cross-realm authentication compared to Version 4.

(b) Monoalphabetic Cipher vs Caesar Cipher

Caesar cipher is a special case of monoalphabetic cipher with a fixed shift. Monoalphabetic cipher allows random substitution, making it slightly stronger.

(c) Transposition Cipher

In transposition cipher, letters are rearranged without changing them.
Example:
Plaintext: ATTACK
Ciphertext: ATCTKA (based on a fixed pattern)

(d) Hill Cipher

Hill cipher uses matrix multiplication for encryption.
For message “AT” and key matrix

K=[5334]K = \begin{bmatrix}5 & 3 \\ 3 & 4\end{bmatrix}K=[53​34​]

The ciphertext is obtained by multiplying plaintext vector with key matrix modulo 26.

(e) IP Security Architecture

IPSec provides secure communication at IP layer using Authentication Header (AH) and Encapsulating Security Payload (ESP) for integrity and confidentiality.

(f) Euclidean Algorithm

Using Euclidean Algorithm:
GCD(1970, 1066) = 2
It is obtained through successive division.

(g) Fermat’s Theorem

Fermat’s Little Theorem states:
If p is prime and gcd(a,p)=1, then

ap−1≡1 (mod p)a^{p-1} \equiv 1 \ (\text{mod } p)ap−1≡1 (mod p)

Using this,

3201mod  11=13^{201} \mod 11 = 13201mod11=1 

(h) Requirements of MAC

MAC must be secure, resistant to forgery, efficient to compute, and must depend on both message and secret key.

SECTION – C

(Attempt any two – long answers)

(3) Short Notes

(i) Secure Electronic Transaction (SET)
SET is a protocol designed to secure online credit card transactions using encryption and digital certificates.

(ii) Firewalls
Firewalls filter traffic, prevent unauthorized access, and protect internal networks.

(iii) Intrusion Detection System (IDS)
IDS monitors network traffic and system activities to detect malicious behavior.

(4) RSA Key Generation Steps

Select two large prime numbers p and q

Compute n = p × q

Compute φ(n)

Choose e such that gcd(e, φ(n)) = 1

Compute private key d

Public key = (e, n), Private key = (d, n)

(5) AES Structure, Strengths & Weaknesses

AES uses substitution, permutation, mixing, and key addition stages.
Strengths: Strong security, efficient, resistant to attacks
Weaknesses: Implementation complexity, side-channel attack risks