What are common examiner traps in Network Security?

Using (n) = n - 1 instead of (p - 1)(q - 1) when computing RSA private exponent d. Confusing the sender's private key with the receiver's public key (confusing digital signing/authenticity with confidentiality). Assuming a third-party attacker or receiver can launch a birthday attack on an already signed message (which actually requires a second preimage attack of complexity O(2n), whereas only the sender can pre-generate collisions before signing).

Network Security

3 Qs · 2016–2017 · 5 marks · 0.2 marks/paperStandard yield

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Network Security in GATE CS is tested primarily through core concepts of public-key cryptography, digital signature protocols, and cryptographic attacks. Questions focus on the mec… Guide

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GATE CSE 2017 Set 1Q15MCQ1 markMediumA sender S sends a message $m$ to receiver R, which is digitally signed by S with its private key. In this scenario, one or more of the following security violations can take place. (I) S can launch a birthday attack to replace $m$ with a fraudulent message. (II) A third party attacker can launch a birthday…
GATE CSE 2017 Set 1Q44NAT2 marksEasyIn a RSA cryptosystem, a participant A uses two prime numbers $p = 13$ and $q = 17$ to generate her public and private keys. If the public key of A is 35, then the private key of A is __________.
GATE CSE 2016 Set 1Q52MCQ2 marksEasyConsider that B wants to send a message $m$ that is digitally signed to A. Let the pair of private and public keys for A and B be denoted by $K_x^-$ and $K_x^+$ for $x = A, B$ , respectively. Let $K_x(m)$ represent the operation of encrypting $m$ with a key $K_x$ and $H(m)$ represent the message digest.…

Topic guide

What does GATE test in Network Security?

Network Security in GATE CS is tested primarily through core concepts of public-key cryptography, digital signature protocols, and cryptographic attacks. Questions focus on the mechanics of asymmetric encryption (specifically RSA key generation and modular arithmetic) and the correct application of private/public keys for digital signing, message digests, and vulnerability analysis.

Core concepts

RSA Public and Private Key Generation
Modular Multiplicative Inverse (Extended Euclidean Algorithm)
Digital Signatures and Message Integrity via Hashing
Sender Authentication and Non-repudiation
Collision Attacks vs. Second Preimage Resistance (Birthday Paradox)

Recurring question patterns

RSA Key Computation
common · NAT · 2 marks · 2017
Given two prime numbers $p$ and $q$ along with the public exponent $e$ , compute Euler's totient function $\phi(n) = (p-1)(q-1)$ and determine the private key decryption exponent $d$ via the modular multiplicative inverse $e \cdot d \equiv 1 \pmod{\phi(n)}$ .
Digital Signature Scheme & Protocol Correctness
common · MCQ · 2 marks · 2016
Identify the standard symbolic representation of a message accompanied by its digital signature, testing whether signing requires encrypting the hash digest with the sender's private key versus other permutations of keys.
Cryptographic Attacks & Collision Vulnerabilities
occasional · MCQ · 1 marks · 2017
Evaluate scenarios where attackers (sender, receiver, or third party) attempt specific cryptographic attacks (such as Birthday attack vs. Preimage attack) against signed messages or hash digests.

Formulas GATE uses

Euler's Totient Function for RSA Modulus

\phi(n) = (p-1)(q-1)

Used when calculating the totient modulus for finding private exponent $d$ in an RSA cryptosystem with distinct primes $p$ and $q$ .

Modular Multiplicative Inverse for RSA Private Key

e \cdot d \equiv 1 \pmod{\phi(n)}

Used to compute the private key exponent $d$ given public exponent $e$ and $\phi(n)$ .

Digital Signature Generation

\text{Signature} = K_{\text{sender}}^-(H(m))

Used to construct an authenticatable message package $\{m, K_{\text{sender}}^-(H(m))\}$ using a cryptographic hash function $H$ and sender's private key.

Examiner traps in Network Security

Using (n) = n - 1 instead of (p - 1)(q - 1) when computing RSA private exponent d.
Confusing the sender's private key with the receiver's public key (confusing digital signing/authenticity with confidentiality).
Assuming a third-party attacker or receiver can launch a birthday attack on an already signed message (which actually requires a second preimage attack of complexity O(2n), whereas only the sender can pre-generate collisions before signing).

How questions are asked

Symbolic protocol representation identification
Multi-statement true/false analysis of cryptographic attack feasibility
Numerical computation of RSA decryption key

Paper trends

Questions moved from purely identifying digital signature notation to deeper conceptual attack models (distinguishing birthday collision capabilities) and computational RSA key derivation.
2017, 2016

Difficulty notes

Easy questions ask for standard digital signature notation or basic RSA arithmetic with small prime numbers. Medium questions test nuanced differences between collision attacks and preimage attacks in the context of signature generation and message replacement.