What this topic covers
11 subtopics- Attacks7
- Cryptography7
- RSA7
- Hashes5
- Diffie-Hellman4
- Sockets4
- TLS4
- Firewalls2
- Signatures2
- VPN and IPsec2
- Security goals1
- Beginner 15
- Intermediate 15
- Advanced 15
About this topic
Security sits across every layer: TLS protects HTTP, IPsec protects IP, firewalls filter traffic, and cryptography underpins all of it. GATE asks RSA and Diffie-Hellman numericals; interviews ask how HTTPS works and how to defend against common attacks. Every key and ciphertext here was computed (RSA with modular inverses, DH with modular exponentiation), and the socket programs really ran over localhost.
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Sample questions
Try each one first, then open the answer for the explanation and why the other options are wrong.
Which security goal does encrypting a message mainly provide?
Show answer and explanation
Explanation
Integrity comes from MACs or signatures, authentication from proving identity, availability from defending against DoS.
Why the other options are wrong
- BEncryption alone does not detect tampering.
- CNeeds signatures.
- DKeeping the service up.
RSA with p = 3, q = 11 and public exponent e = 3. What is the private exponent d?
The answer is a number – work it out, then open the solution.
Show answer and explanation
Explanation
n = 33, φ(n) = 2 × 10 = 20. d is the inverse of 3 mod 20: 3 × 7 = 21 ≡ 1, so d = 7.
RSA with p = 5, q = 11, e = 3. Bob receives ciphertext c = 26. What is the plaintext?
The answer is a number – work it out, then open the solution.
Show answer and explanation
Explanation
n = 55, φ = 40, d = 3⁻¹ mod 40 = 27 (3 × 27 = 81 ≡ 1). m = 26²⁷ mod 55 = 16; check: 16³ = 4096 = 74 × 55 + 26, so 16³ mod 55 = 26.