Authentication Protocols
1. What Is a Protocol? (beginner — choreographed handshake!)
Protocol = message sequence proving identity LIVE (not just one seal!). Must defeat replay (old messages resent!) via nonce (one-time random!) or timestamp (clock!). One-way (login!) vs mutual (both prove!).
2. Nonce Challenge-Response (exam pattern!)
Bob → Alice: nonce R (fresh random!)
Alice → Bob: E_K(R) (prove key by transforming!)
Bob: decrypt — equals R? Alice is LIVE (not replay — R never repeats!)
Symmetric version uses shared ; public-key version signs with private!
3. Protocol Attacks (exam stories!)
| Attack | Trick | Defence |
|---|---|---|
| Replay | resend old valid message! | nonce/timestamp + cache! |
| MITM | Eve relays both sides! | authenticate keys (certs/signatures — DH alone fails!) |
| Reflection | bounce challenge back to prover! | direction tags (initiator/responder IDs!) |
| Clock skew | fake timestamps! | synced clocks + window + nonce fallback! |
4. Real Protocols (exam names!)
Kerberos (tickets + timestamps!), Needham-Schroeder (nonce-based, fixed by Lowe!), X.509/certificates (PKI trust!), TLS handshake (certs + nonces + signatures!), STS (DH + signatures = authenticated key!).
5. Exam Points
- Challenge-response + replay/nonce — 7 marks, very frequent.
- Attack table (replay/MITM/reflection) — 5 marks, favourite.