gen-define-forward-secrecy-and-give-one-short-worked-example-4c84

Define forward secrecy and give one short worked example.

# Forward Secrecy **Forward secrecy** (also called perfect forward secrecy) is a property of cryptographic protocols where compromise of long-term keys does not compromise past session keys. Each session uses ephemeral (temporary) keys that are discarded after use, so even if an attacker later obtains the server's private key, they cannot decrypt previously recorded communications. ## Short Worked Example: Diffie-Hellman Key Exchange **Setup:** Alice and Bob agree on public parameters: prime p = 23, generator g = 5 **Session 1:** 1. Alice generates random ephemeral private key: a = 6 - Computes public value: A = 5^6 mod 23 = 8 2. Bob generates random ephemeral private key: b = 15 - Computes public value: B = 5^15 mod 23 = 19 3. They exchange A and B publicly 4. Both compute shared secret: - Alice: K = 19^6 mod 23 = 2 - Bob: K = 8^15 mod 23 = 2 5. **They delete a and b immediately** **Session 2:** They repeat with new random values (e.g., a' = 9, b' = 13) **Why forward secrecy works:** Even if an attacker records all messages and later steals authentication keys, they cannot recover the discarded ephemeral values (a, b), so they cannot recreate K and decrypt past sessions.