Problem Solved
- Ensures that compromising a server’s long-term private key does not allow attackers to decrypt past recorded network traffic.
Core Mechanism
- Decouples the keys used for encryption from the keys linked to the Certificates by using Diffie-Hellman.
- Diffie-Hellman’s ability to generate the ephemeral private secret in RAM itself is what allows forward secrecy. (Nothing to do with not sharing the secret over the network.)
Why Static RSA fails
- Since the server’s long-term key is used to encrypt the pre-master secret, an attacker can record the transaction.
- If the private key is compromised later, they can extract the pre-master secret and derive the session keys to decrypt the recording.
Caution
- Static Diffie-Hellman: Won’t provide forward secrecy.
- RSA Exchange with Ephemeral Keys: Will provide forward secrecy (in theory).