Simple definition
PFS (Perfect Forward Secrecy) protects previous sessions from later decryption if a long-term private key is compromised.
Technical definition
PFS relies on ephemeral key exchanges, commonly based on (EC)DHE. Each session derives a temporary secret that cannot be recovered solely from the server’s long-term private key.
How it works / role
During negotiation, both sides create ephemeral values and derive a session secret. If those temporary secrets are then discarded, later possession of the certificate private key is not enough to reconstruct the old recorded session keys.
What is it used for?
Reduce the impact of a future private-key compromise on encrypted traffic captured in the past.
Practical example
A TLS connection negotiated with ECDHE creates a temporary secret for that session; later compromise of the certificate private key alone does not reveal the old session key.
Common issues
- Legacy cipher suite does not use an ephemeral exchange
- Older device is incompatible with modern suites
- DHE parameters are poorly sized
- Confusion between PFS and TLS encryption in general
Key takeaway: PFS limits the future value of a compromised private key by preventing that key alone from decrypting earlier sessions.