Session Key

A session key is a symmetric key — usually AES-128/256 or ChaCha20 — used to provide confidentiality, integrity, and authenticity for the data exchanged during a single session, and then destroyed when the session ends. Session keys are typically derived from a longer-term shared secret or key-exchange output via a KDF such as HKDF, so a single handshake (TLS, SSH, WireGuard, IPsec, Signal) can produce distinct keys for each direction and purpose (encryption, MAC, key update). Their short lifetime and ephemeral nature limit the amount of data exposed if a session key is compromised, and combined with ephemeral Diffie-Hellman they provide perfect forward secrecy for past traffic.