Sessionavaimen

Sessionavaimen, or session key, is a temporary symmetric cryptographic key used to protect the data exchanged within a single communication session. It is typically created for the duration of one connection and discarded when the session ends. Using a session key allows long-term keys to remain static while the actual data is encrypted with fresh material, improving forward secrecy and limiting the damage from key compromise.

Generation and exchange are usually designed so the session key is not sent directly over the network. Instead, participating parties perform a key exchange, often Diffie-Hellman or Elliptic Curve Diffie-Hellman, to derive a shared session key. In many protocols, such as TLS, a master secret established during the handshake is then processed by a key derivation function to produce separate session keys, initialization vectors, and authentication keys for each direction of communication and for encryption and integrity.

Applications commonly include securing network protocols like TLS, SSH, and VPN connections. The session key is used with symmetric encryption algorithms such as AES-GCM or ChaCha20-Poly1305 to protect data. A session may involve multiple related keys, including separate keys for encryption, decryption, and message authentication.

Security considerations emphasize that session keys have a limited lifetime and should be discarded after the session ends. Forward secrecy ensures that even if a server’s long-term keys are compromised in the future, past sessions remain protected. Proper randomness, entropy, and robust key derivation are essential to prevent key leakage or predictability. The Finnish term Sessionavaimen is commonly used to refer to this concept in Finnish technical literature.