knownkey

Knownkey is a term used in cryptography to describe a security property of key exchange protocols related to the independence of session keys when some keys have been exposed. In discussions of protocol security, knownkey concerns how the compromise of one or more session keys affects the security of other concurrent or future sessions that share the same long-term keys.

In formal terms, a protocol is said to provide knownkey security if, after an attacker learns certain session keys (for example, from some sessions), this knowledge does not enable the attacker to compute fresh session keys in other sessions. The attacker may be allowed to obtain keys from existing sessions or perform active or passive attacks, but the security definition requires that other sessions remain secure under these conditions. The precise model varies by author, including which keys the attacker can obtain and which sessions must remain secure.

Knownkey security is often discussed alongside other security properties for key exchange, such as forward secrecy and authentication. It is typically weaker than forward secrecy, which requires that past session keys remain secure even if long-term keys are later compromised. Knownkey security focuses on preventing collateral compromise across sessions after partial key exposure, rather than protecting past keys against future compromises.

In practice, security proofs for key exchange protocols may aim to establish knownkey security, sometimes in combination with other properties. This notion helps evaluate resilience in environments with concurrent sessions and potential key exposures, informing the design of robust protocols used in systems like TLS and IPsec. See also: key exchange, known-key attack, forward secrecy, perfect forward secrecy.