In public-key cryptography, the Station-to-Station (STS) protocol is a cryptographic key agreement scheme. The protocol is based on classic Diffie–Hellman, and provides mutual key and entity authentication. Unlike the classic Diffie–Hellman, which is not secure against a man-in-the-middle attack, this protocol assumes that the parties have signature keys, which are used to sign messages, thereby providing security against man-in-the-middle attacks.

In addition to protecting the established key from an attacker, the STS protocol uses no timestamps and provides perfect forward secrecy. It also entails two-way explicit key confirmation, making it an authenticated key agreement with key confirmation (AKC) protocol.

STS was originally presented in 1987 in the context of ISDN security (O'Higgins et al. 1987), finalized in 1989 and generally presented by Whitfield Diffie, Paul C. van Oorschot and Michael J. Wiener in 1992. The historical context for the protocol is also discussed in Diffie (1988).