CHAP

computing, the Challenge-Handshake Authentication Protocol (CHAP) is an authentication protocol used to log on a user to an Internet access provider.

It is defined in RFC 1994.

CHAP is used to periodically verify the identity of the peer using a 3-way handshake. This is done upon initial link establishment, and MAY be repeated anytime after the link has been established.

1. After the Link Establishment phase is complete, the authenticator sends a "challenge" message to the peer.
2. The peer responds with a value calculated using a "one-way hash" function.
3. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authentication is acknowledged; otherwise the connection should be terminated.
4. At random intervals, the authenticator sends a new challenge to the peer, and repeats steps 1 to 3.

See also: Password Authentication Protocol