allows for each party to authenticate the other. Working cycle CHAP is an authentication scheme originally used by Point-to-Point Protocol (PPP) servers to validate the identity of remote clients. CHAP periodically verifies the identity of the client by using a three-way handshake. This happens at the time of establishing the initial link (LCP), and may happen again at any time afterwards. The verification is based on a shared secret (such as the client's password). After the completion of the link establishment phase, the authenticator sends a "challenge" message to the peer. The peer responds with a value calculated using a one-way hash function on the challenge and the secret combined. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authenticator acknowledges the authentication; otherwise it should terminate the connection. In PPP, the authenticator may send a new challenge at random intervals to the peer and repeats steps 1 through 3. However, when CHAP is used in most situations (e.g. RADIUS), this step is not performed. CHAP packets The ID chosen for the random challenge is also used in the corresponding response, success, and failure packets. A new challenge with a new ID must be different from the last challenge with another ID. If the success or failure is lost, the same response can be sent again, and it triggers the same success or failure indication. For MD5 as hash the response value is MD5(ID||secret||challenge), the MD5 for the concatenation of ID, secret, and challenge. See also List of authentication protocols Password Authentication Protocol Challenge–response authentication Cryptographic hash function References External links PPP Challenge Handshake Authentication Protocol (CHAP) Remote Authentication Dial In User Service (RADIUS): uses PAP or CHAP Extensible Authentication Protocol (EAP): discusses CHAP Internet protocols Password authentication Authentication protocols