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(CHAP) Anauthentication scheme used by PPP servers to validate theidentity of the originator of the connection upon connectionor any time later.CHAP applies a three-way handshaking procedure. After thelink is established, the server sends a "challenge" message tothe originator. The originator responds with a valuecalculated using a one-way hash function. The server checksthe response against its own calculation of the expected hashvalue. If the values match, the authentication isacknowledged; otherwise the connection is usually terminated.CHAP provides protection against playback attack through theuse of an incrementally changing identifier and a variablechallenge value. The authentication can be repeated any timewhile the connection is open limiting the time of exposure toany single attack, and the server is in control of thefrequency and timing of the challenges. As a result, CHAPprovides greater security then PAP.CHAP is defined in RFC 1334.