Authentication protocol

The Challenge-Handshake Authentication Protocol (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.