Challenge-Handshake Authentication Protocol

Challenge-Handshake Authentication Protocol
CHAP

   <networking, security, standard, protocol> (CHAP) An
   {authentication} scheme used by {PPP} servers to validate the
   identity of the originator of the connection upon connection
   or any time later.

   CHAP applies a three-way {handshaking} procedure.  After the
   link is established, the server sends a "challenge" message to
   the originator.  The originator responds with a value
   calculated using a {one-way hash function}.  The server checks
   the response against its own calculation of the expected hash
   value.  If the values match, the authentication is
   acknowledged; otherwise the connection is usually terminated.

   CHAP provides protection against {playback} attack through the
   use of an incrementally changing identifier and a variable
   challenge value.  The authentication can be repeated any time
   while the connection is open limiting the time of exposure to
   any single attack, and the server is in control of the
   frequency and timing of the challenges.  As a result, CHAP
   provides greater security then {PAP}.

   CHAP is defined in {RFC} 1334.

   (1996-03-05)