CHAP stands for Challenge Handshake Authentication Protocol.
MS-CHAP is the Microsoft version of the Challenge-Handshake Authentication
Protocol, CHAP. The protocol exists in two versions, MS-CHAPv1 (defined
in RFC 2433) and MS-CHAPv2 (defined in RFC 2759). MS-CHAPv2 provides mutual
authentication between peers by piggybacking a peer challenge on
the Response packet and an authenticator response on the Success
packet.