Challenge Handshake Authentication Protocol (CHAP) is a remote access authentication protocol used in conjunction with Point to Point Protocol (PPP) to provide security and authentication to users of remote resources. CHAP is described in RFC 1994, which can be viewed from http://www.rfc-editor.org/. Challenge Handshake Authentication Protocol (CHAP) uses a challenge method for authentication. Challenge Handshake Authentication Protocol (CHAP) doesn’t use a user ID/password mechanism. In Challenge Handshake Authentication Protocol (CHAP), the initiator sends a logon request to the server. The server sends a challenge back to the client. The challenge is encrypted and then sent back to the server. The server compares the value from the client and, if the information matches, grants the session. If the response fails, the session is denied, and the request phase starts over.

Challenge Handshake Authentication Protocol (CHAP) periodically verify the identity of the peer using a three-way handshake. The verification the identity of the peer is done initially, and may be repeated anytime after the link has been established.

Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP) is the Microsoft implementation of Challenge Handshake Authentication Protocol (CHAP). There are two versions of Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP),  MS-CHAPv1 and MS-CHAPv2. Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP) has some additional features, such as providing a method for changing passwords and retrying in the event of a failure.