MS-CHAP is the Microsoft version of the Challenge-handshake authentication protocol, CHAP. The protocol exist in two versions, MS-CHAPv1 (defined in RFC 2433) and MS-CHAPv2 (defined in RFC 2759). MS-CHAPv2 was introduced with Windows 2000. Windows Vista drops support for MS-CHAPv1.

Compared with CHAP, MS-CHAP:

• is enabled by negotiating CHAP Algorithm 0x80 (0x81 for MS-CHAPv2) in LCP option 3, Authentication Protocol
• provides an authenticator-controlled password change mechanism
• provides an authenticator-controlled authentication retry mechanism
• defines failure codes returned in the Failure packet message field

MS-CHAPv2 provides mutual authentication between peers by piggybacking a peer challenge on the Response packet and an authenticator response on the Success packet.

Security Vulnerabilities and Cryptanalysis

• Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2), co-written by Bruce Schneier

References

• RFC 1994 - PPP Challenge Handshake Authentication Protocol (CHAP)
• RFC 2433 - MS-CHAPv1
• RFC 2548 - RADIUS Encapsulation of MS-CHAPv1 and MS-CHAPv2
• RFC 2759 - MS-CHAPv2