High Quality Content by WIKIPEDIA articles! 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 was introduced with Windows NT 4.0 SP4 and was added to Windows 98 in the "Windows 98 Dial-Up Networking Security Upgrade Release" and Windows 95 in the "Dial Up Networking 1.3 Performance Security Update for MS Windows 95" upgrade. Windows Vista dropped support for MS-CHAPv1.