MS-CHAP

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"^[1] 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.

MS-CHAP is used as one authentication option in Microsoft's implementation of the PPTP protocol for virtual private networks. It is also used as an authentication option with RADIUS^[2] servers which are used for WiFi security using the WPA-Enterprise protocol. It is further used as the main authentication option of the Protected Extensible Authentication Protocol (PEAP).

Compared with CHAP,^[3] MS-CHAP:^[4][5]

• 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.

Cryptanalysis

Several weaknesses have been found in MS-CHAPv2, some of which severely reduce the complexity of brute-force attacks making them feasible with modern hardware.^[6][7]

MS-CHAP and MS-CHAPv2 uses the same weak 56-bit DES encryption as NTLMv1 to encrypt the NTLM password hash. 56-bit encryption had been well known as weak for years, but CloudCracker now provides a service targeted at cracking this weak DES encryption.^[8]

See also

• EFF DES cracker

References

1. "Windows 98 Dial-Up Networking Security Upgrade Release Notes (August 1998)". Support. Microsoft. August 1998.
2. Microsoft Vendor-specific RADIUS Attributes. RFC 2548. https://tools.ietf.org/html/rfc2548. 
3. PPP Challenge Handshake Authentication Protocol (CHAP). RFC 1994. https://tools.ietf.org/html/rfc1994. 
4. Microsoft PPP CHAP Extensions. RFC 2433. https://tools.ietf.org/html/rfc2433. 
5. Microsoft PPP CHAP Extensions, Version 2. RFC 2759. https://tools.ietf.org/html/rfc2759. 
6. Schneier, Bruce; Mudge; Wagner, David (19 October 1999). "Cryptanalysis of Microsoft's PPTP Authentication Extensions (MS-CHAPv2)" (PDF). schneier.com.
7. Eisinger, Jochen (23 July 2001). "Exploiting known security holes in Microsoft's PPTP Authentication Extensions (MS-CHAPv2)" (PDF). penguin-breeder.org.
8. Marlinspike, Moxie; Hulton, David (29 July 2012). "Divide and Conquer: Cracking MS-CHAPv2 with a 100% success rate". Cloud Cracler.