Network Working Group G. Zorn Internet-Draft Microsoft Corporation Category: Informational September 1998 <draft-ietf-pppext-mschapv1-keys-00.txt> Deriving MPPE Keys From MS-CHAP V1 Credentials 1. Status of this Memo This document is an Internet-Draft. Internet-Drafts are working docu- ments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working doc- uments as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress''. To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. The distribution of this memo is unlimited. It is filed as <draft-ietf-pppext- mschapv1-keys-00.txt> and expires March 24, 1999. Please send comments to the PPP Extensions Working Group mailing list (ietf-ppp@merit.edu) or to the author (glennz@microsoft.com). 2. Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links. The PPP Compression Control Protocol [2] provides a method to negotiate and utilize compression protocols over PPP encapsulated links. The Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP) [3] is a Microsoft-proprietary PPP authentication protocol, providing the functionality to which LAN-based users are accustomed while integrating the encryption and hashing algorithms used on Windows networks. Microsoft Point to Point Encryption (MPPE) [4] is a means of represent- ing PPP packets in an encrypted form. MPPE uses the RSA RC4 [5] Zorn [Page 1]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 algorithm to provide data confidentiality. The length of the session key to be used for initializing encryption tables can be negotiated. MPPE currently supports 40-bit and 128-bit session keys. MPPE session keys are changed frequently; the exact frequency depends upon the options negotiated, but may be every packet. MPPE is negotiated within option 18 [6] in the Compression Control Protocol. This document describes the method used to derive the initial MPPE ses- sion keys from MS-CHAP credentials. The algorithm used to change ses- sion keys during a session is described in [4]. 3. Specification of Requirements In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as described in [7]. 4. Deriving Session Keys from MS-CHAP Credentials The following sections detail the methods used to derive initial session keys (both 40- and 128-bit) from MS-CHAP credentials. Implementation Note The initial session key in both directions is derived from the cre- dentials of the peer that initiated the call and the challenge used (if any) is the challenge from the first authentication. This is true for both unilateral and bilateral authentication, as well as for each link in a multilink bundle. In the multi-chassis multilink case, implementations are responsible for ensuring that the correct keys are generated on all participating machines. 4.1. Generating 40-bit Session Keys MPPE uses a derivative of the peer's LAN Manager password as the 40-bit session key used for initializing the RC4 encryption tables. The first step is to obfuscate the peer's password using the LmPassword- Hash() function (described in [3]). The first 8 octets of the result are used as the basis for the session key generated in the following way: /* * PasswordHash is the basis for the session key * SessionKey is a copy of PasswordHash and is the generative session key Zorn [Page 2]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 * 8 is the length (in octets) of the key to be generated. * */ Get_Key(PasswordHash, SessionKey, 8) /* * The effective length of the key is reduced to 40 bits by * replacing the first three bytes as follows: */ SessionKey[0] = 0xD1 ; SessionKey[1] = 0x26 ; SessionKey[2] = 0x9E ; 4.2. Generating 128-bit Session Keys MPPE uses a derivative of the peer's Windows NT password as the 128-bit session key used for initializing encryption tables. The first step is to obfuscate the peer's password using NtPassword- Hash() function as described in [3]. The first 16 octets of the result are then hashed again using the MD4 algorithm. The first 16 octets of the second hash are used as the basis for the session key generated in the following way: /* * Challenge (as described in [7]) is sent by the PPP authenticator * during authentication and is 8 octets long. * NtPasswordHashHash is the basis for the session key. * On return, InitialSessionKey contains the initial session * key to be used. */ Get_Start_Key(Challenge, NtPasswordHashHash, InitialSessionKey) /* * CurrentSessionKey is a copy of InitialSessionKey * and is the generative session key. * Length (in octets) of the key to generate is 16. * */ Get_Key(InitialSessionKey, CurrentSessionKey, 16) 4.3. Key Derivation Functions The following procedures are used to derive the session key. /* Zorn [Page 3]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 * Pads used in key derivation */ SHApad1[40] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}; SHApad2[40] = {0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2, 0xF2}; /* * SHAInit(), SHAUpdate() and SHAFinal() functions are an * implementation of Secure Hash Algorithm (SHA-1) [8]. These are * available in public domain or can be licensed from * RSA Data Security, Inc. * * 1) InitialSessionKey is 8 octets long for 40 bit session keys, * 16 octets long for 128 bit session keys. * 2) CurrentSessionKey is same as InitialSessionKey when this * routine is called for the first time for the session. */ Get_Key( IN InitialSessionKey, IN/OUT CurrentSessionKey IN LengthOfDesiredKey ) { SHAInit(Context) SHAUpdate(Context, InitialSessionKey, LengthOfDesiredKey) SHAUpdate(Context, SHAPad1, 40) SHAUpdate(Context, CurrentSessionKey, LengthOfDesiredKey) SHAUpdate(Context, SHAPad2, 40) SHAFinal(Context, Digest) memcpy(CurrentSessionKey, Digest, LengthOfDesiredKey) } Get_Start_Key( IN Challenge, IN NtPasswordHashHash, OUT InitialSessionKey) { SHAInit(Context) SHAUpdate(Context, NtPasswordHashHash, 16) Zorn [Page 4]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 SHAUpdate(Context, NtPasswordHashHash, 16) SHAUpdate(Context, Challenge, 8) SHAFinal(Context, Digest) memcpy(InitialSessionKey, Digest, 16) } 5. Security Considerations Because of the way in which 40-bit keys are derived, the initial 40-bit session key will be identical in all sessions established under the same peer credentials. For this reason, and because RC4 with a 40-bit key length is believed to be a relatively weak cipher, peers SHOULD NOT use 40-bit keys derived from the LAN Manager password hash (as described above) if it can be avoided. 6. References [1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994 [2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996 [3] Zorn, G. & Cobb, S., "Microsoft PPP CHAP Extensions", draft-ietf- pppext-mschap-00.txt (work in progress), March 1998 [4] Pall, G. S., & Zorn, G., "Microsoft Point-to-Point Encryption (MPPE) Protocol", draft-ietf-pppext-mppe-02.txt, September 1998 [5] RC4 is a proprietary encryption algorithm available under license from RSA Data Security Inc. For licensing information, contact: RSA Data Security, Inc. 100 Marine Parkway Redwood City, CA 94065-1031 [6] Pall, G., "Microsoft Point-to-Point Compression (MPPC) Protocol", RFC 2118, March 1997 [7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [8] "Secure Hash Standard", Federal Information Processing Standards Publication 180-1, National Institute of Standards and Technology, April 1995 Zorn [Page 5]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 7. Acknowledgements Anthony Bell, Richard B. Ward, Terence Spies and Thomas Dimitri, all of Microsoft Corporation, significantly contributed to the design and development of MPPE. Additional thanks to Robert Friend (rfriend@hifn.com), Joe Davies (josephd@microsoft.com), Jody Terrill (jodyt@extendsys.com), Archie Cobbs (archie@whistle.com), Mark Deuser (deuser@us.ibm.com), and Jeff Haag (jeff_haag@3com.com) for useful feedback. 8. Chair's Address The PPP Extensions Working Group can be contacted via the current chair: Karl Fox Ascend Communications 3518 Riverside Drive Suite 101 Columbus, OH 43221 Phone: +1 614 326 6841 Email: karl@ascend.com 9. Author's Address Questions about this memo can also be directed to: Glen Zorn Microsoft Corporation One Microsoft Way Redmond, Washington 98052 Phone: +1 425 703 1559 FAX: +1 425 936 7329 EMail: glennz@microsoft.com 10. Expiration Date This memo is filed as <draft-ietf-pppext-mschapv1-keys-00.txt> and expires on March 24, 1999. Appendix A - Sample Key Derivations Zorn [Page 6]
INTERNET-DRAFT MS-CHAP V1 Keys for MPPE September 1998 The following sections illustrate both 40- and 128-bit key derivations. All intermediate values are in hexadecimal. Appendix A.1 - Sample 40-bit Key Derivation Initial Values Password = "clientPass" Step 1: LmPasswordHash(Password, PasswordHash) PasswordHash = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2 Step 2: Copy PasswordHash to SessionKey SessionKey = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2 Step 3: GetKey(PasswordHash, SessionKey, 8) SessionKey = d8 08 01 53 8c ec 4a 08 Step 4: Reduce the effective key length to 40 bits SessionKey = d1 26 9e 53 8c ec 4a 08 Appendix A.2 - Sample 128-bit Key Derivation Initial Values Password = "clientPass" Challenge = 10 2d b5 df 08 5d 30 41 Step 1: NtPasswordHash(Password, PasswordHash) PasswordHash = 44 eb ba 8d 53 12 b8 d6 11 47 44 11 f5 69 89 ae Step 2: PasswordHashHash = MD4(PasswordHash) PasswordHashHash = 41 c0 0c 58 4b d2 d9 1c 40 17 a2 a1 2f a5 9f 3f Step 2: GetStartKey(Challenge, PasswordHashHash, InitialSessionKey) InitialSessionKey = a8 94 78 50 cf c0 ac ca d1 78 9f b6 2d dc dd b0 Step 3: Copy InitialSessionKey to CurrentSessionKey CurrentSessionKey = a8 94 78 50 cf c0 ac ca d1 78 9f b6 2d dc dd b0 Step 4: GetKey(InitialSessionKey, CurrentSessionKey, 16) CurrentSessionKey = 59 d1 59 bc 09 f7 6f 1d a2 a8 6a 28 ff ec 0b 1e Zorn [Page 7]
