Network Working Group G. Zorn Internet-Draft Microsoft Corporation Category: Informational November 1998 <draft-ietf-pppext-mschapv2-keys-02.txt> Deriving MPPE Keys From MS-CHAP V2 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- mschapv2-keys-02.txt> and expires May 15, 1999. Please send comments to the PPP Extensions Working Group mailing list (ietf-ppp@merit.edu) or to the author (gwz@acm.org). 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. Version 2 of the Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP-2) [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 Zorn [Page 1]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 representing PPP packets in an encrypted form. MPPE uses the RSA RC4 [5] algorithm to provide data confidentiality. The length of the ses- sion key to be used for initializing encryption tables can be negoti- ated. 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-2 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-2 Credentials The following sections detail the methods used to derive initial session keys from MS-CHAP-2 credentials. Both 40- and 128-bit keys are derived using the same algorithm from the authenticating peer's Windows NT pass- word. The only difference is in the length of the keys and their effec- tive strength: 40-bit keys are 8 octets in length, while 128-bit keys are 16 octets long. Separate keys are derived for the send and receive directions of the session. Implementation Note The initial session keys in both directions are derived from the cre- dentials of the peer that initiated the call and the challenges used are those from the first authentication. This is true as well 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 When used in conjunction with MS-CHAP-2 authentication, the initial MPPE session keys are derived from the peer's Windows NT password. The first step is to obfuscate the peer's password using NtPassword- Hash() function as described in [3]. Zorn [Page 2]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 NtPasswordHash(Password, PasswordHash) The first 16 octets of the result are then hashed again using the MD4 algorithm. PasswordHashHash = md4(PasswordHash) The first 16 octets of this second hash are used together with the NT- Response field from the MS-CHAP-2 Response packet [3] as the basis for the master session key: GetMasterKey(PasswordHashHash, NtResponse, MasterKey) Once the master key has been generated, it is used to derive two 40-bit session keys, one for sending and one for receiving: GetAsymmetricStartKey(MasterKey, MasterSendKey, 8, TRUE, TRUE) GetAsymmetricStartKey(MasterKey, MasterReceiveKey, 8, FALSE, TRUE) The master session keys are never used to encrypt or decrypt data; they are only used in the derivation of transient session keys. The initial transient session keys are obtained by calling the function Get- NewKeyFromSHA() (described in [4]): GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 8, SendSessionKey) GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 8, ReceiveSessionKey) Next, the effective strength of both keys is reduced by setting the first three octets to known constants: SendSessionKey[0] = ReceiveSessionKey[0] = 0xD1 SendSessionKey[1] = ReceiveSessionKey[1] = 0x26 SendSessionKey[2] = ReceiveSessionKey[2] = 0x9E Finally, the RC4 tables are initialized using the new session keys: rc4_key(SendRC4key, 8, SendSessionKey) rc4_key(ReceiveRC4key, 8, ReceiveSessionKey) 4.2. Generating 128-bit Session Keys When used in conjunction with MS-CHAP-2 authentication, the initial MPPE session keys are derived from the peer's Windows NT password. The first step is to obfuscate the peer's password using NtPassword- Hash() function as described in [3]. Zorn [Page 3]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 NtPasswordHash(Password, PasswordHash) The first 16 octets of the result are then hashed again using the MD4 algorithm. PasswordHashHash = md4(PasswordHash) The first 16 octets of this second hash are used together with the NT- Response field from the MS-CHAP-2 Response packet [3] as the basis for the master session key: GetMasterKey(PasswordHashHash, NtResponse, MasterKey) Once the master key has been generated, it is used to derive two 128-bit master session keys, one for sending and one for receiving: GetAsymmetricStartKey(MasterKey, MasterSendKey, 16, TRUE, TRUE) GetAsymmetricStartKey(MasterKey, MasterReceiveKey, 16, FALSE, TRUE) The master session keys are never used to encrypt or decrypt data; they are only used in the derivation of transient session keys. The initial transient session keys are obtained by calling the function Get- NewKeyFromSHA() (described in [4]): GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 16, SendSessionKey) GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 16, ReceiveSessionKey) Finally, the RC4 tables are initialized using the new session keys: rc4_key(SendRC4key, 16, SendSessionKey) rc4_key(ReceiveRC4key, 16, ReceiveSessionKey) 4.3. Key Derivation Functions The following procedures are used to derive the session key. /* * Pads used in key derivation */ SHSpad1[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}; SHSpad2[40] = Zorn [Page 4]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 {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}; /* * "Magic" constants used in key derivations */ Magic1[27] = {0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68, 0x65, 0x20, 0x4D, 0x50, 0x50, 0x45, 0x20, 0x4D, 0x61, 0x73, 0x74, 0x65, 0x72, 0x20, 0x4B, 0x65, 0x79}; Magic2[84] = {0x4F, 0x6E, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6C, 0x69, 0x65, 0x6E, 0x74, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2C, 0x20, 0x74, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68, 0x65, 0x20, 0x73, 0x65, 0x6E, 0x64, 0x20, 0x6B, 0x65, 0x79, 0x3B, 0x20, 0x6F, 0x6E, 0x20, 0x74, 0x68, 0x65, 0x20, 0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2C, 0x20, 0x69, 0x74, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68, 0x65, 0x20, 0x72, 0x65, 0x63, 0x65, 0x69, 0x76, 0x65, 0x20, 0x6B, 0x65, 0x79, 0x2E}; Magic3[84] = {0x4F, 0x6E, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6C, 0x69, 0x65, 0x6E, 0x74, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2C, 0x20, 0x74, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68, 0x65, 0x20, 0x72, 0x65, 0x63, 0x65, 0x69, 0x76, 0x65, 0x20, 0x6B, 0x65, 0x79, 0x3B, 0x20, 0x6F, 0x6E, 0x20, 0x74, 0x68, 0x65, 0x20, 0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2C, 0x20, 0x69, 0x74, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68, 0x65, 0x20, 0x73, 0x65, 0x6E, 0x64, 0x20, 0x6B, 0x65, 0x79, 0x2E}; GetMasterKey( IN 16-octet PasswordHashHash, IN 24-octet NTResponse, OUT 16-octet MasterKey ) { 20-octet Digest ZeroMemory(Digest, sizeof(Digest)); /* * SHSInit(), SHSUpdate() and SHSFinal() Zorn [Page 5]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 * are an implementation of the Secure Hash Standard [8]. */ SHSInit(Context); SHSUpdate(Context, PasswordHashHash, 16); SHSUpdate(Context, NTResponse, 24); SHSUpdate(Context, Magic1, 27); SHSFinal(Context, Digest); MoveMemory(MasterKey, Digest, 16); } VOID GetAsymetricStartKey( IN 16-octet MasterKey, OUT 8-to-16 octet SessionKey, IN INTEGER SessionKeyLength, IN BOOLEAN IsSend, IN BOOLEAN IsServer ) { 20-octet Digest; ZeroMemory(Digest, 20); if (IsSend) { if (IsServer) { s = Magic3 } else { s = Magic2 } } else { if (IsServer) { s = Magic2 } else { s = Magic3 } } /* * SHSInit(), SHSUpdate() and SHSFinal() * are an implementation of the Secure Hash Standard [8]. */ SHSInit(Context); SHSUpdate(Context, MasterKey, 16); SHSUpdate(Context, SHSpad1, 40); SHSUpdate(Context, s, 84); Zorn [Page 6]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 SHSUpdate(Context, SHSpad2, 40); SHSFinal(Context, Digest); MoveMemory(SessionKey, Digest, SessionKeyLength); } 5. Sample Key Derivations The following sections illustrate both 40- and 128-bit key derivations. All intermediate values are in hexadecimal. 5.1. Sample 40-bit Key Derivation Initial Values UserName = "User" = 55 73 65 72 Password = "clientPass" = 63 00 6C 00 69 00 65 00 6E 00 74 00 50 00 61 00 73 00 73 00 AuthenticatorChallenge = 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C 60 21 32 26 26 28 PeerChallenge = 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E Challenge = D0 2E 43 86 BC E9 12 26 NT-Response = 82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33 11 4A 3D 85 D6 DF 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: Derive the master key (GetMasterKey()) MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31 Step 3: Derive the master send session key (GetAsymmetricStartKey()) SendStartKey40 = 8B 7C DC 14 9B 99 3A 1B Step 4: Derive the intial send session key (GetNewKeyFromSHA()) SendSessionKey40 = D1 26 9E C4 9F A6 2E 3E Sample Enrypted Message rc4(SendSessionKey40, "test message") = 92 91 37 91 7E 58 03 D6 68 D7 58 98 5.2. Sample 128-bit Key Derivation Initial Values UserName = "User" = 55 73 65 72 Zorn [Page 7]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 Password = "clientPass" = 63 00 6C 00 69 00 65 00 6E 00 74 00 50 00 61 00 73 00 73 00 AuthenticatorChallenge = 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C 60 21 32 26 26 28 PeerChallenge = 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E Challenge = D0 2E 43 86 BC E9 12 26 NT-Response = 82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33 11 4A 3D 85 D6 DF 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: Derive the master key (GetMasterKey()) MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31 Step 3: Derive the send master session key (GetAsymmetricStartKey()) SendStartKey128 = 8B 7C DC 14 9B 99 3A 1B A1 18 CB 15 3F 56 DC CB Step 4: Derive the intial send session key (GetNewKeyFromSHA()) SendSessionKey128 = 40 5C B2 24 7A 79 56 E6 E2 11 00 7A E2 7B 22 D4 Sample Enrypted Message rc4(SendSessionKey128, "test message") = 81 84 83 17 DF 68 84 62 72 FB 5A BE 6. Security Considerations Since the MPPE session keys are derived from user passwords, care should be taken to ensure the selection of strong passwords and passwords should be changed frequently. 7. 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., "Microsoft PPP CHAP Extensions, Version 2", draft-ietf- pppext-mschap-v2-01.txt (work in progress), October 1998 [4] Pall, G. S., & Zorn, G., "Microsoft Point-to-Point Encryption (MPPE) Protocol", draft-ietf-pppext-mppe-02.txt (work in progress), July 1998 Zorn [Page 8]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 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. S., "Microsoft Point-to-Point Compression (MPPC) Proto- col", 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 8. Acknowledgements Thanks (in no particular order) to Tony Bell, Paul Leach, Terence Spies, Robert Friend, Joe Davies, Jody Terrill, Archie Cobbs, Mark Deuser, Brad Robel-Forrest and Jeff Haag for useful suggestions and feedback. 9. 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 10. 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 Zorn [Page 9]
INTERNET-DRAFT MS-CHAP V2 Keys for MPPE November 1998 FAX: +1 425 936 7329 EMail: gwz@acm.org 11. Expiration Date This memo is filed as <draft-ietf-pppext-mschapv1-keys-02.txt> and expires on May 15, 1999. Zorn [Page 10]
