Network Working Group G. Zorn Internet-Draft Microsoft Corporation Category: Informational September 1999 <draft-ietf-pppext-mppe-keys-01.txt> MPPE Key Derivation 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 except for the right to produce derivative works. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. To view the list of Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. 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-mppe- keys-01.txt> and expires March 13, 2000. 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. Microsoft Point to Point Encryption (MPPE) [4] is a means of representing PPP packets in an encrypted form. MPPE uses the RSA RC4 [5] algorithm to provide data confidentiality. The length of the Zorn [Page 1]
INTERNET-DRAFT MPPE Key Derivation September 1999 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 initial MPPE session keys from a variety of credential types. It is expected that this memo will be updated whenever Microsoft defines a new key derivation method for MPPE, since its primary purpose is to provide an open, easily accessible reference for third-parties wishing to interoperate with Microsoft products. The algorithm used to change session 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 Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP-1) [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. The following sections detail the methods used to derive initial session keys (both 40- and 128-bit) from MS-CHAP-1 credentials. Implementation Note The initial session key in both directions is derived from the credentials 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. Zorn [Page 2]
INTERNET-DRAFT MPPE Key Derivation September 1999 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 LmPasswordHash() 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 * 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 NtPasswordHash() 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 [10]) 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) /* Zorn [Page 3]
INTERNET-DRAFT MPPE Key Derivation September 1999 * 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. /* * 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) Zorn [Page 4]
INTERNET-DRAFT MPPE Key Derivation September 1999 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) SHAUpdate(Context, NtPasswordHashHash, 16) SHAUpdate(Context, Challenge, 8) SHAFinal(Context, Digest) memcpy(InitialSessionKey, Digest, 16) } 4.4. Sample Key Derivations The following sections illustrate both 40- and 128-bit key derivations. All intermediate values are in hexadecimal. 4.4.1. 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 4.4.2. Sample 128-bit Key Derivation Initial Values Password = "clientPass" Zorn [Page 5]
INTERNET-DRAFT MPPE Key Derivation September 1999 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 3: GetStartKey(Challenge, PasswordHashHash, InitialSessionKey) InitialSessionKey = a8 94 78 50 cf c0 ac ca d1 78 9f b6 2d dc dd b0 Step 4: Copy InitialSessionKey to CurrentSessionKey CurrentSessionKey = a8 94 78 50 cf c0 ac c1 d1 78 9f b6 2d dc dd b0 Step 5: GetKey(InitialSessionKey, CurrentSessionKey, 16) CurrentSessionKey = 59 d1 59 bc 09 f7 6f 1d a2 a8 6a 28 ff ec 0b 1e 5. Deriving Session Keys from MS-CHAP-2 Credentials Version 2 of the Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP-2) [9] 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. 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 password. The only difference is in the length of the keys and their effective 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 credentials 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. Zorn [Page 6]
INTERNET-DRAFT MPPE Key Derivation September 1999 5.1. 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 NtPasswordHash() function as described in [9]. 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 [9] 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 GetNewKeyFromSHA() (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) Zorn [Page 7]
INTERNET-DRAFT MPPE Key Derivation September 1999 5.2. 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 NtPasswordHash() function as described in [9]. 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 [9] 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 GetNewKeyFromSHA() (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) 5.3. Key Derivation Functions The following procedures are used to derive the session key. /* * Pads used in key derivation */ Zorn [Page 8]
INTERNET-DRAFT MPPE Key Derivation September 1999 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] = {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 ) Zorn [Page 9]
INTERNET-DRAFT MPPE Key Derivation September 1999 { 20-octet Digest ZeroMemory(Digest, sizeof(Digest)); /* * SHSInit(), SHSUpdate() and SHSFinal() * 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() Zorn [Page 10]
INTERNET-DRAFT MPPE Key Derivation September 1999 * are an implementation of the Secure Hash Standard [8]. */ SHSInit(Context); SHSUpdate(Context, MasterKey, 16); SHSUpdate(Context, SHSpad1, 40); SHSUpdate(Context, s, 84); SHSUpdate(Context, SHSpad2, 40); SHSFinal(Context, Digest); MoveMemory(SessionKey, Digest, SessionKeyLength); } 5.4. Sample Key Derivations The following sections illustrate both 40- and 128-bit key derivations. All intermediate values are in hexadecimal. 5.4.1. 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 3: Derive the master key (GetMasterKey()) MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31 Step 4: Derive the master send session key (GetAsymmetricStartKey()) SendStartKey40 = 8B 7C DC 14 9B 99 3A 1B Zorn [Page 11]
INTERNET-DRAFT MPPE Key Derivation September 1999 Step 5: Derive the intial send session key (GetNewKeyFromSHA()) SendSessionKey40 = D1 26 9E C4 9F A6 2E 3E Sample Encrypted Message rc4(SendSessionKey40, "test message") = 92 91 37 91 7E 58 03 D6 68 D7 58 98 5.4.2. Sample 128-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 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 Encrypted Message rc4(SendSessionKey128, "test message") = 81 84 83 17 DF 68 84 62 72 FB 5A BE Zorn [Page 12]
INTERNET-DRAFT MPPE Key Derivation September 1999 6. Deriving MPPE Session Keys from TLS Session Keys The Extensible Authentication Protocol (EAP) [11] is a PPP extension that provides support for additional authentication methods within PPP. Transport Level Security (TLS) [12] provides for mutual authentication, integrity-protected ciphersuite negotiation and key exchange between two endpoints. EAP-TLS [13] is an EAP authentication type which allows the use of TLS within the PPP authentication framework. The following sections describe the methods used to derive initial session keys from TLS session keys. Both 40- and 128-bit keys are derived using the same algorithm. The only difference is in the length of the keys and their effective 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. 6.1. Generating 40-bit Session Keys When MPPE is used in conjunction with EAP-TLS authentication, the TLS master secret is used as the master session key. The algorithm used to derive asymmetrical master session keys from the TLS master secret is described in [13]. The master session keys are never used to encrypt or decrypt data; they are only used in the derivation of transient session keys. Implementation Note If the asymmetrical master keys are less than 8 octets in length, they MUST be padded on the left with zeroes before being used to derive the initial transient session keys. Conversely, if the asymmetrical master keys are more than 8 octets in length, they must be truncated to 8 octets before being used to derive the initial transient session keys. The initial transient session keys are obtained by calling the function GetNewKeyFromSHA() (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 Zorn [Page 13]
INTERNET-DRAFT MPPE Key Derivation September 1999 Finally, the RC4 tables are initialized using the new session keys: rc4_key(SendRC4key, 8, SendSessionKey) rc4_key(ReceiveRC4key, 8, ReceiveSessionKey) 6.2. Generating 128-bit Session Keys When MPPE is used in conjunction with EAP-TLS authentication, the TLS master secret is used as the master session key. The algorithm used to derive asymmetrical master session keys from the TLS master secret is described in [13]. Note that the send key on one side is the receive key on the other. The master session keys are never used to encrypt or decrypt data; they are only used in the derivation of transient session keys. Implementation Note If the asymmetrical master keys are less than 16 octets in length, they MUST be padded on the left with zeroes before being used to derive the initial transient session keys. Conversely, if the asymmetrical master keys are more than 16 octets in length, they must be truncated to 16 octets before being used to derive the initial transient session keys. The initial transient session keys are obtained by calling the function GetNewKeyFromSHA() (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) 7. Security Considerations 7.1. MS-CHAP Credentials Because of the way in which 40-bit keys are derived from MS-CHAP-1 credentials, 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 Zorn [Page 14]
INTERNET-DRAFT MPPE Key Derivation September 1999 Manager password hash (as described above) if it can be avoided. Since the MPPE session keys are derived from user passwords (in the MS- CHAP-1 and MS-CHAP-2 cases), care should be taken to ensure the selection of strong passwords and passwords should be changed frequently. 7.2. EAP-TLS Credentials The strength of the session keys is dependent upon the security of the TLS protocol. The EAP server may be on a separate machine from the PPP authenticator; if this is the case, adequate care must be taken in the transmission of the EAP-TLS master keys to the authenticator. 8. 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. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433, October 1998 [4] Pall, G. and G. Zorn, "Microsoft Point-to-Point Encryption (MPPE) Protocol", draft-ietf-pppext-mppe-03.txt (work in progress), May 1999 [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 15]
INTERNET-DRAFT MPPE Key Derivation September 1999 [9] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", draft-ietf- pppext-mschap-v2-03.txt (work in progress), April 1999 [10] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996 [11] Blunk, L., Vollbrecht, J., "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998 [12] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999 [13] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol", draft-ietf-pppext-eaptls-06.txt (work in progress), August 1999 9. 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, Joe Davies, Jody Terrill, Archie Cobbs, Mark Deuser, Vijay Baliga, Brad Robel-Forrest and Jeff Haag for useful feedback. 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 FAX: +1 425 936 7329 EMail: gwz@acm.org 11. Expiration Date This memo is filed as <draft-ietf-pppext-mppe-keys-01.txt> and expires on March 13, 2000. Zorn [Page 16]