Network Working Group G. Zorn
Internet-Draft Microsoft Corporation
Category: Informational October 1998
<draft-ietf-pppext-mschapv2-keys-01.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
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or to cite them other than as ``work in progress''.
To learn the current status of any Internet-Draft, please check the
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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-01.txt> and expires April 15, 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.
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
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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].
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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].
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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] =
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{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()
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* 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);
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SHSUpdate(Context, SHSpad2, 40);
SHSFinal(Context, Digest);
MoveMemory(SessionKey, Digest, SessionKeyLength);
}
5. 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.
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., "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
[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
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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), Brad Robel-
Forrest (brad@watchguard.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-01.txt> and
expires on April 15, 1999.
Appendix A - Sample Key Derivations
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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
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
Appendix A.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
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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
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