One document matched: draft-ietf-pppext-mschapv1-keys-00.txt
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]
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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
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* 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.
/*
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* 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)
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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
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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), 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
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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
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
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