rfc3079.txt

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Network Working Group                                            G. Zorn
Request for Comments: 3079                                 cisco Systems
Category: Informational                                       March 2001


 Deriving Keys for use with Microsoft Point-to-Point Encryption (MPPE)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Compression Control Protocol provides a method to negotiate
   and utilize compression protocols over PPP encapsulated links.

   Microsoft Point to Point Encryption (MPPE) is a means of representing
   PPP packets in an encrypted form.  MPPE uses the RSA RC4 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, 56-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 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.

   MPPE itself (including the protocol used to negotiate its use, the
   details of the encryption method used and the algorithm used to
   change session keys during a session) is described in RFC 3078.







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RFC 3079                  MPPE Key Derivation                 March 2001


Table of Contents

   1.  Specification of Requirements ............................... 2
   2.  Deriving Session Keys from MS-CHAP Credentials .............. 2
   2.1.  Generating 40-bit Session Keys ............................ 3
   2.2.  Generating 56-bit Session Keys ............................ 3
   2.3.  Generating 128-bit Session Keys ........................... 4
   2.4.  Key Derivation Functions .................................. 5
   2.5.  Sample Key Derivations .................................... 6
   2.5.1.  Sample 40-bit Key Derivation ............................ 6
   2.5.2.  Sample 56-bit Key Derivation ............................ 6
   2.5.3.  Sample 128-bit Key Derivation ........................... 7
   3.  Deriving Session Keys from MS-CHAP-2 Credentials ............ 7
   3.1.  Generating 40-bit Session Keys ............................ 8
   3.2.  Generating 56-bit Session Keys ............................ 9
   3.3.  Generating 128-bit Session Keys ...........................10
   3.4.  Key Derivation Functions ..................................11
   3.5.  Sample Key Derivations ....................................13
   3.5.1.  Sample 40-bit Key Derivation ............................13
   3.5.2.  Sample 56-bit Key Derivation ............................14
   3.5.3.  Sample 128-bit Key Derivation ...........................15
   4.  Deriving MPPE Session Keys from TLS Session Keys ............16
   4.1.  Generating 40-bit Session Keys ............................16
   4.2.  Generating 56-bit Session Keys ............................17
   4.3.  Generating 128-bit Session Keys ...........................17
   5.  Security Considerations .....................................18
   5.1.  MS-CHAP Credentials .......................................18
   5.2.  EAP-TLS Credentials .......................................19
   6.  References ..................................................19
   7.  Acknowledgements ............................................20
   8.  Author's Address ............................................20
   9.  Full Copyright Statement ....................................21

1.  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 [6].

2.  Deriving Session Keys from MS-CHAP Credentials

   The Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP-1)
   [2] is a Microsoft-proprietary PPP [1] authentication protocol,
   providing the functionality to which LAN-based users are accustomed
   while integrating the encryption and hashing algorithms used on
   Windows networks.





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RFC 3079                  MPPE Key Derivation                 March 2001


   The following sections detail the methods used to derive initial
   session keys (40-, 56- 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.

2.1.  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 [2]).  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 ;

2.2.  Generating 56-bit Session Keys

   MPPE uses a derivative of the peer's LAN Manager password as the 56-
   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 [2]).  The first 8 octets of
   the result are used as the basis for the session key generated in the
   following way:



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RFC 3079                  MPPE Key Derivation                 March 2001


/*
* 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 56 bits by
* replacing the first byte as follows:
*/
SessionKey[0] = 0xd1 ;

2.3.  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 [2].  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 [9]) 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)










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RFC 3079                  MPPE Key Derivation                 March 2001


2.4.  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) [7]. These are
 * available in public domain or can be licensed from
 * RSA Data Security, Inc.
 *
 * 1) InitialSessionKey is 8 octets long for 56- and 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,



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RFC 3079                  MPPE Key Derivation                 March 2001


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)
}

2.5.  Sample Key Derivations

   The following sections illustrate 40-, 56- and 128-bit key
   derivations.  All intermediate values are in hexadecimal.

2.5.1.  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

2.5.2.  Sample 56-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




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RFC 3079                  MPPE Key Derivation                 March 2001


   Step 4: Reduce the effective key length to 56 bits
      SessionKey = d1 08 01 53 8c ec 4a 08

2.5.3.  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 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

3.  Deriving Session Keys from MS-CHAP-2 Credentials

   Version 2 of the Microsoft Challenge-Handshake Authentication
   Protocol (MS-CHAP-2) [8] 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.  40-, 56- and 128-bit keys
   are all 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- and 56-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.



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