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Versions: 00 01 02 03 RFC 3079

Network Working Group                                            G. Zorn
Internet-Draft                                             cisco Systems
Category: Informational                                     October 2000
<draft-ietf-pppext-mppe-keys-03.txt>

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


1.  Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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-03.txt> and expires May 3, 2001.  Please send comments to the PPP
Extensions Working Group mailing list (ietf-ppp@merit.edu) or to the
author (gwz@cisco.com).


2.  Copyright Statement

Copyright (C) The Internet Society (2000).  All Rights Reserved.


3.  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.



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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
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 [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].


4.  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].


5.  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 (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.




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5.1.  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 ;


5.2.  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 [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 56 bits by
   * replacing the first byte as follows:



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   */
   SessionKey[0] = 0xd1 ;


5.3.  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 [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)

   /*
   * 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)


5.4.  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] =



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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};

/*
 * 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 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,
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)
}








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5.5.  Sample Key Derivations

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


5.5.1.  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


5.5.2.  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

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


5.5.3.  Sample 128-bit Key Derivation

Initial Values
   Password = "clientPass"
   Challenge = 10 2d b5 df 08 5d 30 41




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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



6.  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.  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.


6.1.  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.




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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)


6.2.  Generating 56-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.




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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 56-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 octet to a known constant:

   SendSessionKey[0] = ReceiveSessionKey[0] = 0xd1

Finally, the RC4 tables are initialized using the new session keys:

   rc4_key(SendRC4key, 8, SendSessionKey)
   rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)


6.3.  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].



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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 [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)


6.4.  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);
      }


6.5.  Sample Key Derivations

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


6.5.1.  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 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

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




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6.5.2.  Sample 56-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())
   SendStartKey56 = 8B 7C DC 14 9B 99 3A 1B

Step 5: Derive the intial send session key (GetNewKeyFromSHA())
   SendSessionKey56 = D1 5C 00 C4 9F A6 2E 3E

Sample Encrypted Message
   rc4(SendSessionKey40, "test message") = 3F 10 68 33 FA 44 8D A8 42 BC 57 58


6.5.3.  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



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   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


7.  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.  56-, 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: 56- and 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.


7.1.  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



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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

Finally, the RC4 tables are initialized using the new session keys:

   rc4_key(SendRC4key, 8, SendSessionKey)
   rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)


7.2.  Generating 56-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



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     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
initial octet to a known constant:

   SendSessionKey[0] = ReceiveSessionKey[0] = 0xD1

Finally, the RC4 tables are initialized using the new session keys:

   rc4_key(SendRC4key, 8, SendSessionKey)
   rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)


7.3.  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:




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   rc4_key(SendRC4key, 16, SendSessionKey)
   rc4_key(ReceiveRC4key, 16, ReceiveSessionKey)


8.  Security Considerations

8.1.  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
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.


8.2.  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.


9.  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-05.txt (work in progress),
     October 1999

[5]  RC4 is a proprietary encryption algorithm available under license
     from RSA Data Security Inc.  For licensing information, contact:



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        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

[9]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759,
     January 2000

[10] Simpson, W., "PPP Challenge Handshake Authentication Protocol
     (CHAP)", RFC 1994, August 1996

[11] Blunk, L. and J. Vollbrecht, "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", RFC
     2716, October 1999


10.  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.

The technical portions of this memo were completed while the author was
employed by Microsoft Corporation.

11.  Author's Address

Questions about this memo can also be directed to:

   Glen Zorn



Zorn                                                           [Page 19]


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   cisco Systems
   500 108th Avenue N.E.
   Suite 500
   Bellevue, Washington 98004
   USA

   Phone: +1 425 438 8218
   FAX:   +1 425 438 1848
   EMail: gwz@cisco.com


12.  Full Copyright Statement

Copyright (C) The Internet Society (2000).  All Rights Reserved.  This
document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist
in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.  However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.  The limited permissions granted above are
perpetual and will not be revoked by the Internet Society or its
successors or assigns.  This document and the information contained
herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


13.  Expiration Date

This memo is filed as <draft-ietf-pppext-mppe-keys-03.txt> and expires
on May 3, 2001.












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