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rfc:rfc2433

Network Working Group G. Zorn Request for Comments: 2433 S. Cobb Category: Informational Microsoft Corporation

                                                          October 1998
                   Microsoft PPP CHAP Extensions

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 (1998).  All Rights Reserved.

IESG Note

 The protocol described here has significant vulnerabilities.  People
 planning on implementing or using this protocol should read section
 12, "Security Considerations".

1. Abstract

 The Point-to-Point Protocol (PPP) [1] provides a standard method for
 transporting multi-protocol datagrams over point-to-point links.  PPP
 defines an extensible Link Control Protocol and a family of Network
 Control Protocols (NCPs) for establishing and configuring different
 network-layer protocols.
 This document describes Microsoft's PPP CHAP dialect (MS-CHAP), which
 extends the user authentication functionality provided on Windows
 networks to remote workstations.  MS-CHAP is closely derived from the
 PPP Challenge Handshake Authentication Protocol described in RFC 1994
 [2], which the reader should have at hand.
 The algorithms used in the generation of various MS-CHAP protocol
 fields are described in an appendix.

2. Introduction

 Microsoft created MS-CHAP to authenticate remote Windows
 workstations, providing the functionality to which LAN-based users
 are accustomed while integrating the encryption and hashing
 algorithms used on Windows networks.

Zorn & Cobb Informational [Page 1] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

 Where possible, MS-CHAP is consistent with standard CHAP.  Briefly,
 the differences between MS-CHAP and standard CHAP are:
  • MS-CHAP is enabled by negotiating CHAP Algorithm 0x80 in LCP

option 3, Authentication Protocol.

  • The MS-CHAP Response packet is in a format designed for

compatibility with Microsoft's Windows NT 3.5, 3.51 and 4.0, and

      Windows95 networking products.  The MS-CHAP format does not
      require the authenticator to store a clear-text or reversibly
      encrypted password.
  • MS-CHAP provides authenticator-controlled authentication retry

and password changing mechanisms.

  • MS-CHAP defines a set of reason-for-failure codes returned in

the Failure packet Message field.

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 [2].

4. LCP Configuration

 The LCP configuration for MS-CHAP is identical to that for standard
 CHAP, except that the Algorithm field has value 0x80, rather than the
 MD5 value 0x05.  PPP implementations which do not support MS-CHAP,
 but correctly implement LCP Config-Rej, should have no problem
 dealing with this non-standard option.

5. Challenge Packet

 The MS-CHAP Challenge packet is identical in format to the standard
 CHAP Challenge packet.
 MS-CHAP authenticators send an 8-octet challenge Value field.  Peers
 need not duplicate Microsoft's algorithm for selecting the 8-octet
 value, but the standard guidelines on randomness [1,2,7] SHOULD be
 observed.
 Microsoft authenticators do not currently provide information in the
 Name field.  This may change in the future.

Zorn & Cobb Informational [Page 2] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

6. Response Packet

 The MS-CHAP Response packet is identical in format to the standard
 CHAP Response packet.  However, the Value field is sub-formatted
 differently as follows:
    24 octets: LAN Manager compatible challenge response
    24 octets: Windows NT compatible challenge response
     1 octet : "Use Windows NT compatible challenge response" flag
 The LAN Manager compatible challenge response is an encoded function
 of the password and the received challenge as output by the routine
 LmChallengeResponse() (see section A.1, below).  LAN Manager
 passwords are limited to 14 case-insensitive OEM characters.  Note
 that use of the LAN Manager compatible challenge response has been
 deprecated; peers SHOULD NOT generate it, and the sub-field SHOULD be
 zero-filled.  The algorithm used in the generation of the LAN Manager
 compatible challenge response is described here for informational
 purposes only.
 The Windows NT compatible challenge response is an encoded function
 of the password and the received challenge as output by the routine
 NTChallengeResponse() (see section A.5, below).  The Windows NT
 password is a string of 0 to (theoretically) 256 case-sensitive
 Unicode [8] characters.  Current versions of Windows NT limit
 passwords to 14 characters, mainly for compatibility reasons; this
 may change in the future.
 The "use Windows NT compatible challenge response" flag, if 1,
 indicates that the Windows NT response is provided and should be used
 in preference to the LAN Manager response.  The LAN Manager response
 will still be used if the account does not have a Windows NT password
 hash, e.g.  if the password has not been changed since the account
 was uploaded from a LAN Manager 2.x account database.  If the flag is
 0, the Windows NT response is ignored and the LAN Manager response is
 used.  Since the use of LAN Manager authentication has been
 deprecated, this flag SHOULD always be set (1) and the LAN Manager
 compatible challenge response field SHOULD be zero-filled.
 The Name field identifies the peer's user account name.  The Windows
 NT domain name may prefix the user's account name (e.g.
 "BIGCO\johndoe" where "BIGCO" is a Windows NT domain containing the
 user account "john-doe").  If a domain is not provided, the backslash
 should also be omitted, (e.g. "johndoe").

Zorn & Cobb Informational [Page 3] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

7. Success Packet

 The Success packet is identical in format to the standard CHAP
 Success packet.

8. Failure Packet

 The Failure packet is identical in format to the standard CHAP
 Failure packet.  There is, however, formatted text stored in the
 Message field which, contrary to the standard CHAP rules, affects the
 protocol.  The Message field format is:
       "E=eeeeeeeeee R=r C=cccccccccccccccc V=vvvvvvvvvv"
    where
       The "eeeeeeeeee" is the decimal error code (need not be 10
       digits) corresponding to one of those listed below, though
       implementations should deal with codes not on this list
       gracefully.
          646 ERROR_RESTRICTED_LOGON_HOURS
          647 ERROR_ACCT_DISABLED
          648 ERROR_PASSWD_EXPIRED
          649 ERROR_NO_DIALIN_PERMISSION
          691 ERROR_AUTHENTICATION_FAILURE
          709 ERROR_CHANGING_PASSWORD
       The "r" is a flag set to "1" if a retry is allowed, and "0" if
       not.  When the authenticator sets this flag to "1" it disables
       short timeouts, expecting the peer to prompt the user for new
       credentials and resubmit the response.
       The "cccccccccccccccc" is 16 hexadecimal digits representing an
       ASCII representation of a new challenge value.  This field is
       optional.  If it is not sent, the authenticator expects the
       resubmitted response to be calculated based on the previous
       challenge value plus decimal 23 in the first octet, i.e. the
       one immediately following the Value Size field.  Windows 95
       authenticators may send this field.  Windows NT authenticators
       do not, but may in the future.  Both systems implement peer
       support of this field.
       The "vvvvvvvvvv" is the decimal version code (need not be 10
       digits) indicating the MS-CHAP protocol version supported on
       the server.  Currently, this is interesting only in selecting a
       Change Password packet type.  If the field is not present the
       version should be assumed to be 1; since use of the version 1

Zorn & Cobb Informational [Page 4] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

       Change Password packet has been deprecated, this field SHOULD
       always contain a value greater than or equal to 2.
 Implementations should accept but ignore additional text they do not
 recognize.

9. Change Password Packet (version 1)

 The version 1 Change Password packet does not appear in standard
 CHAP.  It allows the peer to change the password on the account
 specified in the previous Response packet.  The version 1 Change
 Password packet should be sent only if the authenticator reports
 ERROR_PASSWD_EXPIRED (E=648) and V is either missing or equal to one
 in the Message field of the Failure packet.
 The use of the Change Password Packet (version 1) has been
 deprecated; the format of the packet is described here for
 informational purposes, but peers SHOULD NOT transmit it.
 The format of this packet is as follows:
     1 octet : Code (=5)
     1 octet : Identifier
     2 octets: Length (=72)
    16 octets: Encrypted LAN Manager Old password Hash
    16 octets: Encrypted LAN Manager New Password Hash
    16 octets: Encrypted Windows NT Old Password Hash
    16 octets: Encrypted Windows NT New Password Hash
     2 octets: Password Length
     2 octets: Flags
    Code
       5
    Identifier
       The Identifier field is one octet and aids in matching requests
       and replies.  The value is the Identifier of the received
       Failure packet to which this packet responds plus 1.
    Length
       72
    Encrypted LAN Manager New Password Hash
    Encrypted LAN Manager Old Password Hash
       These fields contain the LAN Manager password hash of the new
       and old passwords encrypted with the last received challenge
       value, as output by the routine LmEncryptedPasswordHash() (see
       section A.8, below).

Zorn & Cobb Informational [Page 5] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

    Encrypted Windows NT New Password Hash
    Encrypted Windows NT Old Password Hash
       These fields contain the Windows NT password hash of the new
       and old passwords encrypted with the last received challenge
       value, as output by the pseudo-code routine
       NtEncryptedPasswordHash() (see section A.10, below).
    Password Length
       The length in octets of the LAN Manager compatible form of the
       new password.  If this value is greater than or equal to zero
       and less than or equal to 14 it is assumed that the encrypted
       LAN Manager password hash fields are valid.  Otherwise, it is
       assumed these fields are not valid, in which case the Windows
       NT compatible passwords MUST be provided.
    Flags
       This field is two octets in length.  It is a bit field of
       option flags where 0 is the least significant bit of the 16-bit
       quantity:
          Bit 0
             If this bit is set (1), it indicates that the encrypted
             Windows NT hashed passwords are valid and should be used.
             If this bit is cleared (0), the Windows NT fields are not
             used and the LAN Manager fields must be provided.
          Bits 1-15
             Reserved, always clear (0).

10. Change Password Packet (version 2)

 The version 2 Change Password packet does not appear in standard
 CHAP.  It allows the peer to change the password on the account
 specified in the preceding Response packet.  The version 2 Change
 Password packet should be sent only if the authenticator reports
 ERROR_PASSWD_EXPIRED (E=648) and a version of 2 or greater in the
 Message field of the Failure packet.
 This packet type is supported by Windows NT 3.51, 4.0 and recent
 versions of Windows 95.  It is not supported by Windows NT 3.5 or
 early versions of Windows 95.
    The format of this packet is as follows:
         1 octet  : Code
         1 octet  : Identifier
         2 octets : Length
       516 octets : Password Encrypted with Old NT Hash

Zorn & Cobb Informational [Page 6] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

        16 octets : Old NT Hash Encrypted with New NT Hash
       516 octets : Password Encrypted with Old LM Hash
        16 octets : Old LM Hash Encrypted With New NT Hash
        24 octets : LAN Manager compatible challenge response
        24 octets : Windows NT compatible challenge response
         2-octet  : Flags
    Code
       6
    Identifier
       The Identifier field is one octet and aids in matching requests
       and replies.  The value is the Identifier of the received
       Failure packet to which this packet responds plus 1.
    Length
       1118
    Password Encrypted with Old NT Hash
       This field contains the PWBLOCK form of the new Windows NT
       password encrypted with the old Windows NT password hash, as
       output by the NewPasswordEncryptedWithOldNtPasswordHash()
       routine (see section A.11, below).
    Old NT Hash Encrypted with New NT Hash
       This field contains the old Windows NT password hash encrypted
       with the new Windows NT password hash, as output by the
       OldNtPasswordHashEncryptedWithNewNtPasswordHash() routine (see
       section A.14, below).
    Password Encrypted with Old LM Hash
       This field contains the PWBLOCK form of the new Windows NT
       password encrypted with the old LAN Manager password hash, as
       output by the NewPasswordEncryptedWithOldLmPasswordHash()
       routine described in section A.15, below.  Note, however, that
       the use of this field has been deprecated: peers SHOULD NOT
       generate it, and this field SHOULD be zero-filled.
    Old LM Hash Encrypted With New NT Hash
       This field contains the old LAN Manager password hash encrypted
       with the new Windows NT password hash, as output by the
       OldLmPasswordHashEncryptedWithNewNtPasswordHash() routine (see
       section A.16, below).  Note, however, that the use of this
       field has been deprecated: peers SHOULD NOT generate it, and
       this field SHOULD be zero-filled.

Zorn & Cobb Informational [Page 7] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

    LAN Manager compatible challenge response
    Windows NT compatible challenge response
       The challenge response field (as described in the Response
       packet description), but calculated on the new password and the
       same challenge used in the last response.  Note that use of the
       LAN Manager compatible challenge response has been deprecated;
       peers SHOULD NOT generate it, and the field SHOULD be zero-
       filled.
    Flags
       This field is two octets in length.  It is a bit field of
       option flags where 0 is the least significant bit of the 16-bit
       quantity.  The format of this field is illustrated in the
       following diagram:
                 1
       5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           | |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Bit 0
             The "use Windows NT compatible challenge response" flag
             as described in the Response packet.
          Bit 1
             Set (1) indicates that the "Password Encrypted with Old
             LM Hash" and "Old LM Hash Encrypted With New NT Hash"
             fields are valid and should be used.  Clear (0) indicates
             these fields are not valid.  This bit SHOULD always be
             clear (0).
          Bits 2-15
             Reserved, always clear (0).

11. Security Considerations

 As an implementation detail, the authenticator SHOULD limit the
 number of password retries allowed to make brute-force password
 guessing attacks more difficult.
 Because the challenge value is encrypted using the password hash to
 form the response and the challenge is transmitted in clear-text
 form, both passive known-plaintext and active chosen-plaintext
 attacks against the password hash are possible.  Suitable precautions
 (i.e., frequent password changes) SHOULD be taken in environments
 where eavesdropping is likely.

Zorn & Cobb Informational [Page 8] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

 The Change Password (version 1) packet is vulnerable to a passive
 eavesdropping attack which can easily reveal the new password hash.
 For this reason, it MUST NOT be sent if eavesdropping is possible.

12. References

 [1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
     1661, July 1994.
 [2] Simpson, W., "PPP Challenge Handshake Authentication Protocol
     (CHAP)", RFC 1994, August 1996.
 [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.
 [4] "Data Encryption Standard (DES)", Federal Information Processing
     Standard Publication 46-2, National Institute of Standards and
     Technology, December 1993.
 [5] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, April 1992.
 [6] 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
 [7] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
     Recomnendations for Security", RFC 1750, December 1994.
 [8] "The Unicode Standard, Version 2.0", The Unicode Consortium,
     Addison-Wesley, 1996. ISBN 0-201-48345-9.
 [9] "DES Modes of Operation", Federal Information Processing
     Standards Publication 81, National Institute of Standards and
     Technology, December 1980

13. Acknowledgements

 Thanks (in no particular order) to Jeff Haag (Jeff_Haag@3com.com),
 Bill Palter (palter@network-alchemy.com), Bruce Johnson
 (bjohnson@microsoft.com), Tony Bell (tonybe@microsoft.com), Benoit
 Martin (ehlija@vircom.com), and Joe Davies (josephd@microsoft.com)
 for useful suggestions and feedback.

Zorn & Cobb Informational [Page 9] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

14. 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

15. Authors' Addresses

 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
 Steve Cobb
 Microsoft Corporation
 One Microsoft Way
 Redmond, Washington 98052
 EMail: stevec@microsoft.com

Zorn & Cobb Informational [Page 10] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

Appendix A - Pseudocode

 The routines mentioned in the text are described in pseudocode below.

A.1 LmChallengeResponse()

 LmChallengeResponse(
 IN  8-octet          Challenge,
 IN  0-to-14-oem-char Password,
 OUT 24-octet         Response )
 {
    LmPasswordHash( Password, giving PasswordHash )
    ChallengeResponse( Challenge, PasswordHash, giving Response )
 }

A.2 LmPasswordHash()

 LmPasswordHash(
 IN  0-to-14-oem-char Password,
 OUT 16-octet         PasswordHash )
 {
    Set UcasePassword to the uppercased Password
    Zero pad UcasePassword to 14 characters
    DesHash( 1st 7-octets of UcasePassword,
             giving 1st 8-octets of PasswordHash )
    DesHash( 2nd 7-octets of UcasePassword,
             giving 2nd 8-octets of PasswordHash )
 }

A.3 DesHash()

 DesHash(
 IN  7-octet Clear,
 OUT 8-octet Cypher )
 {
    /*
     * Make Cypher an irreversibly encrypted form of Clear by
     * encrypting known text using Clear as the secret key.
     * The known text consists of the string
     *
     *              KGS!@#$%
     */
    Set StdText to "KGS!@#$%"

Zorn & Cobb Informational [Page 11] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

    DesEncrypt( StdText, Clear, giving Cypher )
 }

A.4 DesEncrypt()

 DesEncrypt(
 IN  8-octet Clear,
 IN  7-octet Key,
 OUT 8-octet Cypher )
 {
    /*
     * Use the DES encryption algorithm [4] in ECB mode [9]
     * to encrypt Clear into Cypher such that Cypher can
     * only be decrypted back to Clear by providing Key.
     * Note that the DES algorithm takes as input a 64-bit
     * stream where the 8th, 16th, 24th, etc.  bits are
     * parity bits ignored by the encrypting algorithm.
     * Unless you write your own DES to accept 56-bit input
     * without parity, you will need to insert the parity bits
     * yourself.
     */
 }

A.5 NtChallengeResponse()

 NtChallengeResponse(
 IN  8-octet               Challenge,
 IN  0-to-256-unicode-char Password,
 OUT 24-octet              Response )
 {
    NtPasswordHash( Password, giving PasswordHash )
    ChallengeResponse( Challenge, PasswordHash, giving Response )
 }

A.6 NtPasswordHash()

 NtPasswordHash(
 IN  0-to-256-unicode-char Password,
 OUT 16-octet              PasswordHash )
 {
    /*
     * Use the MD4 algorithm [5] to irreversibly hash Password
     * into PasswordHash.  Only the password is hashed without
     * including any terminating 0.
     */

Zorn & Cobb Informational [Page 12] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

 }

A.7 ChallengeResponse()

 ChallengeResponse(
 IN  8-octet  Challenge,
 IN  16-octet PasswordHash,
 OUT 24-octet Response )
 {
    Set ZPasswordHash to PasswordHash zero-padded to 21 octets
    DesEncrypt( Challenge,
                1st 7-octets of ZPasswordHash,
                giving 1st 8-octets of Response )
    DesEncrypt( Challenge,
                2nd 7-octets of ZPasswordHash,
                giving 2nd 8-octets of Response )
    DesEncrypt( Challenge,
                3rd 7-octets of ZPasswordHash,
                giving 3rd 8-octets of Response )
 }

A.8 LmEncryptedPasswordHash()

 LmEncryptedPasswordHash(
 IN  0-to-14-oem-char Password,
 IN  8-octet          KeyValue,
 OUT 16-octet         Cypher )
 {
    LmPasswordHash( Password, giving PasswordHash )
    PasswordHashEncryptedWithBlock( PasswordHash,
                                    KeyValue,
                                    giving Cypher )
 }

A.9 PasswordHashEncryptedWithBlock()

 PasswordHashEncryptedWithBlock(
 IN  16-octet PasswordHash,
 IN  8-octet  Block,
 OUT 16-octet Cypher )
 {

Zorn & Cobb Informational [Page 13] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

    DesEncrypt( 1st 8-octets PasswordHash,
                1st 7-octets Block,
                giving 1st 8-octets Cypher )
    DesEncrypt( 2nd 8-octets PasswordHash,
                1st 7-octets Block,
                giving 2nd 8-octets Cypher )
 }

A.10 NtEncryptedPasswordHash()

 NtEncryptedPasswordHash(  IN   0-to-14-oem-char  Password IN  8-octet
 Challenge OUT 16-octet         Cypher ) {
    NtPasswordHash( Password, giving PasswordHash )
    PasswordHashEncryptedWithBlock( PasswordHash,
                                    Challenge,
                                    giving Cypher )
 }

A.11 NewPasswordEncryptedWithOldNtPasswordHash()

 datatype-PWBLOCK
 {
    256-unicode-char Password
    4-octets         PasswordLength
 }
 NewPasswordEncryptedWithOldNtPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT datatype-PWBLOCK      EncryptedPwBlock )
 {
    NtPasswordHash( OldPassword, giving PasswordHash )
    EncryptPwBlockWithPasswordHash( NewPassword,
                                    PasswordHash,
                                    giving EncryptedPwBlock )
 }

A.12 EncryptPwBlockWithPasswordHash()

 EncryptPwBlockWithPasswordHash(
 IN  0-to-256-unicode-char Password,
 IN  16-octet              PasswordHash,

Zorn & Cobb Informational [Page 14] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

 OUT datatype-PWBLOCK      PwBlock )
 {
    Fill ClearPwBlock with random octet values
    PwSize = lstrlenW( Password ) * sizeof( unicode-char )
    PwOffset = sizeof( ClearPwBlock.Password ) - PwSize
    Move PwSize octets to (ClearPwBlock.Password + PwOffset ) from Password
    ClearPwBlock.PasswordLength = PwSize
    Rc4Encrypt( ClearPwBlock,
                sizeof( ClearPwBlock ),
                PasswordHash,
                sizeof( PasswordHash ),
                giving PwBlock )
 }

A.13 Rc4Encrypt()

 Rc4Encrypt(
 IN  x-octet Clear,
 IN  integer ClearLength,
 IN  y-octet Key,
 IN  integer KeyLength,
 OUT x-octet Cypher )
 {
    /*
     * Use the RC4 encryption algorithm [6] to encrypt Clear of
     * length ClearLength octets into a Cypher of the same length
     * such that the Cypher can only be decrypted back to Clear
     * by providing a Key of length KeyLength octets.
     */
 }

A.14 OldNtPasswordHashEncryptedWithNewNtPasswordHash()

 OldNtPasswordHashEncryptedWithNewNtPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT 16-octet              EncryptedPasswordHash )
 {
    NtPasswordHash( OldPassword, giving OldPasswordHash )
    NtPasswordHash( NewPassword, giving NewPasswordHash )
    NtPasswordHashEncryptedWithBlock( OldPasswordHash,
                                      NewPasswordHash,
                                      giving EncryptedPasswordHash )
 }

Zorn & Cobb Informational [Page 15] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

A.15 NewPasswordEncryptedWithOldLmPasswordHash()

 NewPasswordEncryptedWithOldLmPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT datatype-PWBLOCK      EncryptedPwBlock )
 {
    LmPasswordHash( OldPassword, giving PasswordHash )
    EncryptPwBlockWithPasswordHash( NewPassword, PasswordHash,
                                    giving EncryptedPwBlock )
 }

A.16 OldLmPasswordHashEncryptedWithNewNtPasswordHash()

 OldLmPasswordHashEncryptedWithNewNtPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT 16-octet              EncryptedPasswordHash )
 {
    LmPasswordHash( OldPassword, giving OldPasswordHash )
    NtPasswordHash( NewPassword, giving NewPasswordHash )
    NtPasswordHashEncryptedWithBlock( OldPasswordHash, NewPasswordHash,
                                    giving EncrytptedPasswordHash )
 }

A.17 NtPasswordHashEncryptedWithBlock()

 NtPasswordHashEncryptedWithBlock(
 IN  16-octet PasswordHash,
 IN  16-octet Block,
 OUT 16-octet Cypher )
 {
    DesEncrypt( 1st 8-octets PasswordHash,
                1st 7-octets Block,
                giving 1st 8-octets Cypher )
    DesEncrypt( 2nd 8-octets PasswordHash,
                2nd 7-octets Block,
                giving 2nd 8-octets Cypher )
 }

Zorn & Cobb Informational [Page 16] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

Appendix B - Examples

B.1 Negotiation Examples

 Here are some examples of typical negotiations.  The peer is on the
 left and the authenticator is on the right.
 The packet sequence ID is incremented on each authentication retry
 Response and on the change password response.  All cases where the
 packet sequence ID is updated are noted below.
 Response retry is never allowed after Change Password.  Change
 Password may occur after Response retry.  The implied challenge form
 is shown in the examples, though all cases of "first challenge+23"
 should be replaced by the "C=cccccccccccccccc" challenge if
 authenticator supplies it in the Failure packet.

B.1.1 Successful authentication

          <- Challenge
      Response ->
          <- Success

B.1.2 Failed authentication with no retry allowed

          <- Challenge
      Response ->
          <- Failure (E=691 R=0)

B.1.3 Successful authentication after retry

          <- Challenge
      Response ->
          <- Failure (E=691 R=1), disable short timeout
      Response (++ID) to first challenge+23 ->
          <- Success

B.1.4 Failed hack attack with 3 attempts allowed

          <- Challenge
      Response ->
          <- Failure (E=691 R=1), disable short timeout
      Response (++ID) to first challenge+23 ->
          <- Failure (E=691 R=1), disable short timeout
      Response (++ID) to first challenge+23+23 ->

Zorn & Cobb Informational [Page 17] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

          <- Failure (E=691 R=0)

B.1.5 Successful authentication with password change

          <- Challenge
      Response ->
          <- Failure (E=648 R=0 V=2), disable short timeout
      ChangePassword (++ID) to first challenge ->
          <- Success

B.1.6 Successful authentication with retry and password change

          <- Challenge
      Response ->
          <- Failure (E=691 R=1), disable short timeout
      Response (++ID) to first challenge+23 ->
          <- Failure (E=648 R=0 V=2), disable short timeout
      ChangePassword (++ID) to first challenge+23 ->
          <- Success

B.2 Hash Example

Intermediate values for password "MyPw".

 8-octet Challenge:
 10 2D B5 DF 08 5D 30 41
 0-to-256-unicode-char NtPassword:
 4D 00 79 00 50 00 77 00
 16-octet NtPasswordHash:
 FC 15 6A F7 ED CD 6C 0E DD E3 33 7D 42 7F 4E AC
 24-octet NtChallengeResponse:
 4E 9D 3C 8F 9C FD 38 5D 5B F4 D3 24 67 91 95 6C
 A4 C3 51 AB 40 9A 3D 61

B.3 Example of DES Key Generation

DES uses 56-bit keys, expanded to 64 bits by the insertion of parity bits. After the parity of the key has been fixed, every eighth bit is a parity bit and the number of bits that are set (1) in each octet is odd; i.e., odd parity. Note that many DES engines do not check parity, however, simply stripping the parity bits. The following example illustrates the values resulting from the use of the 16-octet NTPasswordHash shown in Appendix B.2 to generate a pair of DES keys

Zorn & Cobb Informational [Page 18] RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998

(e.g., for use in the NtPasswordHashEncryptedWithBlock() described in Appendix A.17).

 16-octet NtPasswordHash:
 FC 15 6A F7 ED CD 6C 0E DD E3 33 7D 42 7F 4E AC
 First "raw" DES key (initial 7 octets of password hash):
 FC 15 6A F7 ED CD 6C
 First parity-corrected DES key (eight octets):
 FD 0B 5B 5E 7F 6E 34 D9
 Second "raw" DES key (second 7 octets of password hash)
 0E DD E3 33 7D 42 7F
 Second parity-corrected DES key (eight octets):
 0E 6E 79 67 37 EA 08 FE

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Zorn & Cobb Informational [Page 20]

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