Microsoft PPP CHAP Extensions, Version 2
draft-ietf-pppext-mschap-v2-04

Network Working Group G. Zorn
Internet-Draft Microsoft Corporation
Category: Informational October 1999
<draft-ietf-pppext-mschap-v2-04.txt>
 Microsoft PPP CHAP Extensions, Version 2
Status of this Memo
 This document is an Internet-Draft and is in full conformance with
 all provisions of Section 10 of RFC2026 except that the right to
 produce derivative works is not granted.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF), its areas, and its working groups. Note that
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 Drafts.
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 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-mschap-v2-04.txt> and expires April 10, 2000. Please
 send comments to the PPP Extensions Working Group mailing list
 (ietf-ppp@merit.edu) or to the author (gwz@acm.org).
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 version two of Microsoft's PPP CHAP dialect
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 (MS-CHAP-V2). MS-CHAP-V2 is similar to, but incompatible with, MS-
 CHAP version one (MS-CHAP-V1, described in [9]). In particular,
 certain protocol fields have been deleted or reused but with
 different semantics. In addition, MS-CHAP-V2 features mutual
 authentication.
 The algorithms used in the generation of various MS-CHAP-V2
 protocol fields are described in section 8. Negotiation and hash
 generation examples are provided in section 9.
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 [3].
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 Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. LCP Configuration . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Response Packet . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Success Packet . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Failure Packet . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Change-Password Packet . . . . . . . . . . . . . . . . . . . . . 8
8. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. GenerateNTResponse() . . . . . . . . . . . . . . . . . . . . . 9
8.2. ChallengeHash() . . . . . . . . . . . . . . . . . . . . . . . . 10
8.3. NtPasswordHash() . . . . . . . . . . . . . . . . . . . . . . . 11
8.4. HashNtPasswordHash() . . . . . . . . . . . . . . . . . . . . . 11
8.5. ChallengeResponse() . . . . . . . . . . . . . . . . . . . . . . 11
8.6. DesEncrypt() . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.7. GenerateAuthenticatorResponse() . . . . . . . . . . . . . . . . 12
8.8. CheckAuthenticatorResponse() . . . . . . . . . . . . . . . . . 13
8.9. NewPasswordEncryptedWithOldNtPasswordHash() . . . . . . . . . . 14
8.10. EncryptPwBlockWithPasswordHash() . . . . . . . . . . . . . . . 14
8.11. Rc4Encrypt() . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.12. OldNtPasswordHashEncryptedWithNewNtPasswordHash() . . . . . . 15
8.13. NtPasswordHashEncryptedWithBlock() . . . . . . . . . . . . . . 16
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Negotiation Examples . . . . . . . . . . . . . . . . . . . . . 16
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9.1.1. Successful authentication . . . . . . . . . . . . . . . . . . 16
9.1.2. Authenticator authentication failure . . . . . . . . . . . . 16
9.1.3. Failed authentication with no retry allowed . . . . . . . . . 17
9.1.4. Successful authentication after retry . . . . . . . . . . . . 17
9.1.5. Failed hack attack with 3 attempts allowed . . . . . . . . . 17
9.1.6. Successful authentication with password change . . . . . . . 17
9.1.7. Successful authentication with retry and password change
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.2. Hash Example . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.3. Example of DES Key Generation . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
14. Expiration Date . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Where possible, MS-CHAP-V2 is consistent with both MS-CHAP-V1 and
standard CHAP. Briefly, the differences between MS-CHAP-V2 and MS-CHAP-
V1 are:
 * MS-CHAP-V2 is enabled by negotiating CHAP Algorithm 0x81 in LCP
 option 3, Authentication Protocol.
 * MS-CHAP-V2 provides mutual authentication between peers by
 piggybacking a peer challenge on the Response packet and an
 authenticator reponse on the Success packet.
 * The calculation of the "Windows NT compatible challenge
 response" sub-field in the Response packet has been changed
 to include the peer challenge and the user name.
 * In MS-CHAP-V1, the "LAN Manager compatible challenge response"
 sub-field was always sent in the Response packet. This field
 has been replaced in MS-CHAP-V2 by the Peer-Challenge field.
 * The format of the Message field in the Failure packet has
 been changed.
 * The Change Password (version 1) and Change Password (version 2)
 packets are no longer supported. They have been replaced with a
 single Change-Password packet.
2. LCP Configuration
The LCP configuration for MS-CHAP-V2 is identical to that for standard
CHAP, except that the Algorithm field has value 0x81, rather than the
MD5 value 0x05. PPP implementations which do not support MS-CHAP-V2,
but correctly implement LCP Config-Rej, should have no problem dealing
with this non-standard option.
3. Challenge Packet
The MS-CHAP-V2 Challenge packet is identical in format to the standard
CHAP Challenge packet.
MS-CHAP-V2 authenticators send an 16-octet challenge Value field. Peers
need not duplicate Microsoft's algorithm for selecting the 16-octet
value, but the standard guidelines on randomness [1,2,7] SHOULD be
observed.
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Microsoft authenticators do not currently provide information in the
Name field. This may change in the future.
4. Response Packet
The MS-CHAP-V2 Response packet is identical in format to the standard
CHAP Response packet. However, the Value field is sub-formatted
differently as follows:
 16 octets: Peer-Challenge
 8 octets: Reserved, must be zero
 24 octets: NT-Response
 1 octet : Flags
The Peer-Challenge field is a 16-octet random number. As the name
implies, it is generated by the peer and is used in the calculation of
the NT-Response field, below. Peers need not duplicate Microsoft's
algorithm for selecting the 16-octet value, but the standard guidelines
on randomness [1,2,7] SHOULD be observed.
The NT-Response field is an encoded function of the password, the user
name, the contents of the Peer-Challenge field and the received
challenge as output by the routine GenerateNTResponse() (see section
8.1, 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. When computing
the NT-Response field contents, only the user name is used, without any
associated Windows NT domain name. This is true regardless of whether a
Windows NT domain name is present in the Name field (see below).
The Flag field is reserved for future use and MUST be zero.
The Name field is a string of 0 to (theoretically) 256 case-sensitive
ASCII characters which 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 "johndoe"). If a domain is not provided, the backslash should
also be omitted, (e.g. "johndoe").
5. Success Packet
The Success packet is identical in format to the standard CHAP Success
packet. However, the Message field contains a 42-octet authenticator
response string and a printable message. The format of the message
field is illustrated below.
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 "S=<auth_string> M=<message>"
The <auth_string> quantity is a 20 octet number encoded in ASCII as 40
hexadecimal digits. The hexadecimal digits A-F (if present) MUST be
uppercase. This number is derived from the challenge from the Challenge
packet, the Peer-Challenge and NT-Response fields from the Response
packet, and the peer password as output by the routine
GenerateAuthenticatorResponse() (see section 8.7, below). The
authenticating peer MUST verify the authenticator response when a
Success packet is received. The method for verifying the authenticator
is described in section 8.8, below. If the authenticator response is
either missing or incorrect, the peer MUST end the session.
The <message> quantity is human-readable text in the appropriate charset
and language [12].
6. 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, does affect the operation of
the protocol. The Message field format is:
 "E=eeeeeeeeee R=r C=cccccccccccccccccccccccccccccccc V=vvvvvvvvvv
M=<msg>"
 where
 The "eeeeeeeeee" is the ASCII representation of a 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 an ASCII 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 "cccccccccccccccccccccccccccccccc" is the ASCII representation
 of a hexadecimal challenge value. This field MUST be exactly 32
 octets long and MUST be present.
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 The "vvvvvvvvvv" is the ASCII representation of a decimal version
 code (need not be 10 digits) indicating the password changing
 protocol version supported on the server. For MS-CHAP-V2, this
 value SHOULD always be 3.
 <msg> is human-readable text in the appropriate charset and
 language [12].
7. Change-Password Packet
The Change-Password packet does not appear in either standard CHAP or
MS-CHAP-V1. It allows the peer to change the password on the account
specified in the preceding Response packet. The Change-Password packet
should be sent only if the authenticator reports ERROR_PASSWD_EXPIRED
(E=648) in the Message field of the Failure packet.
This packet type is supported by recent versions of Windows NT 4.0,
Windows 95 and Windows 98. It is not supported by Windows NT 3.5,
Windows NT 3.51, or early versions of Windows NT 4.0, Windows 95 and
Windows 98.
 The format of this packet is as follows:
 1 octet : Code
 1 octet : Identifier
 2 octets : Length
 516 octets : Encrypted-Password
 16 octets : Encrypted-Hash
 16 octets : Peer-Challenge
 8 octets : Reserved
 24 octets : NT-Response
 2-octet : Flags
 Code
 7
 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
 586
 Encrypted-Password
 This field contains the PWBLOCK form of the new Windows NT
 password encrypted with the old Windows NT password hash, as
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 output by the NewPasswordEncryptedWithOldNtPasswordHash() routine
 (see section 8.9, below).
 Encrypted-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 8.12, below).
 Peer-Challenge
 A 16-octet random quantity, as described in the Response packet
 description.
 Reserved
 8 octets, must be zero.
 NT-Response
 The NT-Response field (as described in the Response packet
 description), but calculated on the new password and the challenge
 received in the Failure packet.
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Bits 0-15
 Reserved, always clear (0).
8. Pseudocode
The routines mentioned in the text above are described in pseudocode in
the following sections.
8.1. GenerateNTResponse()
 GenerateNTResponse(
 IN 16-octet AuthenticatorChallenge,
 IN 16-octet PeerChallenge,
 IN 0-to-256-char UserName,
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 IN 0-to-256-unicode-char Password,
 OUT 24-octet Response )
 {
 8-octet Challenge
 16-octet PasswordHash
 ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName,
 giving Challenge)
 NtPasswordHash( Password, giving PasswordHash )
 ChallengeResponse( Challenge, PasswordHash, giving Response )
 }
8.2. ChallengeHash()
 ChallengeHash(
 IN 16-octet PeerChallenge,
 IN 16-octet AuthenticatorChallenge,
 IN 0-to-256-char UserName,
 OUT 8-octet Challenge
 {
 /*
 * SHAInit(), SHAUpdate() and SHAFinal() functions are an
 * implementation of Secure Hash Algorithm (SHA-1) [11]. These are
 * available in public domain or can be licensed from
 * RSA Data Security, Inc.
 */
 SHAInit(Context)
 SHAUpdate(Context, PeerChallenge, 16)
 SHAUpdate(Context, AuthenticatorChallenge, 16)
 /*
 * Only the user name (as presented by the peer and
 * excluding any prepended domain name)
 * is used as input to SHAUpdate().
 */
 SHAUpdate(Context, UserName, strlen(Username))
 SHAFinal(Context, Digest)
 memcpy(Challenge, Digest, 8)
 }
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8.3. 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.
 */
 }
8.4. HashNtPasswordHash()
 HashNtPasswordHash(
 IN 16-octet PasswordHash,
 OUT 16-octet PasswordHashHash )
 {
 /*
 * Use the MD4 algorithm [5] to irreversibly hash
 * PasswordHash into PasswordHashHash.
 */
 }
8.5. 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 )
 }
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8.6. DesEncrypt()
 DesEncrypt(
 IN 8-octet Clear,
 IN 7-octet Key,
 OUT 8-octet Cypher )
 {
 /*
 * Use the DES encryption algorithm [4] in ECB mode [10]
 * 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.
 */
 }
8.7. GenerateAuthenticatorResponse()
 GenerateAuthenticatorResponse(
 IN 0-to-256-unicode-char Password,
 IN 24-octet NT-Response,
 IN 16-octet PeerChallenge,
 IN 16-octet AuthenticatorChallenge,
 IN 0-to-256-char UserName,
 OUT 42-octet AuthenticatorResponse )
 {
 16-octet PasswordHash
 16-octet PasswordHashHash
 8-octet Challenge
 /*
 * "Magic" constants used in response generation
 */
 Magic1[39] =
 {0x4D, 0x61, 0x67, 0x69, 0x63, 0x20, 0x73, 0x65, 0x72, 0x76,
 0x65, 0x72, 0x20, 0x74, 0x6F, 0x20, 0x63, 0x6C, 0x69, 0x65,
 0x6E, 0x74, 0x20, 0x73, 0x69, 0x67, 0x6E, 0x69, 0x6E, 0x67,
 0x20, 0x63, 0x6F, 0x6E, 0x73, 0x74, 0x61, 0x6E, 0x74};
 Magic2[41] =
 {0x50, 0x61, 0x64, 0x20, 0x74, 0x6F, 0x20, 0x6D, 0x61, 0x6B,
 0x65, 0x20, 0x69, 0x74, 0x20, 0x64, 0x6F, 0x20, 0x6D, 0x6F,
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 0x72, 0x65, 0x20, 0x74, 0x68, 0x61, 0x6E, 0x20, 0x6F, 0x6E,
 0x65, 0x20, 0x69, 0x74, 0x65, 0x72, 0x61, 0x74, 0x69, 0x6F,
 0x6E};
 /*
 * Hash the password with MD4
 */
 NtPasswordHash( Password, giving PasswordHash )
 /*
 * Now hash the hash
 */
 HashNtPasswordHash( PasswordHash, giving PasswordHashHash)
 SHAInit(Context)
 SHAUpdate(Context, PasswordHashHash, 16)
 SHAUpdate(Context, NTResponse, 24)
 SHAUpdate(Context, Magic1, 39)
 SHAFinal(Context, Digest)
 ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName,
 giving Challenge)
 SHAInit(Context)
 SHAUpdate(Context, Digest, 20)
 SHAUpdate(Context, Challenge, 8)
 SHAUpdate(Context, Magic2, 41)
 SHAFinal(Context, Digest)
 /*
 * Encode the value of 'Digest' as "S=" followed by
 * 40 ASCII hexadecimal digits and return it in
 * AuthenticatorResponse.
 * For example,
 * "S=0123456789ABCDEF0123456789ABCDEF01234567"
 */
 }
8.8. CheckAuthenticatorResponse()
 CheckAuthenticatorResponse(
 IN 0-to-256-unicode-char Password,
 IN 24-octet NtResponse,
 IN 16-octet PeerChallenge,
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 IN 16-octet AuthenticatorChallenge,
 IN 0-to-256-char UserName,
 IN 42-octet ReceivedResponse,
 OUT Boolean ResponseOK )
 {
 20-octet MyResponse
 set ResponseOK = FALSE
 GenerateAuthenticatorResponse( Password, NtResponse, PeerChallenge,
 AuthenticatorChallenge, UserName,
 giving MyResponse)
 if (MyResponse = ReceivedResponse) then set ResponseOK = TRUE
 return ResponseOK
 }
8.9. 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 )
 }
8.10. EncryptPwBlockWithPasswordHash()
 EncryptPwBlockWithPasswordHash(
 IN 0-to-256-unicode-char Password,
 IN 16-octet PasswordHash,
 OUT datatype-PWBLOCK PwBlock )
 {
 Fill ClearPwBlock with random octet values
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 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 )
 }
8.11. 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.
 */
 }
8.12. 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 )
 }
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8.13. 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 )
 }
9. Examples
The following sections include protocol negotiation and hash generation
examples.
9.1. 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.
9.1.1. Successful authentication
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- Success/Authenticator Response
(Authenticator Response verification succeeds, call continues)
9.1.2. Authenticator authentication failure
 <- Authenticator Challenge
 Peer Response/Challenge ->
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 <- Success/Authenticator Response
(Authenticator Response verification fails, peer disconnects)
9.1.3. Failed authentication with no retry allowed
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- Failure (E=691 R=0)
(Authenticator disconnects)
9.1.4. Successful authentication after retry
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- Failure (E=691 R=1), disable short timeout
 Response (++ID) to challenge in failure message ->
 <- Success/Authenticator Response
(Authenticator Response verification succeeds, call continues)
9.1.5. Failed hack attack with 3 attempts allowed
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- Failure (E=691 R=1), disable short timeout
 Response (++ID) to challenge in Failure message ->
 <- Failure (E=691 R=1), disable short timeout
 Response (++ID) to challenge in Failure message ->
 <- Failure (E=691 R=0)
9.1.6. Successful authentication with password change
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- Failure (E=648 R=0 V=3), disable short timeout
 ChangePassword (++ID) to challenge in Failure message ->
 <- Success/Authenticator Response
(Authenticator Response verification succeeds, call continues)
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9.1.7. Successful authentication with retry and password change
 <- Authenticator Challenge
 Peer Response/Challenge ->
 <- 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/Authenticator Response
(Authenticator Response verification succeeds, call continues)
9.2. Hash Example
Intermediate values for user name "User" and password "clientPass". All
numeric values are hexadecimal.
 0-to-256-char UserName:
 55 73 65 72
 0-to-256-unicode-char Password:
 63 00 6C 00 69 00 65 00 6E 00 74 00 50 00 61 00 73 00 73 00
 16-octet AuthenticatorChallenge:
 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C 60 21 32 26 26 28
 16-octet PeerChallenge:
 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E
 8-octet Challenge:
 D0 2E 43 86 BC E9 12 26
 16-octet PasswordHash:
 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE
 24 octet 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
 16-octet PasswordHashHash:
 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F
 42-octet AuthenticatorResponse:
 "S=407A5589115FD0D6209F510FE9C04566932CDA56"
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9.3. 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 password "MyPw" to
generate a pair of DES keys (e.g., for use in the
NtPasswordHashEncryptedWithBlock() described in section 8.13).
 0-to-256-unicode-char Password:
 4D 79 50 77
 16-octet PasswordHash:
 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
10. 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.
11. 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
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[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., et. al., "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] Zorn, G. and Cobb, S., "Microsoft PPP CHAP Extensions", RFC 2433,
 October 1998
[10] "DES Modes of Operation", Federal Information Processing Standards
 Publication 81, National Institute of Standards and Technology,
 December 1980
[11] "Secure Hash Standard", Federal Information Processing Standards
 Publication 180-1, National Institute of Standards and Technology,
 April 1995
[12] Zorn, G., "PPP LCP Internationalization Configuration Option", RFC
 2484, January 1999
12. Acknowledgements
Thanks (in no particular order) to Bruce Johnson, Tony Bell, Paul Leach,
Terence Spies, Dan Simon, Narendra Gidwani, Gurdeep Singh Pall, Jody
Terrill, Brad Robel-Forrest, and Joe Davies for useful suggestions and
feedback.
13. Author's Address
Questions about this memo can also be directed to:
 Glen Zorn
 Microsoft Corporation
 One Microsoft Way
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 Redmond, Washington 98052
 Phone: +1 425 703 1559
 FAX: +1 425 936 7329
 EMail: gwz@acm.org
14. Expiration Date
This memo is filed as <draft-ietf-pppext-mschap-v2-04.txt> and expires
on April 10, 2000.
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