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Network Working Group S. CobbInformational Memo MicrosoftRevision 1.3 March 1997
Microsoft PPP CHAP Extensions
Status of this Memo
This document has no official Internet Engineering Task Force status. It is submitted as an example of one vendor's working solution to several authentication issues not yet standardized by the Point-to-Point Working Group.
The protocol described is implemented in Microsoft Windows NT 3.5 and 3.51 and in Microsoft Windows95. Differences between the platforms are noted in the text. This information, plus that in the references, is believed sufficient to implement an interoperating peer.
Distribution of this memo is unlimited.
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 1334 [2], which the reader should have at hand.
History
Rev 1.21: (Sect 6) Fix error in implicit challenge description Rev 1.22: (Sect 7) Fix error in sub-field table ordering Rev 1.3: (Sect 10) Added hash example section
Cobb [Page 1] Memo Microsoft PPP CHAP Extensions March 1997
Table Of Contents
1. Introduction................................................3 2. LCP Configuration...........................................4 3. Challenge Packet............................................4 4. Response Packet.............................................4 5. Success Packet..............................................8 6. Failure Packet..............................................8 7. Change Password Packet (version 1)..........................9 8. Change Password Packet (version 2).........................12 9. Negotiation Examples.......................................16 10. Hash Example...............................................16
REFERENCES.....................................................18 CHAIR'S ADDRESS................................................19 AUTHOR'S ADDRESS...............................................19
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1. Introduction
Microsoft created MS-CHAP to authenticate remote Windows workstations, providing the functionality to which LAN-based users are accustomed.
The closest fit available in standard PPP is CHAP which is primarily used for mutual secure authentication between WAN-aware routers. Unfortunately, CHAP is not widely used in support of remote workstations where providers commonly require an insecure text login session in place of PPP authentication protocols. To date, several remote workstation issues have not been adequately addressed in CHAP. MS-CHAP addresses these issues and also integrates the encryption and hashing algorithms used on Windows networks.
Where possible, MS-CHAP is consistent with standard CHAP, and the differences are easily modularized. Microsoft implements MS-CHAP as extensions to it's standard CHAP code base. Briefly, 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 Windows NT 3.5 and 3.51, Microsoft Windows95, and Microsoft LAN Manager 2.x networking products. The MS-CHAP format does not require the authenticator to store a clear or reversibly encrypted password.
* MS-CHAP provides an authenticator controlled authentication retry mechanism.
* MS-CHAP provides an authenticator controlled change password mechanism.
* MS-CHAP defines a set of reason-for-failure codes returned in the Failure packet Message field.
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2. 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. Non-MS-CHAP-aware implementations that correctly implement LCP Config-Rej have no problem dealing with this non-standard option.
Microsoft currently negotiates authentication only on the server->workstation configuration. Mutual authentication may be supported in the future.
3. 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. It is not necessary for peers to duplicate Microsoft's algorithm for selecting the 8-octet value, but the CHAP guidelines on randomness should be observed.
Microsoft authenticators do not currently provide information in the Name field. This may change in the future.
4. 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 pseudo-code routine LmChallengeResponse below. LAN Manager passwords are limited to 14 case-insensitive OEM characters.
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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 ) }
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 ) }
DesHash( IN 7-octet Clear, OUT 8-octet Cypher ) { Make Cypher an irreversibly encrypted form of Clear by encrypting known text [6] using Clear as the secret key, that is...
DesEncrypt( StdText, Clear, giving Cypher ) }
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DesEncrypt( IN 8-octet Clear, IN 7-octet Key, OUT 8-octet Cypher ) { Use the DES encryption algorithm [3] 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. }
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 ) }
The Windows NT compatible challenge response is an encoded function of the password and the received challenge as output by the NtChallengeResponse routine below. The Windows NT password is a string of 0 to (theoretically) 256 case-sensitive Unicode characters. Current versions of Windows NT limit passwords to 14 characters, mainly for compatibility reasons, though this may change in the future.
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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 ) }
NtPasswordHash( IN 0-to-256-unicode-char Password, OUT 16-octet PasswordHash ) { Use the MD4 algorithm [4] to irreversibly hash Password into PasswordHash. Only the password is hashed without including any terminating 0. }
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. The LAN Manager response need not be provided (set to 0's) if the implementation expects all user accounts to be stored only in fresh Windows NT account databases or ones where all uploaded passwords have been changed. However, doing so may sacrifice downward compatibility with non-Windows-NT servers.
If the flag is 0, the Windows NT response is ignored and the LAN Manager response is used. If the password is LAN Manager compatible, interoperability may be achieved without providing the Windows NT challenge response (set to 0's), and providing only the LAN Manager response. This is what Microsoft Windows95 does, though this may change in the future. Doing so may sacrifice interoperability with OEM-specific versions of Windows NT designed for maximum security in Windows-NT-only networks.
Implementors seeking the broadest possible interoperability are advised to supply both responses when the password is LAN Manager compatible. This is what Microsoft Windows NT does.
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The Name field identifies the authenticatee's user account name. The Windows NT domain name may prefix the user's account name in the typical Windows NT format, e.g. "redmond\stevec" where "redmond" is a Windows NT domain containing the user account "stevec". If a domain is not provided, the backslash should also be omitted, e.g. "stevec".
5. Success Packet
The Success packet is identical in format to the standard CHAP Success packet.
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 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 authenticator sets this flag to "1" it disables short timeouts, expecting the authenticatee to prompt the user for new credentials and resubmit the response.
The "cccccccccccccccc" is 16 hex digits representing an ASCII representation of a new challenge value. This field is optional. If it is not sent, 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. Windows95 authenticators may send this field. Windows NT authenticators do not, but may in the future. Both systems implement authenticatee support of this field.
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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 1.
Implementations should accept but ignore additional text they do not recognize.
7. Change Password Packet (version 1)
The version 1 Change Password packet does not appear in standard CHAP. It allows the authenticatee 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) in the Message field of the Failure packet.
This packet type is supported by Windows NT 3.5 and 3.51. It is not supported by Windows95, though this may change in the future. See also Change Password Packet (version 2).
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
Cobb [Page 9] Memo Microsoft PPP CHAP Extensions March 1997
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 an 8-octet key value [6], as output by the pseudo-code routine LmEncryptedPasswordHash below.
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 ) }
PasswordHashEncryptedWithBlock( IN 16-octet PasswordHash, IN 7-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, 1st 7-octets Block, giving 2nd 8-octets Cypher ) }
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 an 8-octet key value [6], as output by the pseudo-code routine NtEncryptedPasswordHash below.
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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 ) }
Password Length
The length in octets of the LAN Manager compatible form of the new password. If this value is 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
Bit field of option flags where 0 is the least significant bit of the 16-bit quantity:
0 : Set 1 indicates that the encrypted Windows NT hashed passwords are valid and should be used. If 0, the Windows NT fields are not used and the LAN Manager fields must be provided.
For the broadest possible interoperability, implementations are encouraged to provide both the Windows NT and LAN Manager fields when the password is LAN Manager compatible. This is what Windows NT does.
1-15 : Reserved, always set 0.
Cobb [Page 11] Memo Microsoft PPP CHAP Extensions March 1997
8. Change Password Packet (version 2)
The version 2 Change Password packet does not appear in standard CHAP. It allows the authenticatee to change the password on the account specified in the previous 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 more in the Message field of the Failure packet.
This packet type is supported by Windows NT 3.51. It is not supported by Windows NT 3.5 or Windows95, though the latter may change in the future. The version 2 change password packet type is preferable to the version 1 type and should be offered and accepted where possible.
The format of this packet is as follows:
1 octet : Code (=6) 1 octet : Identifier 2 octet : Length (=1070) 516 octets : Password Encrypted with Old NT Hash 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 below:
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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 ) }
EncryptPwBlockWithPasswordHash( IN 0-to-256-unicode-char Password, IN 16-octet PasswordHash, OUT datatype-PWBLOCK PwBlock ) { Fill ClearPwBlock with random octet values lstrcpyW( to ClearPwBlock.Password, from Password ) ClearPwBlock.PasswordLength = lstrlenW( Password )
Rc4Encrypt( ClearPwBlock, sizeof( ClearPwBlock ), PasswordHash, sizeof( PasswordHash ), giving PwBlock ) }
Rc4Encrypt( IN x-octet Clear, IN integer ClearLength, IN y-octet Key, IN integer KeyLength, OUT x-octet Cypher ) { Use the RC4 encryption algorithm [5] 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. }
Cobb [Page 13] Memo Microsoft PPP CHAP Extensions March 1997
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 below:
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 )
PasswordHashEncryptedWithBlock( OldPasswordHash, NewPasswordHash, giving EncrytptedPasswordHash ) }
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 below:
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 ) }
Cobb [Page 14] Memo Microsoft PPP CHAP Extensions March 1997
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 below:
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 )
PasswordHashEncryptedWithBlock( OldPasswordHash, NewPasswordHash, giving EncrytptedPasswordHash ) }
LAN Manager compatible challenge response Windows NT compatible challenge response
The challenge response fields as described in the Response packet description, but calculated on the new password and the same challenge used in the last response.
Flags
Bit field of option flags:
0 : The "use Windows NT compatible challenge response" flag as described in the Response packet.
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. Set 0 indicates these fields are not valid.
For the broadest possible interoperability, implementations are encouraged to provide both the Windows NT and LAN Manager fields when the password is LAN Manager compatible. This is what Windows NT does.
2-15 : Reserved, always set 0.
Cobb [Page 15] Memo Microsoft PPP CHAP Extensions March 1997
9. Negotiation Examples
Here are some examples of typical negotiations. The authenticatee 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 either 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.
Successful authentication
<- Challenge Response -> <- Success
Failed authentication with no retry allowed
<- Challenge Response -> <- Failure (E=691 R=0)
Successful authentication after retry
<- Challenge Response -> <- Failure (E=691 R=1), disable short timeout Response (++ID) to first challenge+23 -> <- Success
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 -> <- Failure (E=691 R=0)
Cobb [Page 16] Memo Microsoft PPP CHAP Extensions March 1997
Successful authentication with password change
<- Challenge Response -> <- Failure (E=648 R=0), disable short timeout ChangePassword (++ID) to first challenge -> <- Success
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), disable short timeout ChangePassword (++ID) to first challenge+23 -> <- Success
10. Hash Example
Intermediate values for password "MyPw".
8-octet Challenge: 10 2D B5 DF 08 5D 30 41
0-to-14-oem-char LmPassword: 4D 59 50 57
16-octet LmPasswordHash: 75 BA 30 19 8E 6D 19 75 AA D3 B4 35 B5 14 04 EE
24-octet LmChallengeResponse: 91 88 1D 01 52 AB 0C 33 C5 24 13 5E C2 4A 95 EE 64 E2 3C DC 2D 33 34 7D
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
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REFERENCES
[1] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC 1331, Daydreamer, May 1992
[2] LLoyd, B and Simpson, W., "PPP Authentication Protocols", RFC 1334, L&A and Daydreamer respectively, Octobet 1992
[3] "Data Encryption Standard (DES)" is Federal Information Processing Standard publication 46, National Institute of Standard and Techology.
[4] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, MIT Laboratory for Computer Science and RSA Data Security, Inc., April 1992.
[5] RC4 is an encryption standard available from RSA Data Security Inc.
[6] The 8-octet StdText string used in the LAN Manager compatible password hashing and the 8-octet KeyValue used in the Change Password (version 1) packet are not available for public distribution at this time. Contact the Microsoft Developer Relations group (at time of writing [email protected]) for details on obtaining these values. On this particular point the author can't help you.
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CHAIR'S ADDRESS
The working group can be contacted via the current chair:
Fred Baker Email: [email protected]
AUTHOR'S ADDRESS
The author is a developer in Microsoft's Windows NT Internetworking group, which monitors the [email protected] discussions. Questions can also be directed as below, where email is preferred.
Steve Cobb Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399
Email: [email protected]
The author maintains an informal mailing list of persons interested in MS-CHAP and other news regarding Windows NT support for PPP authentication protocols. Send email if interested.
Cobb [Page 19]
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