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Network Working Group S. Cobb
Informational Memo Microsoft
Revision 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
Cobb [Page 2]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 3]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 4]
Memo Microsoft PPP CHAP Extensions March 1997
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 )
}
Cobb [Page 5]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 6]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 7]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 8]
Memo Microsoft PPP CHAP Extensions March 1997
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.
Cobb [Page 10]
Memo Microsoft PPP CHAP Extensions March 1997
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:
Cobb [Page 12]
Memo Microsoft PPP CHAP Extensions March 1997
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
Cobb [Page 17]
Memo Microsoft PPP CHAP Extensions March 1997
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 dbeaver@microsoft.com) for
details on obtaining these values. On this particular point
the author can't help you.
Cobb [Page 18]
Memo Microsoft PPP CHAP Extensions March 1997
CHAIR'S ADDRESS
The working group can be contacted via the current chair:
Fred Baker
Email: fred@cisco.com
AUTHOR'S ADDRESS
The author is a developer in Microsoft's Windows NT
Internetworking group, which monitors the ietf-ppp@merit.edu
discussions. Questions can also be directed as below, where email
is preferred.
Steve Cobb
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Email: stevec@microsoft.com
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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