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".
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.
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.
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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.
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as
described in [2].
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.
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.
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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
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").
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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
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
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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.
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).
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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).
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
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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.
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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).
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.
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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.
[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
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.
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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
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
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
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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
Appendix A - Pseudocode
The routines mentioned in the text are described in pseudocode below.
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 using Clear as the secret key.
* The known text consists of the string
*
* KGS!@#$%
*/
Set StdText to "KGS!@#$%"
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DesEncrypt( StdText, Clear, giving Cypher )
}
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.
*/
}
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.
*/
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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
}
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 )
}
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.
*/
}
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.
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
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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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RFC 2433 Microsoft PPP CHAP Extensions Ocotober 1998
Full Copyright Statement
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