PPPEXT Working Group Ryan Hurst
INTERNET-DRAFT Ashwin Palekar
Category: Informational Microsoft Corporation
Expires: December 25, 2007 12 June 2007
Microsoft EAP CHAP Extensions
draft-kamath-pppext-eap-mschapv2-02.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document defines the Microsoft EAP CHAP Extensions Protocol,
Version 2, which encapsulates the MS-CHAPv2 protocol defined in RFC
2759, within EAP as defined in RFC 3748.
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Table of Contents
1. Introduction .......................................... 3
1.1 Requirements language ........................... 3
1.2 Terminology ..................................... 3
2. EAP MS-CHAP-v2 Packet Format .......................... 4
2.1. Challenge packet ................................ 5
2.2. Response packet ................................. 7
2.3. Success Request packet .......................... 9
2.4. Success Response packet ......................... 11
2.5. Failure Request packet .......................... 12
2.6. Failure Response packet ......................... 14
2.7. Change-Password packet .......................... 15
2.8. Alternative failure behavior .................... 17
2.9. Known bugs ...................................... 18
3. Security claims .......................................... 18
4. References ............................................... 19
4.1 Normative references ............................ 19
4.2 Informative references .......................... 20
Appendix A - Examples ........................................ 22
Acknowledgments .............................................. 25
Author Addresses ............................................. 25
Full Copyright Statement ..................................... 25
Intellectual Property ........................................ 26
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1. Introduction
The Extensible Authentication Protocol (EAP), described in [RFC3748],
provides a standard mechanism for support of multiple authentication
methods. Through the use of EAP, support for a number of
authentication schemes may be added, including smart cards, Kerberos,
Public Key, One Time Passwords, and others.
This document defines the Microsoft EAP CHAP Extensions Protocol,
Version 2, which encapsulates the MS-CHAP-v2 protocol, defined in
[RFC2759], within EAP. As with MS-CHAP-v2, EAP-MSCHAPv2 supports
mutual authentication and key derivation. The way EAP-MSCHAPv2
derived keys are used with the Microsoft Point to Point Encryption
(MPPE) cipher is described in [RFC3079].
EAP MS-CHAP-V2 provides mutual authentication between peers by
piggybacking a peer challenge on the Response packet and an
authenticator response on the Success packet.
1.1. Requirements language
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
1.2. Terminology
This document frequently uses the following terms:
Authenticator
The end of the link requiring the authentication.
Peer The other end of the point-to-point link; the end which is being
authenticated by the authenticator.
silently discard
This means the implementation discards the packet without further
processing. The implementation SHOULD provide the capability of
logging the error, including the contents of the silently discarded
packet, and SHOULD record the event in a statistics counter.
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2. EAP MS-CHAP-v2 Packet Format
A summary of the EAP MS-CHAP-V2 packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The Identifier field is one octet and aids in matching responses with
requests.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
CHAPv2-ID, MS-Length and Data fields. Octets outside the range of
the Length field should be treated as Data Link Layer padding and
should be ignored on reception.
Type
26 - EAP MS-CHAP-V2
OpCode
The OpCode field is one octet and identifies the type of EAP MS-CHAP-
v2 packet. OpCodes are assigned as follows:
1 Challenge
2 Response
3 Success
4 Failure
7 Change-Password
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MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Data
The format of the Data field is determined by the OpCode field.
2.1. Challenge packet
The Challenge packet is used to begin the EAP MS-CHAP-V2 protocol.
The authenticator MUST transmit an EAP Request packet with Type=26,
and the OpCode field set to 1 (Challenge). The format of the EAP MS-
CHAP-v2 Challenge packet is shown below. The fields are transmitted
from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Value-Size | Challenge...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
Identifier
The Identifier field is one octet. The Identifier field MUST be the
same if a Request packet is retransmitted due to a timeout while
waiting for a Response. Any new (non-retransmission) Requests MUST
modify the Identifier field. If a peer receives a duplicate Request
for which it has already sent a Response, it MUST resend it's
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Response. If a peer receives a duplicate Request before it has sent
a Response to the initial Request (i.e. it's waiting for user input),
it MUST silently discard the duplicate Request.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
CHAPv2-ID, MS-Length, Value-Size, Challenge, and Name fields. Octets
outside the range of the Length field should be treated as Data Link
Layer padding and should be ignored on reception.
Type
26 - EAP MS-CHAP-V2
OpCode
1 - Challenge
MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Value-Size
This field is one octet and indicates the length of the Challenge
field. Since EAP MS-CHAPv2 utilizes a 16 octet Challenge field, it
is set to 0x10 (16 decimal).
Challenge
The Challenge field is 16 octets. The most significant octet is
transmitted first. The Challenge MUST be changed each time a
Challenge is sent.
Name
The Name field is one or more octets representing the identification
of the system transmitting the packet. There are no limitations on
the content of this field. The Name should not be NUL or CR/LF
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terminated. The size of the Name field is equal to Length - Value-
Size - 10.
2.2. Response packet
The format of the EAP MS-CHAP-v2 Response packet is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Value-Size | Response...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 - Response
Identifier
The Identifier field is one octet and contains the value included in
the EAP Request to which it responds.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
CHAPv2-ID, MS-Length, Value-Size, Response, and Name fields. Octets
outside the range of the Length field should be treated as Data Link
Layer padding and should be ignored on reception.
Type
26 - EAP MS-CHAP-V2
OpCode
2 - Response
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MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Value-Size
This field is one octet and indicates the length of the Response
field. It is set to 0x31 (Decimal 49).
Response
The Response field is 49 octets. The most significant octet is
transmitted first. It is sub-formatted 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 [RFC1750] SHOULD be observed.
The NT-Response field is an encoded function of the password, the
Name field of the Response packet, the contents of the Peer-Challenge
field and the received Challenge as output by the routine
GenerateNTResponse() defined in [RFC2759], Section 8.1.
The Windows NT password is a string of 0 to (theoretically) 256 case-
sensitive Unicode [UNICODE] 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.
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Whenever a Response packet is received, the authenticator compares
the Response Value with its own calculation of the expected value. If
the values match, then the authenticator MUST send a Success-Request
packet, as described in Section 2.3. If the values do not match, and
if the error is retryable, then a Failure-Request packet MUST be sent
as described in Section 2.5. If the values do not match, and the
error is not retryable, then a Failure-Request packet (described in
Section 2.5) SHOULD be sent, or alternatively, the authentication MAY
be terminated (as described in Section 2.8) such as by sending an
EAP Failure.
Name
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). The Name SHOULD NOT be NUL or CR/LF
terminated. The size of the Name field is determined from the Length
- Value-Size - 10.
2.3. Success Request packet
If the value received in the Response field of the EAP MS-CHAP-V2
Response packet is equal to the expected value, then the
implementation MUST transmit an EAP MS-CHAP-V2 Request packet with
the OpCode field set to 3 (Success).
The format of the EAP MS-CHAP-v2 Success Request packet is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Message...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
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Identifier
The Identifier field is one octet. The Identifier field MUST be the
same if a Request packet is retransmitted due to a timeout while
waiting for a Response. Any new (non-retransmission) Requests MUST
modify the Identifier field. If a peer receives a duplicate Request
for which it has already sent a Response, it MUST resend it's
Response. If a peer receives a duplicate Request before it has sent
a Response to the initial Request (i.e. it's waiting for user input),
it MUST silently discard the duplicate Request.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
CHAPv2-ID, MS-Length, and Message fields. Octets outside the range
of the Length field should be treated as Data Link Layer padding and
should be ignored on reception.
Type
26 - EAP MS-CHAP-V2
OpCode
3 - Success
MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Message
The Message field contains a 42-octet authenticator response string
and a printable message. The format of the message field is
illustrated below.
"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
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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() defined in [RFC2759], Section 8.7.
The authenticating peer MUST verify the authenticator response when a
Success packet is received. The method for verifying the
authenticator is described in [RFC2759], section 8.8. If the
authenticator response is either missing or incorrect, the peer MUST
end the session without sending a response.
The <message> quantity is human-readable text in the appropriate
charset and language [RFC2484].
2.4. Success Response packet
In the peer successfully validates the EAP MS-CHAP-V2 Success Request
packet sent by the authenticator, then it MUST respond with an EAP
MS-CHAP-V2 Success Response packet with the OpCode field set to 3
(Success).
The format of the EAP MS-CHAP-v2 Success Response packet is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 - Response
Identifier
The Identifier field is one octet and contains the value included in
the EAP Request to which it responds.
Length
6
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Type
26 - EAP MS-CHAP-V2
OpCode
3 - Success
2.5. Failure Request packet
If the Value received in a Response is not equal to the expected
value, and the error is retryable, then the implementation MUST
transmit an EAP MS-CHAP-v2 Request packet with the OpCode field set
to 4 (Failure). If the error is not retryable, then the
implementation SHOULD transmit an EAP MS-CHAP-v2 Failure Request
packet, or it MAY terminate the authentication (e.g. send an EAP
Failure packet). The former approach is preferable, since this
enables the cause of the error to be communicated.
The format of the EAP MS-CHAP-v2 Failure Request packet is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Message...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
Identifier
The Identifier field is one octet. The Identifier field MUST be the
same if a Request packet is retransmitted due to a timeout while
waiting for a Response. Any new (non-retransmission) Requests MUST
modify the Identifier field. If a peer receives a duplicate Request
for which it has already sent a Response, it MUST resend it's
Response. If a peer receives a duplicate Request before it has sent
a Response to the initial Request (i.e. it's waiting for user input),
it MUST silently discard the duplicate Request.
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Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
CHAPv2-ID, MS-Length, and Message fields. Octets outside the range
of the Length field should be treated as Data Link Layer padding and
should be ignored on reception.
Type
26 - EAP MS-CHAP-V2
OpCode
4 - Failure
MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Message
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
corresponding to one of those listed below, though implementations
should deal with codes not on this list gracefully. The error code
need not be 10 digits long.
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 single character ASCII flag set to '1' if a retry is
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allowed, and '0' if not. Typically, errors 646, 647, and 649 are
non-retryable (R=0). 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.
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 EAP MS-CHAP-V2, this
value MUSTalways be 3.
<msg> is human-readable text in the appropriate charset and language
[RFC2484].
2.6. Failure Response packet
When the peer receives a Failure Request packet that is retryable
(R=1), the authentication MAY be retried. For example, a new
Response packet, or Change Password packet MAY be sent. In these
cases a Failure Response packet is not sent.
However, if the EAP MS-CHAPv2 Failure Request is non-retryable (R=0),
then the peer SHOULD transmit an EAP MS-CHAP-v2 Response packet with
the OpCode field set to 4 (Failure). The format of the EAP MS-CHAP-v2
Failure Response packet is shown below. The fields are transmitted
from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 - Response
Identifier
The Identifier field is one octet and contains the value included in
the EAP Request to which it responds.
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Length
6
Type
26 - EAP MS-CHAP-V2
OpCode
4 - Failure
2.7. 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.
The format of the EAP MS-CHAP-v2 Change Password packet is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | OpCode | MS-CHAPv2-ID | MS-Length...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MS-Length | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2 - Response
Identifier
The Identifier field is one octet and aids in matching responses with
requests. The value is the Identifier of the received Failure packet
to which this packet responds.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, OpCode, MS-
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CHAPv2-ID, MS-Length and Data fields. Octets outside the range of
the Length field should be treated as Data Link Layer padding and
should be ignored on reception. For the Change Password packet, the
length = 591.
Type
26 - EAP MS-CHAP-V2
OpCode
7 - Change Password
MS-CHAPv2-ID
The MS-CHAPv2-ID field is one octet and aids in matching MSCHAP-v2
responses with requests. Typically, the MS-CHAPv2-ID field is the
same as the Identifier field.
MS-Length
The MS-Length field is two octets and MUST be set to the value of the
Length field minus 5.
Data
The Data field is 582 octets in length, and is subdivided as follows:
516 octets : Encrypted-Password
16 octets : Encrypted-Hash
16 octets : Peer-Challenge
8 octets : Reserved
24 octets : NT-Response
2-octet : Flags
Encrypted-Password
The Encrypted-Password field is 516 octets in length, and contains
the PWBLOCK form of the new Windows NT password encrypted with the
old Windows NT password hash, as output by the
NewPasswordEncryptedWithOldNtPasswordHash() routine defined in
[RFC2759], Section 8.9.
Encrypted-Hash
The Encrypted-Hash field is 16 octets in length and contains the old
Windows NT password hash encrypted with the new Windows NT password
hash, as output by the
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OldNtPasswordHashEncryptedWithNewNtPasswordHash() routine, defined in
[RFC2759], Section 8.12.
Peer-Challenge
The Peer-Challenge field is 16 octets in length, and contains a
16-octet random quantity, as described in the Response packet
description.
Reserved
8 octets, must be zero.
NT-Response
The NT-Response field is 24 octets in length and is as described in
the Response packet description. However it is calculated on the new
password and the challenge received in the Failure packet.
Flags
The Flags 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).
2.8. Alternative failure behavior
Rather than sending a Failure Request as described in Section 2.5, if
the error is non-retryable (e.g. R=0), or if the maximum number of
retries has been exhausted, then the Authenticator MAY terminate the
authentication conversation. Where EAP MS-CHAP-V2 is running
standalone (e.g. without PEAP), this will result in transmission of
an EAP Failure message to the authenticator. Since EAP Failure
packets do not carry additional data, no error message may be
transmitted to the peer.
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2.9. Known bugs
In Windows XP SP1, Failure Request packets are only sent where the
error is retryable (R=1). Rather than sending a Failure Request with
a non-retryable error (R=0), a Windows XP SP1 authenticator will
terminate authentication. This is undesirable, because it prevents
non-retryable error messages from being received by the peer. A
Windows XP SP1 host, on receiving a Failure Request packet with a
non-retryable error (R=0), will silently discard the packet.
Since a Windows XP SP1 peer will respond to a retryable (R=1) Failure
Request by retrying authentication (such as by sending a Response or
Change-Password packet), and non-retryable (R=0) Failure Requests are
silently discarded, Windows XP SP1 peers do not send Failure Response
packets. If a Windows XP SP1 authenticator receives a Failure
Response packet, it will be silently discarded.
3. Security Claims
EAP security claims are defined in [RFC3748] Section 7.2.1. Using
the terms defined there, the security properties of the Microsoft EAP
MS-CHAP-v2 protocol are as follows:
Auth. mechanism: Password
Ciphersuite negotiation: No
Mutual authentication: Yes
Integrity protection: Yes
Replay protection: Yes
Confidentiality: No
Key derivation: Yes
Key strength: Depends on password policy
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: Depends on password policy
Fragmentation: No
Channel binding: No
The Microsoft EAP MS-CHAP-v2 protocol is based on MS-CHAP-v2 as
defined in [RFC2759]. MS-CHAP-v2 is a password-based authentication
method that supports mutual authentication. While backward
compatibility with MS-CHAP-v1 is supported, this does not really
constitute a protected ciphersuite negotiation, since the
cryptographic algorithms are largely fixed.
Integrity and replay protection are supported. As described in
Section 2.2, the NT-Response field is an encoded function of the
password, the Name field of the Response packet, the contents of the
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Peer-Challenge field and the received Challenge. The inclusion of
both the Peer-Challenge and received challenge provides replay
protection. Fields within the EAP header (Code, Identifier, Length,
Type) are not protected.
Confidentiality is not supported; the Name field in both the
Challenge and Response packets are sent in the clear.
While Key Derivation is supported, the key strength is limited by the
password policy. As noted in Section 2.2, in practice the password
may be limited to 14 octets. If these octets are randomly chosen
from the ASCII character set, then an effective key strength of 98
bits can be obtained. However, if the octets are only chosen from an
English language dictionary, then an effective key strength of 2.2
bits per octet or 31 bits will obtain.
Session independence also depends on password policy. Where the
password is weak, it may be obtained via dictionary attack, in which
case future and past keys can be calculated. However, if the
password is strong then the inclusion of nonces in both directions
provides for session independence, absent invalidation of a
cryptographic assumption.
As noted in [PPTPv1] and [PPTPv2], the MS-CHAP-v2 protocol is subject
to dictionary attack. It is advised that this method only be used
when protected from snooping by a tunnel method such as [PEAP]; this
will also mask potential key strength issues.
As the protocol exchanges fit within the minimum EAP MTU size defined
in [RFC3748], there is no need for fragmentation support. Fast
reconnect and Channel binding are not supported.
4. References
4.1. Normative references
[RFC1320] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, April
1992.
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol
(CHAP)", RFC 1994, August 1996.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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INTERNET-DRAFT EAP MS-CHAPv2 12 June 2007
[RFC2433] Zorn, G. and Cobb, S., "Microsoft PPP CHAP Extensions", RFC
2433, October 1998.
[RFC2484] Zorn, G., "PPP LCP Internationalization Configuration Option",
RFC 2484, January 1999.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC
2759, January 2000.
[RFC3748] Blunk, L., "Extensible Authentication Protocol (EAP)", RFC
3748, April 2004.
[RC4] 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
[IEEE8021X]
IEEE Standards for Local and Metropolitan Area Networks: Port
Based Network Access Control, IEEE Std 802.1X-2001, June 2001.
[SHA1] "Secure Hash Standard", Federal Information Processing
Standards Publication 180-1, National Institute of Standards
and Technology, April 1995.
[UNICODE] "The Unicode Standard, Version 2.0", The Unicode Consortium,
Addison-Wesley, 1996. ISBN 0-201-48345-9.
4.2. Informative references
[RFC1570] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January
1994.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994.
[DES] "Data Encryption Standard (DES)", Federal Information
Processing Standard Publication 46-2, National Institute of
Standards and Technology, December 1993.
[DESMODES]
"DES Modes of Operation", Federal Information Processing
Standards Publication 81, National Institute of Standards and
Technology, December 1980.
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[RFC3079] Zorn, G., "Deriving Keys for use with Microsoft Point-to-Point
Encryption (MPPE)", RFC 3079, March 2001.
[PEAP] Palekar, A., et al., "Protected EAP Protocol (PEAP) Version
2", draft-josefsson-pppext-eap-tls-eap-08.txt, Internet draft
(work in progress), April 2004.
[PPTPv1] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's Point-
to- Point Tunneling Protocol", Proceedings of the 5th ACM
Conference on Communications and Computer Security, ACM Press,
November 1998.
[PPTPv2] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's PPTP
Authentication Extensions (MS-CHAPv2)", CQRE '99, Springer-
Verlag, 1999, pp. 192-203.
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Appendix A - Examples
In the case where the EAP-MS-CHAP-V2 authentication is successful,
the conversation will appear as follows:
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=EAP-MS-CHAP-V2
(Success)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Success) ->
<- EAP-Success
In the case where the EAP MS-CHAP-V2 authentication is unsuccessful,
due to a retryable error, the conversation will appear as follows
(assuming a maximum of two retries):
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=EAP-MS-CHAP-V2
(Failure, R=1)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response) ->
<- EAP-Request/
EAP-Type=EAP-MS-CHAP-V2
(Failure, R=1)
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EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response) ->
<- EAP-Failure
In the case where the EAP MS-CHAP-V2 authentication is unsuccessful,
due to a non-retryable error, the conversation will appear as follows
(Windows XP SP1):
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Failure
In the case where the EAP MS-CHAP-V2 authentication is unsuccessful,
due to a non-retryable error, and a Failure Request packet is sent,
the conversation will appear as follows (behavior not exhibited by
Windows XP SP1):
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Failure, R=0)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Failure)->
<- EAP-Failure
In the case where the EAP MS-CHAP-V2 authentication is initially
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unsuccessful due to password expiration, but the subsequent Change
Password operation succeeds, the conversation will appear as follows:
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=MS-CHAP-V2
(Failure, R=1,
Message=ERROR_PASSWD_EXPIRED (E=648))
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Change-Password) ->
<- EAP-Request/
EAP-Type=MS-CHAP-V2
(Success)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Success) ->
<- EAP-Success
In the case where the EAP MS-CHAP-V2 authentication is unnsuccessful
due to password failure and a successful retry occurs, the
conversation appears as follows:
Peer Authenticator
---- -------------
<- EAP-Request/Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP MS-CHAP-V2
(Challenge)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=MS-CHAP-V2
(Failure, R=1,
Message=ERROR_AUTHENTICATION_FAILURE (E=691)
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EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Response)->
<- EAP-Request/
EAP-Type=MS-CHAP-V2
(Success)
EAP-Response/
EAP-Type=EAP-MS-CHAP-V2
(Success) ->
<- EAP-Success
Acknowledgments
Thanks to Vivek Kamath, Mark Wodrich and Narendra Gidwani for
discussions, comments and text relating to this document.
Authors' Addresses
Vivek Kamath
Ashwin Palekar
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: {vivek, ashwinp}@microsoft.com
Phone: +1 425 882 8080
Fax: +1 425 936 7329
Full Copyright Statement
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