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PPPEXT Working Group                                        Vivek Kamath
INTERNET-DRAFT                                            Ashwin Palekar
Category: Informational                                        Microsoft
<draft-kamath-pppext-eap-mschapv2-01.txt>
5 April 2004



                     Microsoft EAP CHAP Extensions


This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.


Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet- Drafts.


Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."


The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt


The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.


Copyright Notice


Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract


This document defines the Microsoft EAP CHAP Extensions Protocol,
Version 2, which encapsulates the MS-CHAP-v2 protocol, defined in
[RFC2759], within EAP, defined in [RFC3748].
















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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 ......................................   17
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





























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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.  These words are often capitalized.  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 [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-
   CHAPv2-ID, MS-Length and Data  fields.  Octets outside the range of




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   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
   OldNtPasswordHashEncryptedWithNewNtPasswordHash() routine, defined in




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   [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.


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




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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 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.





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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.


[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.





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[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.


[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.





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[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 Mark Wodrich and Narendra Gidwani of Microsoft for discussions
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


Copyright (C) The Internet Society (2004).  All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.  However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.  The limited permissions granted above are
perpetual and will not be revoked by the Internet Society or its
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herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE




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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."


Expiration Date


This memo is filed as <draft-kamath-pppext-eap-mschapv2-01.txt>,  and
expires October 4, 2004.













































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