[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12

PPPEXT Working Group                                       Bernard Aboba
INTERNET-DRAFT                                                 Dan Simon
Category: Experimental                                         Microsoft
<draft-aboba-pppext-eapgss-11.txt>
13 February 2002

                    EAP GSS Authentication Protocol

Status of this Memo

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

This document is an Internet-Draft.  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 (2002).  All Rights Reserved.

Abstract

The Extensible Authentication Protocol (EAP) provides a standard
mechanism for support of multiple authentication methods, including
public key, smart cards, One Time Passwords, and others.

This document describes the EAP GSS protocol, which enables the use of
GSS-API mechanisms within EAP. As a result, any GSS-API mechanism
providing initial authentication can be used with EAP GSS.









Aboba                         Experimental                      [Page 1]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Table of Contents

1.     Introduction ..........................................    3
   1.1       Requirements language ...........................    3
   1.2       Terminology .....................................    3
2.     Protocol overview ...................... ..............    4
   2.1       EAP server as GSS-API initiator .................    4
   2.2       Peer as GSS-API initiator .......................    6
3.     Detailed description of EAP GSS protocol ..............    9
   3.1       EAP GSS packet format ...........................    9
   3.2       EAP GSS Request packet ..........................   10
   3.3       EAP GSS Response packet .........................   12
   3.4       Options .........................................   13
   3.5       Fragmentation ....... ...........................   15
   3.6       Retry behavior ..................................   18
   3.7       Identity verification ...........................   18
   3.8       Use of addresses ................................   19
4.     Key management ........................................   19
   4.1       Key derivation algorithm ........................   19
5.     Security considerations ...............................   21
   5.1       Dictionary attacks ..............................   21
   5.2       Certificate revocation  .........................   23
   5.3       Mutual authentication ...........................   23
   5.4       Credential reuse ................................   23
   5.5       Key transmission issues .........................   25
   5.6       Protected negotiation ...........................   26
6.     Normative References ..................................   27
7.     Informative References ................................   28
IANA Considerations ..........................................   30
Acknowledgments ..............................................   30
Author's Addresses ...........................................   30
Appendix A - Example IAKERB topologies .......................   31
   A.1       RADIUS+KDC backend ..............................   32
   A.2       Kerberos KDC backend ............................   34
Appendix B - The Pseudorandom Function .......................   36
Intellectual Property Statement ..............................   38
Full Copyright Statement .....................................   38














Aboba                         Experimental                      [Page 2]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


1.  Introduction

The Extensible Authentication Protocol (EAP) [RFC2284] provides a
standard mechanism for support of multiple authentication methods,
including public key [RFC2716], smart cards, One Time Passwords
[RFC2284], and others. EAP may run directly over the link layer without
requiring IP and therefore includes its own support for in-order
delivery and re-transmission, but not fragmentation and reassembly,
which is the responsibility of individual EAP methods.  While EAP was
originally developed for use with PPP [RFC1661], it is also now in use
with IEEE 802 [IEEE802]. The encapsulation of EAP on IEEE 802 links is
described in [IEEE8021X].

The Generic Security Service API (GSS-API), is described in [RFC2743].
This document describes the EAP GSS protocol, which supports
fragmentation and reassembly and enables the use of  GSS-API mechanisms
within EAP. As a result, any GSS-API mechanism providing initial
authentication can be used with EAP GSS.

Supporting GSS-API authentication methods within EAP is desirable
because this enables developers creating GSS-API authentication methods
to leverage their development efforts.  Since the EAP Type field is a
finite (one octet) resource, EAP GSS allows GSS-API methods to
automatically be supported within EAP without having to consume an EAP
Type for each GSS-API method.

1.1.  Requirements language

In this document, the key words "MAY", "MUST,  "MUST  NOT",  "OPTIONAL",
"RECOMMENDED",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [RFC2119].

1.2.  Terminology

This document frequently uses the following terms:

Authentication Server
          An Authentication Server is an entity that provides an
          Authentication Service to an NAS. This service verifies from
          the credentials provided by the peer, the claim of identity
          made by the peer.

Master key
          The key derived between the EAP client and EAP server during
          the EAP authentication process.

Master session key
          The keys derived from the master key that are subsequently



Aboba                         Experimental                      [Page 3]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


          used in generation of the transient session keys for
          authentication, encryption, and IV-generation. So that the
          master session keys are usable with any ciphersuite, they are
          longer than is necessary, and are truncated to fit.

NAS       The end of the link requiring the authentication. In IEEE
          802.1X, this end is known as the Authenticator.

Peer      The other end of the point-to-point link (PPP), point-to-point
          LAN segment (IEEE 802.1X) or 802.11 wireless link, which being
          authenticated by the NAS. In IEEE 802.1X, this end is known as
          the Supplicant.

Transient session keys
          The chosen ciphersuites uses transient session keys for
          authentication and encryption as well as IVs (if required).
          The transient session keys are derived from the master session
          keys, and are of the appropriate size and type for use with
          the chosen ciphersuite.

2.  Protocol overview

As described in [RFC2284], EAP conversations will typically begin with
the NAS and the peer negotiating EAP.  The NAS will then typically send
an EAP-Request/Identity packet to the peer, and the peer will respond
with an EAP-Response/Identity packet to the NAS, containing the peer's
UserId. Once having received the peer's Identity, the EAP server
responds with an EAP-Request packet of EAP-Type=EAP GSS.  From this
point forward, the EAP GSS conversation may proceed in one of two ways:

[1]  EAP server as GSS-API iniator. Here the EAP server acts as the GSS-
     API initiator, and the peer acts as the GSS-API target.

[2]  EAP client as GSS-API initiator. Here the peer acts as the GSS-API
     initiator, and the EAP server acts as the GSS-API target. This mode
     adds an extra round-trip.

2.1.  EAP server as GSS-API initiator

As described in [RFC2284], the EAP server typically authenticates the
peer using a prearranged method or set of methods. As a result, the EAP
server may have predetermined the use of EAP GSS as well as the GSS-API
method to be used. If that GSS-API method can be initiated by the EAP
Server, then the EAP server MAY act as a GSS-API initiator with the peer
acting as a GSS-API target.  In this case, the EAP Server will indicate
the pre-determined GSS-API method, possibly via SPNEGO, but SHOULD NOT
allow negotiation of a substitute GSS-API method.




Aboba                         Experimental                      [Page 4]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


To initiate the conversation, the EAP-Server sends an EAP-Request packet
with EAP-Type=EAP GSS. This initial packet MUST have the O (Options) bit
set, and MUST include a Nonce Payload option. The data field of the
packet will encapsulate a GSS-API token, created as a result of a call
to GSS_Init_sec_context ().  In this case mutual authentication MUST be
requested (otherwise the peer would not be authenticated to the NAS!) so
that the the mutual_req_flag is set and the call to GSS_Init_sec-
context() returns GSS_S_CONTINUE_NEEDED status.

When it receives the EAP-Request, the peer will de-capsulate the
received GSS-API token within the EAP GSS frame, and will pass it as the
input_token parameter to GSS_Accept_sec_context().  If
GSS_Accept_sec_context indicates GSS_S_COMPLETE status, then the NAS has
been authenticated by the peer, and the NAS's indicated identity is
provided in the src_name result.  The peer then responds with an EAP-
Response packet with EAP-Type= EAP GSS, MUST have the O bit (Options)
set, and contains a Nonce Payload option. The data field of the packet
contains the returned output_token.

The EAP server will then de-capsulate the GSS-API token within the EAP-
Response message and pass it as the input_token parameter to
GSS_Init_sec_context(). If the call returns GSS_S_COMPLETE status, then
the peer has been authenticated to the EAP-Server, then the EAP-Server
responds with an EAP-Success message.  If GSS_S_CONTINUE_NEEDED status
is returned, then the EAP Server encapsulates the returned output_token
with an EAP-Request packet of EAP-Type=EAP GSS, and pass this back to
the peer.
























Aboba                         Experimental                      [Page 5]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


The conversation (which can be completed in a minimum of 2.5 round
trips), appears as follows:

Peer                  NAS
------           -------------
                 EAP/Identity
         <-------Request

EAP/Identity
Response -------->

                  GSS_Init_sec_context(mutual_req_flag)
                  returns GSS_S_CONTINUE_NEEDED,
                  output_token

         <--------EAP Request
                  EAP Type=EAP GSS
                  O bit, Nonce Payload,
                  output_token

GSS_Accept_sec_context(input_token)
returns GSS_S_COMPLETE,
output_token

EAP Response -------->
EAP Type=EAP GSS
O bit, Nonce Payload,
output_token

                  GSS_Init_sec_context(input_token)
                  returns GSS_S_COMPLETE

         <--------EAP Success

2.2.  Peer as GSS-API initiator

If the EAP server is prepared to allow negotiation of the GSS-API method
via SPNEGO [RFC2478], or if the EAP server knows the GSS-API method to
be used, but cannot initiate it (e.g. IAKERB, or Kerberos V), then the
peer MUST act as a GSS-API initiator, with the EAP server acting as the
GSS-API target.

In this case, the EAP server MUST respond with an EAP GSS/Start packet,
which is an EAP-Request packet with EAP-Type=EAP GSS, the Start (S) and
Options (O) bits set, a Nonce Payload option, and no data.

The peer then calls GSS_Init_sec_context(), typically with mutual
authentication requested so that the mutual_req_flag is set and the call



Aboba                         Experimental                      [Page 6]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


returns GSS_S_CONTINUE_NEEDED status. The peer then MUST respond with an
EAP-Response packet with EAP-Type=EAP GSS, the O bit set, the Nonce
Payload option present, and the data field set to the output_token.

If method negotiation is to be used, then an initial negotiation token
for the Simple and Protected GSS-API Negotiation Mechanism (SPNEGO)
[RFC2478] is transferred. This contains an ordered list of mechanisms, a
set of options that should be supported by the selected mechanism and
the initial security token for the mechanism preferred by the peer.  The
inclusion of the initial security token for the preferred method saves a
round-trip, assuming that the NAS agrees to the preferred mechanism.

The EAP server then de-capsulates the GSS-API token contained within the
EAP-Response of EAP-Type=EAP GSS and uses this as the input_token
parameter to a call to GSS_Accept_sec_context(). The output_token
parameter will then contain a token, containing the result of the
negotiation and in the case of accept, the agreed security mechanism and
the response to the initial security token as described in [RFC2478].
This token is then encapsulated within an EAP-Request packet of EAP-
Type=GSS-API, which is sent to the peer. This occurs whether the call
completed with GSS_S_CONTINUE_NEEDED status or GSS_S_COMPLETE status.

The peer then de-capsulates the GSS-API token contained within the EAP-
Request packet with EAP-Type=EAP GSS, and passes the input_token
parameter to GSS_Init_sec_context().  The output_token is encapsulated
within an EAP-Response packet with EAP-Type=EAP GSS and sent to the EAP
server.  This occurs whether the call completed with
GSS_S_CONTINUE_NEEDED status or GSS_S_COMPLETE status.

If the previous call to GSS_Accept_sec_context() returned GSS_S_COMPLETE
status, then the EAP-Server returns an EAP-Success message to the
client. Otherwise, it de-capsulates the GSS-API token contained within
the EAP-Request packet, and the conversation continues.


















Aboba                         Experimental                      [Page 7]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


The conversation (which can be completed in a minimum of 3.5 round
trips), appears as follows:

Authenticating Peer     NAS
-------------------     -------------
                        EAP-Request/
                      <- Identity
EAP-Response/
Identity (MyID) ->
                        EAP-Request/
                        EAP-Type=EAP GSS
                         (GSS Start,
                          S and O bits set),
                      <-  Nonce Payload

GSS_Init_sec_context(mutual_req_flag)
  returns GSS_S_CONTINUE_NEEDED,
  output_token (SPNEGO)

EAP-Response/
EAP-Type=EAP GSS
O bit, Nonce Payload,
output_token ->
                        GSS_Accept_sec_context(input_token)
                         returns GSS_S_COMPLETE,
                         output_token (SPNEGO)

                        EAP-Request/
                          EAP-Type=EAP GSS
                      <- output_token

GSS_Init_sec_context(input_token)
  returns GSS_S_COMPLETE,
  output_token

EAP-Response/
EAP-Type=EAP GSS
output_token ->
                      <- EAP-Success












Aboba                         Experimental                      [Page 8]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


3.  Detailed description of the EAP GSS protocol

3.1.  EAP GSS Packet Format

A summary of the EAP GSS Request/Response 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      |        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, 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

   TBD - EAP GSS

Data

   The format of the Data field is determined by the Code field.












Aboba                         Experimental                      [Page 9]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


3.2.  EAP GSS Request Packet

A summary of the EAP GSS Request 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      |    Version    |    Flags      |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Options...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            GSS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

   1

Identifier

   The Identifier field is one octet and aids in matching responses with
   requests. The Identifier field MUST be changed on each Request
   packet.

Length

   The Length field is two octets and indicates the length of the entire
   EAP packet.

Type

   TBD - EAP GSS

Version

   1












Aboba                         Experimental                     [Page 10]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Flags

   0 1 2 3 4 5 6 7 8
   +-+-+-+-+-+-+-+-+
   |L M S O R R R R|
   +-+-+-+-+-+-+-+-+

   L = Length included
   M = More fragments
   S = EAP GSS start
   O = Options present
   R = Reserved

   The L bit (length included) MUST be set for the first fragment of a
   fragmented  GSS message or set of messages. If set, the GSS Message
   Length option MUST be included.  The M bit (more fragments) is set on
   all but the last fragment. The S bit (EAP GSS start) is set in an EAP
   GSS Start message.  This differentiates the EAP GSS Start message
   from a fragment acknowledgment.  The O bit (Options present) is set
   to indicate the presence of options, including the GSS Message Length
   option. As a result, the O bit MUST be set whenever the L bit is set;
   however, it also may be set when the L bit is not set.

Options

   The Options field is of variable length, and contains type-length-
   value tuples (TLVs). Options are present only if the O bit is set.

GSS data

   The GSS data consists of the encapsulated GSS token.




















Aboba                         Experimental                     [Page 11]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


3.3.  EAP GSS Response Packet

A summary of the EAP GSS Response 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      |    Version    |    Flags      |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Options...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            GSS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

   2

Identifier

   The Identifier field is one octet and MUST match the Identifier field
   from the corresponding request.

Length

   The Length field is two octets and indicates the length of the entire
   EAP packet.

Type

   TBD - EAP GSS

Version

   1













Aboba                         Experimental                     [Page 12]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Flags

   0 1 2 3 4 5 6 7 8
   +-+-+-+-+-+-+-+-+
   |L M S O R R R R|
   +-+-+-+-+-+-+-+-+

   L = Length included
   M = More fragments
   S = EAP GSS start
   O = Options present
   R = Reserved

   The L bit (length included) MUST be set for the first fragment of a
   fragmented  GSS message or set of messages. If set, the GSS Message
   Length option MUST be included.  The M bit (more fragments) is set on
   all but the last fragment. The S bit (EAP GSS start) is set in an EAP
   GSS Start message.  This differentiates the EAP GSS Start message
   from a fragment acknowledgment.  The O bit (Options present) is set
   to indicate the presence of options, including the GSS Message Length
   option. As a result, the O bit MUST be set whenever the L bit is set;
   however, it also may be set when the L bit is not set.

Options

   The Options field is of variable length, and contains type-length-
   value tuples (TLVs). Options are present only if the O bit is set.

GSS data

   The GSS data consists of the encapsulated GSS token.

3.4.  Options

To date, the only options that are defined are the GSS Message Length
option (option 1) and the Nonce Payload option (option 2).  The format
of options are as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






Aboba                         Experimental                     [Page 13]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Type

   A two octet field, denoting the option. Values include:

   1 - GSS Message Length
   2 - Nonce Payload

Length

   The length of the option, including the type, length and value
   fields.

Value

   The value of the option.

3.4.1.  GSS Message Length option

The GSS Message Length option is present only if the L and O bits are
set. It is defined as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Value                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   1

Length

   8

Value

   The value field of the GSS Message Length options is four octets in length.
   This field provides the total length of the GSS message or set of messages
   that is being fragmented.









Aboba                         Experimental                     [Page 14]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


3.4.2.  Nonce Payload option

The Nonce Payload option is defined as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Type             |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Value                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   2

Length

   36

Value

   The value field of the Nonce Payload option is 32 octets in length.
   As in [RFC2246] the first 4 octets of this field are the time of
   day when the message was generated (in seconds since the Unix epoch,
   12:00 midnight, January 1, 1970 GMT) and the other 28 octets are randomly
   generated.

3.5.  Fragmentation

It is possible that EAP GSS messages may exceed the link MTU size, the
maximum RADIUS packet size of  4096 octets, or even the PPP Multilink
Maximum Received Reconstructed Unit (MRRU). As described in [RFC1990],
within PPP the multi-link MRRU is negotiated via the Multilink MRRU LCP
option, which includes an MRRU length field of two octets, and thus can
support MRRUs as large as 64 KB.

In order to protect against reassembly lockup and denial of service
attacks, it may be desirable for an implementation to set a maximum size
for a GSS-API token. Since a typical certificate chain is rarely longer
than a few thousand octets, and no other field is likely to be anywhere
near as long, a reasonable choice of maximum acceptable message length
might be 64 KB.

If this value is chosen, then for PPP links, fragmentation can be
handled via the multi-link PPP fragmentation mechanisms described in
[RFC1990]. While this is desirable, there may be cases in which multi-
link or the MRRU LCP option cannot be negotiated. Also, since EAP



Aboba                         Experimental                     [Page 15]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


methods must also be usable within IEEE 802.1X [IEEE8021X], an EAP GSS
implementation MUST provide its own support for fragmentation and
reassembly.

Since EAP is a simple ACK-NAK protocol, fragmentation support can be
added in a simple manner. In EAP, fragments that are lost or damaged in
transit will be retransmitted, and since sequencing information is
provided by the Identifier field in EAP, there is no need for a fragment
offset field as is provided in IP.

EAP GSS fragmentation support is provided through addition of a flags
octet within the EAP-Response and EAP-Request packets, as well as a GSS
Message Length field of four octets. Flags include the Length included
(L), More fragments (M), Options (O) and EAP GSS Start (S) bits. The L
is set to indicate the presence of the GSS Message Length option, and
MUST be set for the first fragment of a fragmented GSS message or set of
messages. The M flag is set on all but the last fragment. The S flag is
set only within the EAP GSS start message sent from the EAP server to
the peer. The GSS Message Length option provides the total length of the
GSS-API token or set of messages that is being fragmented;  this
simplifies buffer allocation.

When an EAP GSS peer receives an EAP-Request packet with the M bit set,
it MUST respond with an EAP-Response with EAP-Type=EAP GSS and no data.
This serves as a fragment ACK. The EAP server MUST wait until it
receives the EAP-Response before sending another fragment. In order to
prevent errors in processing of fragments, the EAP server MUST increment
the Identifier field for each fragment contained within an EAP-Request,
and the peer MUST include this Identifier value in the fragment ACK
contained within the EAP-Response. Retransmitted fragments will contain
the same Identifier value.

Similarly, when the EAP server receives an EAP-Response with the M bit
set, it MUST respond with an EAP-Request with EAP-Type=EAP GSS and no
data. This serves as a fragment ACK. The EAP peer MUST wait until it
receives the EAP-Request before sending another fragment.  In order to
prevent errors in the processing of fragments, the EAP server MUST use
increment the Identifier value for each fragment ACK contained within an
EAP-Request, and the peer MUST include this Identifier value in the
subsequent fragment contained within an EAP-Response.











Aboba                         Experimental                     [Page 16]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


In the case where the EAP GSS authentication is successful, and
fragmentation is required, the conversation will appear as follows:

Authenticating Peer     NAS
-------------------     -------------
                         EAP-Request/
                         <- Identity
EAP-Response/
Identity (MyID) ->
                         EAP-Request/
                           EAP-Type=EAP GSS
                         <- GSS Start,
                           O and S bit set,
                           Nonce Payload

GSS_Init_sec_context(mutual_req_flag)
  returns GSS_S_CONTINUE_NEEDED,
  output_token (SPNEGO)

EAP-Response/
EAP-Type=EAP GSS
O bit set, Nonce Payload,
output_token ->

                         GSS_Accept_sec_context(input_token)
                          returns GSS_S_COMPLETE,
                          output_token (SPNEGO)

                         EAP-Request/
                           EAP-Type=EAP GSS
                           output_token
                         <- (Fragment 1: L, O, M bits set)
EAP-Response/
EAP-Type=EAP GSS ->
                         EAP-Request/
                           EAP-Type=EAP GSS
                         <- (Fragment 2: M bit set)
EAP-Response/
EAP-Type=EAP GSS ->
                         EAP-Request/
                           EAP-Type=EAP GSS
                         <- (Fragment 3)

GSS_Init_sec_context(input_token)
  returns GSS_S_COMPLETE,
  output_token

EAP-Response/



Aboba                         Experimental                     [Page 17]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


EAP-Type=EAP GSS
output_token
(Fragment 1:
 L, O, M bits set)->
                         EAP-Request/
                         <- EAP-Type=EAP GSS
EAP-Response/
EAP-Type=EAP GSS
(Fragment 2)->
                         <- EAP-Success

3.6.  Retry behavior

As with other EAP protocols, the EAP server is responsible for retry
behavior. This means that if the EAP server does not receive a reply
from the peer, it MUST resend the EAP-Request for which it has not yet
received an EAP-Response. However, the peer MUST NOT resend EAP-Response
packets without first being prompted by the EAP server.

For example, if the initial EAP GSS start packet sent by the EAP server
were to be lost, then the peer would not receive this packet, and would
not respond to it. As a result, the EAP GSS start packet would be resent
by the EAP server. Once the peer received the EAP GSS start packet, it
would send an EAP-Response encapsulating the client_hello message.  If
the EAP-Response were to be lost, then the EAP server would resend the
initial EAP GSS start, and the peer would resend the EAP-Response.

As a result, it is possible that a peer will receive duplicate EAP-
Request messages, and may send duplicate EAP-Responses.  Both the peer
and the EAP-Server should be engineered to handle this possibility.

3.7.  Identity verification

As part of the GSS-API conversation, it is possible that the server may
present a certificate to the peer, or that the peer may present a
certificate to the EAP server.  If the peer has made a claim of identity
in the  EAP-Response/Identity (MyID) packet, the EAP server SHOULD
verify that  the claimed identity corresponds to the certificate
presented by the  peer. Typically this will be accomplished either by
placing the userId within the peer certificate, or by providing a
mapping between the peer certificate and the userId using a directory
service.

Similarly, the peer MUST verify the validity of the EAP server
certificate, and SHOULD also examine the EAP server name presented in
the certificate, in order to determine whether the EAP server can be
trusted. Please note that in the case where the EAP authentication is
remoted that the EAP server will not reside on the same machine as the



Aboba                         Experimental                     [Page 18]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


NAS, and therefore the name in the EAP server's certificate cannot be
expected to match that of the intended destination. In this case, a more
appropriate test might be whether the EAP server's certificate is signed
by a CA controlling the intended destination and whether the EAP server
exists within a target sub-domain.

3.8.  Use of addresses

When using EAP GSS, the EAP client may not be able to include an address
in an EAP-Response message, since prior to obtaining access the EAP
client may not have an IP address.  This limits effective use of EAP GSS
to GSS-API methods that do not require the peer to have an IP address
prior to authentication.

The IAKERB GSS-API method can explicitly handle this situation, as
described in [IAKERB]. However, where the Kerberos V protocol, described
in [RFC1510], is negotiated as a GSS-API method as described in
[RFC1964], the addresses field of the AS_REQ and TGS_REQ SHOULD be blank
and the caddr field of the ticket SHOULD also be left blank.

4.  Key management

As a result of the EAP GSS conversation, the EAP endpoints will mutually
authenticate and derive a master key.  The master key is then used to
derive master session keys for authentication and encryption as well as
initialization vectors in each direction. It is the master session keys
that are provided by the EAP method hosted on the client and AAA server,
and communicated by the AAA server to the NAS.

The master session keys are then used by the client and NAS in order to
derive ciphersuite-specific keys, once the negotiated ciphersuite is
known.  Depending on the negotiated ciphersuite, not all of the master
session keys will be used in this process.

By requiring master session keys (but not ciphersuite-specific keys) to
be derived by the EAP method, it is possible for EAP methods to derive
keying material that can be used by any ciphersuite. This is desirable,
since it avoids having to revise EAP methods each time a new ciphersuite
is deployed for any of the applications using EAP.

4.1.  Key derivation algorithm

EAP TLS, described in [RFC2716], provides a mechanism for deriving
authentication and encryption keys as well as IVs in both directions
from the TLS master key. The key hierarcy of EAP GSS is similar to that
of EAP TLS, with the following differences:





Aboba                         Experimental                     [Page 19]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


[1]  Pseudo-master key derivation.  TLS APIs typically do not enable
     direct access to the master key, for security reasons. As a result,
     EAP TLS utilizes the TLS PRF in the master session key derivation,
     rather than acting on the master key itself.

     For similar reasons, EAP GSS implementations will typically not be
     able to obtain access to the master key via GSS-API. However, GSS-
     API methods can call GSS_Wrap() to encrypt and GSS_GetMIC() to
     generate authentication tokens based on the master key.  Since EAP
     GSS cannot assume direct access to the master key, it is not
     possible to utilize the master key directly in the derivation of
     the master session keys.

     Instead, an intermediate step is required, where a "Pseudo-Master
     Key" K' is derived from the master key, and used in the derivation
     of the master session keys. Note that since the entropy of the GSS-
     API MIC cannot be guaranteed, additional randomness needs to be
     introduced into the derivation at this point.

[2]  Nonce generation. TLS includes an exchange of nonces, and the
     exchanged nonces are used within the keying hierarchy in order to
     ensure liveness in the derived master session keys. GSS-API methods
     such as Kerberos do not include such a nonce exchange, although
     typically some other source of "liveness" is provided, such as a
     counter or a time value. The variation in GSS-API methods makes it
     difficult to come up with a key hierarchy that can be used with any
     GSS-API method, if only GSS-API calls are available. To circumvent
     this limitation, EAP GSS adds a nonce exchange patterned after
     [RFC2246].

In the techniques described here, master session keys are derived from
the master key derived by the GSS-API method, but are never used to
encrypt or decrypt data; they are only used in the derivation of
transient session keys.

The derivation proceeds as follows:

[1]  The "Pseudo-Master Key" K' is derived from the raw master key using
     the formula: K' = HMAC-SHA1("", GSS_GetMIC("EAP GSS pseudo master
     key") | random). Here random is defined as the concatenation of the
     message fields client_nonce and server_nonce (in that order).

[2]  Given the K' value and the pseudorandom function (PRF) defined in
     Appendix B, the value PRF1 = PRF(K', "client EAP encryption",
     random) is computed up to 128 bytes, and the PRF2 = PRF("","client
     EAP encryption", random) is computed up to 64 bytes (where "" is an
     empty string).  Here random is defined as the concatenation of the
     message fields client_nonce and server_nonce (in that order).



Aboba                         Experimental                     [Page 20]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


[3]  The peer encryption key (the one used for encrypting data from peer
     to authenticator) is  obtained by truncating to the correct length
     the first 32 bytes of PRF1.  The authenticator encryption key (the
     one used for encrypting data from authenticator to peer), if
     different from the client encryption key, is obtained by truncating
     to the correct length the second 32 bytes of PRF1.  The peer
     authentication key (the one used for computing MICs for messages
     from peer to authenticator), if used, is obtained by truncating to
     the correct length the third 32 bytes of PRF1.  The authenticator
     authentication key (the one used for computing MICs for messages
     from authenticator to peer), if used, and if different from the
     peer authentication key, is obtained by truncating to the correct
     length the fourth 32 bytes of PRF1.  The peer initialization vector
     (IV), used for messages from peer to authenticator if a block
     cipher has been specified, is obtained by truncating to the
     cipher's block size the first 32 bytes of PRF2.  Finally, the
     authenticator initialization vector (IV), used for messages from
     peer to authenticator if a block cipher has been specified, is
     obtained by truncating to the cipher's block size the second 32
     bytes of PRF2.

The use of these encryption, authentication keys and IVs is specific to
the ciphersuite used. Additional keys or other non-secret values (such
as IVs) can be obtained as needed for future ciphersuites by extending
the outputs of the PRF beyond 128 bytes and 64 bytes, respectively.

A description of the key hierarchy is provided on the next page.

5.  Security Considerations

5.1.  Dictionary attacks

As noted in [KRBATTACK],[KERBLIM],[KERB4WEAK], both Kerberos IV and V
are vulnerable to dictionary attack. These attacks are particularly
potent when carried out in a location where a large number of
authentication exchanges can be collected within a short period of time,
such as with wireless LANs deployed in "hot spots".

As noted in [KRBATTACK], offline dictionary attacks are easily carried
out against the AS_REP, since the key encrypting the enclosed Kerberos
ticket is a function of the password. Such attacks are amenable to
parallelization, and it is therefore possible to crack a large number of
passwords in short time with only modest resources.  The imposition of a
password policy is likely only to decrease the yield, but given access
to sufficient exchanges, large scale password compromise remains
possible.





Aboba                         Experimental                     [Page 21]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


For this reason, when used on wireless networks, EAP GSS SHOULD
negotiate methods invulnerable to offline dictionary attacks. This
includes public key authentication techniques such as [PKINIT], or
password-based techniques such as SRP, described in [RFC2945], EKE,
described in [EKE], or techniques described in [STRONGAUTH] or [DUAL].














































Aboba                         Experimental                     [Page 22]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                     |
             |      Derivation of the pseudo       |
             |          master key from the        |
             |             raw master key          |
             |                                     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               |
                               | K'
                               |
                               V
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                     |
             |          Master Session Key         |
             |              Derivation             |
             |                                     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |                                 |
               | PRF1                            | PRF2
               | 128B                            | 64B
               V                                 V
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             |   |                             |
 |             Key             |   |             IV              |
 |          Derivation         |   |         Derivation          |
 |                             |   |                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | P->A  | A->P  | P->A  | A->P        | P->A            | A->P
   | Enc.  | Enc.  | Auth. | Auth.       | IV              | IV
   | Key   | Key   | Key   | Key         | 32B             | 32B
   | 32B   | 32B   | 32B   | 32B         |                 |
   |       |       |       |             |                 |
   V       V       V       V             V                 V
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                Ciphersuite-Specific Truncation &              |
 |                       Key utilization                         |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Figure 1 - Algorithm for derivation of session keys from the
             GSS-API method master key K.









Aboba                         Experimental                     [Page 23]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Kerberos V SHOULD NOT be used without extensions providing protection
against offline dictionary attacks.  As noted in [KRBATTACK], it has
been proposed that Kerberos V dictionary attack vulnerabilities be
addressed via a pre-authentication exchange.  The vulnerability can also
be addressed by use of public key authentication with Kerberos,
described in [PKINIT].

5.2.  Certificate revocation

Since the EAP server is typically connected to the Internet during the
EAP conversation, it is capable of following a certificate chain or
verifying whether the peer's certificate has been revoked. In contrast,
the peer may or may not have Internet connectivity, and thus while it
can validate the EAP server's certificate based on a pre-configured set
of CAs, it may not be able to follow a certificate chain or verify
whether the EAP server's certificate has been revoked.

In the case where the peer is initiating a voluntary Layer 2 tunnel
using PPTP or L2TP, the peer will typically already have a PPP interface
and Internet connectivity established at the time of tunnel initiation.
As a result, during the EAP conversation it is capable of checking for
certificate revocation.

However, in the case where the peer is initiating a connection, it will
not have Internet connectivity and is therefore not capable of checking
for certificate revocation until after the peer  has access to the
Internet. In this case, the peer SHOULD check for certificate revocation
after connecting to the Internet.

5.3.  Mutual authentication

In order to guard against rogue NAS devices, it is recommended that a
GSS-API method supporting mutual authentication be selected during the
SPNEGO negotiation, as described in [RFC2478].  This also avoids
potential reflection attacks against dual one-way authentication
methods, such as EAP-MD5.

5.4.  Credential reuse

A peer with valid credentials may reuse those credentials in a
subsequent authentication.  Credential reuse improves efficiency in a
number of scenarios.  Where the peer attempts to re-authenticate to an
EAP server within a short period of time, the re-authentication time may
be shortened. Also, where the peer roams to another NAS willing to
accept credentials from a previous NAS, fast-handoff may be achieved.
Credential reuse may also prove useful during multi-link authentication.





Aboba                         Experimental                     [Page 24]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


For example, a peer initially using the IAKERB GSS-API method to obtain
a TGT and a ticket to the NAS may subsequently reuse that ticket in an
AP_REQ/AP_REP exchange. Such an exchange may occur either in-band (e.g.
via use of the Kerberos V GSS-API method) or out-of-band (e.g. via an
802.1X EAPOL-Key message). Typically in-band efficiency savings are
modest (one round-trip saved using the Kerberos V GSS-API method versus
IAKERB), since the authentication typically is remoted to the backend
authentication server. The savings from out-of-band credential reuse can
be more substantial, since in this case the credential validation may
occur on the NAS.

The decision of whether to attempt to reuse credentials is left up to
the peer, which needs to determine whether credential use is likely to
succeed. The decision may be based on out-of-band information (such as
probe/response messages exchanged via 802.11 [IEEE80211]), or the time
elapsed since the previous authentication attempt.

If the peer attempts to reuse credentials that are not valid, then it
will receive an error response and the peer can re-authenticate using
the more complete sequence.  For example, after an initial IAKERB
authentication, the peer will have obtained a TGT from the KDC via the
AS_REP, and a ticket for network access via the TGS_REP. The peer may
subsequently attempt to negotiate the Kerberos V GSS-API method, so as
to reuse the previously obtained credentials. Should a KRB_ERROR be
returned to the peer, then the peer can negotiate IAKERB on its next
attempt instead.

Note that credential reuse for the purpose of "fast handoff" has
significant limitations. For use in "fast handoff", it is desirable for
Kerberos ticket validation to occur on the NAS, rather than remoting the
validation to the backend authentication server, since this will save a
round-trip between the NAS and the backend authentication server.

However, to enable the peer to reuse a Kerberos ticket on a different
NAS, it is necessary for NASen within the same geographic area to share
a key with the KDC. If this is not the case, then peers moving from one
NAS to another will not be able to reuse credentials without either
requiring communication between the NASen, or remoting of the credential
validation to a backend authentication server.

Allowing multiple NASen to share a key with the KDC makes it more likely
that an attacker sniffing the wire will be able to obtain the NAS key,
particularly if the key is derived from a password. Details are provided
within reference [KRBATTACK]. Alternatively, the new NAS can pass the
submitted ticket and authenticator to a backend authentication server or
to the previous NAS for validation. However, requiring communication
between the NAS and backend authentication server for "fast handoff"
adds substantial to the delay. In addition if it is assumed that the



Aboba                         Experimental                     [Page 25]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


NASen support an Inter-Access Point Protocol (IAPP), then EAP-based
"fast handoff" is not necessary at all.

Similarly, if the EAP servers are set up in a rotary or made available
via a round-robin technique, then the credentials also may not be
reusable without communication with a backend authentication server or
previous NAS.

Furthermore, since existing Kerberos implementations do not include AAA
authorizations within the authorization data field of the Kerberos
ticket [RFC1510], even if the credentials can be reused, it may be
necessary for the NAS to obtain the authorization information from the
AAA server before the correct session state can be re-established on the
new NAS. If AAA authorizations are not obtained prior to granting
access, then the new NAS could potentially provide the wrong service to
the peer. For example, where Filter-Id [RFC2865] or tunnel attributes
[RFC2868] were unavailable, a peer might be given unrestricted network
access where this was not intended.

As a result of these considerations, credential reuse for the purpose of
"fast handoff" does not appear to be practical at this time.

5.5.  Key transmission issues

As a result of the EAP GSS conversation, the EAP endpoints will mutually
authenticate and derive a session key, which this specification calls
the "master key".  Within GSS-API, it is possible to use GSSWrap() to
encrypt using the master key, or GSSGetMIC() to produce a messsage
integrity check (MIC) keyed by the master key.  However, for security
reasons, there are no GSS-API calls to obtain the master key itself.

In the most general case, authentication keys, encryption keys and IVs
may be required for use with the chosen ciphersuite. The keys from which
these session keys are derived are known as "master session keys" and
are derived from the master key.

Since the peer and EAP client reside on the same machine, and the master
key cannot be exported, it is necessary for the master session keys to
be derived within EAP GSS.  Once the ciphersuite has been determined,
the master session keys are converted to ciphersuite-specific session
keys and passed to the link layer encryption module.

The situation may be more complex on the NAS, which may or may not
reside on the same machine as the EAP server. In the case where the EAP
server and NAS reside on different machines, there are several
implications for security. Firstly, the mutual authentication defined in
EAP GSS will occur between the peer and the EAP server, not between  the
peer and the NAS.



Aboba                         Experimental                     [Page 26]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


This means that as a result of the EAP GSS conversation, it is not
possible for the peer to validate the identity of the device that it is
speaking to. The second issue is that the master session keys negotiated
between the peer and EAP server will need to be transmitted to the NAS.

Both issues can be addressed via addition of a followon exchange. For
example, where the IAKERB GSS-API method is used for initial
authentication, the Kerberos V GSS-API method can be used to mutually
authenticate the peer and NAS and transfer the master session key from
the peer to the NAS, enclosed in a Kerberos ticket.  The NAS can then
derive the master session keys from the master key. Once the ciphersuite
is determined, the master session keys are converted to ciphersuite-
specific session keys and passed to the link layer encryption module on
the NAS.

5.6.  Protected negotiation

SPNEGO [RFC2478] supports protected method negotiation in the case where
the negotiated method provides authentication and integrity protection.
In contrast, EAP, described in [RFC 2284], does not provide for
protected method negotiation.

Link layer ciphersuite negotiations are also typically unprotected. For
example, ECP, described in [RFC1968], supports unprotected cipher-suite
negotiations within PPP and is thus vulnerable to attack. Similarly,
802.11, described in [IEEE80211], does not support protected ciphersuite
negotiations.

Since peers completing the GSS-API SPNEGO negotiation will typically
implicitly select a ciphersuite, which includes key strength, encryption
and hashing methods, it is tempting to use this protected ciphersuite
negotiation in place of unprotected ciphersuite negotiation mechanisms.

However, use of EAP GSS for protected ciphersuite negotiation presents
substantial difficulties, since available link layer ciphersuites may
not correspond to the ciphersuites implicitly negotiated as part of
SPNEGO.

For example,  802.11 [IEEE80211] supports Wired Equivalency Privacy
(WEP), a flawed cipher based on RC4 which supports confidentiality but
lacks a keyed message integrity check.  As a result, WEP does not
support per-frame authentication and integrity protection.  Similarly,
PPP encryption methods such as DESEbis [RFC2419] and 3DES [RFC2420]
support confidentiality but also do not support per-frame authentication
or integrity protection. Since GSS-API ciphersuites available within
GSS-API methods such as Kerberos V [RFC 1964] provide confidentiality as
well as per-packet integrity protection and authentication, they do not
correspond well to these link layer ciphersuites. As a result, the use



Aboba                         Experimental                     [Page 27]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


of SPNEGO for link-layer ciphersuite negotiation is not recommended.

6.  Normative References

[RFC1510] Kohl, J., Neuman, C., "The Kerberos Network Authentication
          Service (V5)", RFC 1510, September 1993.

[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)." STD
          51, RFC 1661, July 1994.

[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
          1964, June 1996.

[RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)." RFC 1968, June
          1996.

[RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.
          Coradetti, "The PPP Multilink Protocol (MP)." RFC 1990, August
          1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2246] Dierks, T. and Allen, C. "The TLS Protocol Version 1.0", RFC
          2246, November 1998.

[RFC2284] Blunk, L., Vollbrecht, J., "PPP Extensible Authentication
          Protocol (EAP)", RFC 2284, March 1998.

[RFC2478] Baize, E., Pinkas., D., "The Simple and Protected GSS-API
          Negotiation Mechanism", RFC 2478, December 1998.

[RFC2743] Linn, J., "Generic Security Service Application Program
          Interface, Version 2", RFC 2743, January 2000.

[IEEE8021X]
          IEEE Standards for Local and Metropolitan Area Networks: Port
          based Network Access Control, IEEE Std 802.1X-2001, June 2001.

[KERB]    Neuman, B. C., Ts'o, T., "Kerberos: An Authentication Service
          for Computer Networks", IEEE Communications, 32(9):33-38,
          September 1994.

[IAKERB]  Swift, M., Trostle, J., Aboba, B., Zorn, G., "Initial
          Authentication and Pass Through Authentication Using Kerberos
          V5 and the GSS-API (IAKERB)", Internet draft (work in
          progress), draft-ietf-cat-iakerb-08.txt, August 2001.




Aboba                         Experimental                     [Page 28]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


7.  Non-normative References

[RFC2104] Krawczyk, H. et al, "HMAC: Keyed-Hashing for Message
          Authentication", RFC 2104, February 1997.

[RFC2419] Sklower, K., Meyer, G., "The PPP DES Encryption Protocol,
          Version 2 (DESE-bis)", RFC 2419, September 1998.

[RFC2420] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)",
          RFC 2420, September 1998.

[RFC2716] Aboba, B., Simon, S.,"PPP EAP TLS Authentication Protocol",
          RFC 2716, October 1999.

[RFC2865] Rigney, C., Rubens, A., Simpson, W., Willens, S.,  "Remote
          Authentication Dial In User Service (RADIUS)", RFC 2865, June
          2000.

[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC2867] Zorn, G., Mitton, D., Aboba, B., "RADIUS Accounting
          Modifications for Tunnel Protocol Support", RFC 2867, June
          2000.

[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.,
          Goyret, I., "RADIUS Attributes for Tunnel Protocol Support",
          RFC 2868, June 2000.

[RFC2869] Rigney, C., Willats, W., Calhoun, P., "RADIUS Extensions", RFC
          2869, June 2000.

[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", RFC
          2945, September 2000.

[RFC3078] Pall, G. and Zorn, G. "Microsoft Point-to-Point Encryption
          (MPPE) RFC 3078, March 2001.

[RFC3079] Zorn, G. "Deriving Keys for use with Microsoft Point-to-Point
          Encryption (MPPE)," RFC 3079, March 2001.

[IEEE80211]
          Information technology - Telecommunications and information
          exchange between systems - Local and metropolitan area
          networks - Specific Requirements Part 11:  Wireless LAN Medium
          Access Control (MAC) and Physical Layer (PHY) Specifications,
          IEEE Std. 802.11-1997, 1997.





Aboba                         Experimental                     [Page 29]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


[IEEE802] IEEE Standards for Local and Metropolitan Area Networks:
          Overview and Architecture, ANSI/IEEE Std 802, 1990.

[DESFIPS] National Bureau of Standards, "DES Modes of Operation", FIPS
          PUB 81 (December 1980).

[KRBATTACK]
          Wu, T., "A Real-World Analysis of Kerberos Password Security",
          Stanford University Computer Science Department,
          http://theory.stanford.edu/~tjw/krbpass.html

[KRBLIM]  Bellovin, S.M., Merritt, M., "Limitations of the kerberos
          authentication system", Proceedings of the 1991 Winter USENIX
          Conference, pp. 253-267, 1991.

[KERB4WEAK]
          Dole, B., Lodin, S., and Spafford, E., "Misplaced trust:
          Kerberos 4 session keys", Proceedings of the Internet Society
          Network and Distributed System Security Symposium, pp. 60-70,
          March 1997.

[EKE]     Bellovin, S.M., Merritt, M., "Encrypted key exchange:
          Password-based protocols secure against dictionary attacks",
          Proceedings of the 1992 IEEE Computer Society Conference on
          Research in Security and Privacy, pp.  72-84, 1992.

[STRONGAUTH]
          Jablon, D., "Strong password-only authenticated key exchange",
          Computer Communication Review, 26(5):5-26, October 1996.

[DUAL]    Jaspan, B., "Dual-workfactor encrypted key exchange:
          Efficiently preventing password chaining and dictionary
          attacks", Proceedings of the Sixth Annual USENIX Security
          Conference, pp. 43-50, July 1996.

[IEEE80211i]
          IEEE Draft 802.11i/D2, "Draft Supplement to STANDARD FOR
          Telecommunications and Information Exchange between Systems -
          LAN/MAN Specific Requirements - Part 11: Wireless Medium
          Access Control (MAC) and physical layer (PHY) specifications:
          Specification for Enhanced Security", July 2001.

[AESProp] Daemen, J., Rijman, V., "AES Proposal: Rijndael," NIST AES
          Proposal, June 1998.
          http://csrc.nist.gov/encryption/aes/round2/
          AESAlgs/Rijndael/Rijndael.pdf





Aboba                         Experimental                     [Page 30]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


[AESFIPS] Draft FIPS Publication ZZZZ, "Advanced Encryption Standard
          (AES)", U.S. DoC/NIST, summer 2001.

[MODES]   "Symmetric Key Block Cipher Modes of Operation,"
          http://www.nist.gov/modes.

[BLOCK]   "Recommendation for Block Cipher Modes of Operation", National
          Institute of Standards and Technology (NIST) Special
          Publication 800-XX, CODEN: NSPUE2, U.S. Government Printing
          Office, Washington, DC, July 2001.

[PKINIT]  Tung, B. Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S.,
          Wray, J., Trostle, J., "Public Key Cryptography for Initial
          Authentication in Kerberos", draft-ietf-cat-kerberos-pk-
          init-13.txt, August 2001.

IANA Considerations

This document requires assignment of a EAP Type for EAP GSS. It does not
create any new number spaces for IANA administration.

Acknowledgments

Thanks to Paul Leach of Microsoft, Glen Zorn of Cisco Systems, and Jesse
Walker of Intel for useful discussions of this problem space.

Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 425 706 6605
EMail: bernarda@microsoft.com

Dan Simon
Microsoft Research
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: dansimon@microsoft.com
Phone: +1 425 706 6711







Aboba                         Experimental                     [Page 31]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Appendix A - Example IAKERB topologies

Where EAP GSS is used along with the GSS-API IAKERB [IAKERB] or Kerberos
V [RFC1964] mechanisms, two major topologies are possible:

RADIUS+KDC backend
     Here a RADIUS backend is used, along with a Kerberos KDC.  The NAS
     functions as an EAP-pass-through device as described in [RFC2284].
     This involves encapsulating EAP messages received from the peer
     within RADIUS as described in [RFC2869], and passing them on to the
     RADIUS server. In turn, the RADIUS server acts as an IAKERB proxy,
     de-capsulating EAP GSS/IAKERB packets, and passing them on to the
     Kerberos KDC. In turn, the RADIUS server will encapsulate packets
     from the Kerberos KDC in EAP GSS/IAKERB and send them to the NAS.
     EAP-Message attributes received from the RADIUS server are de-
     capsulated by the NAS and sent to the peer. In this topology, the
     NAS need not have knowledge of specific EAP or GSS-API methods,
     while the RADIUS server does require this knowledge.

KDC backend
     In this topology, only a Kerberos KDC is used as a backend, and the
     NAS functions as an IAKERB proxy, de-capsulating EAP GSS/IAKERB
     messages and passing them on to the KDC. Messages from the KDC are
     encapsulated within EAP GSS/IAKERB by the NAS and sent to the peer.
     In this case, the NAS needs to understand the EAP GSS, GSS-API
     IAKERB, as well as GSS-API Kerberos V mechanisms.  In addition,
     where the peer already has a valid TGT and ticket to the NAS, it
     may choose to use the Kerberos V mechanism within EAP. Note that in
     the case of 802.11, the Kerberos AP_REQ/AP_REP messages may be
     carried in messages outside the conventional EAP exchange, such as
     those defined in [IEEE8021X] so that use of the Kerberos V
     mechanism within EAP is not necessary.

In the examples below,  each topology is discussed.  While nominally the
EAP conversation occurs between the NAS and the peer, the NAS MAY act as
a pass-through device, with the EAP packets received from the peer being
encapsulated for transmission to a RADIUS server.  In the discussion
that follows,  we will use the term  "EAP server" to denote the ultimate
endpoint conversing with the peer.












Aboba                         Experimental                     [Page 32]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


A.1 RADIUS+KDC backend

In this topology, the NAS will act as an EAP pass-through, and the
RADIUS server acts as an IAKERB proxy. A successful EAP GSS/IAKERB
authentication will appear as follows:

Peer                 NAS                  RADIUS                  KDC
------           -------------           ---------               ------
                 EAP/Identity
                <-Request

EAP/Identity
Response ->

                 EAP/Identity
                 Response  ->
                                         Access-Challenge
                                         EAP GSS Request
                                       <- (Start)

                <-EAP GSS Request(Empty)

EAP GSS
Response [1]
(SPNEGO) ->

                 EAP GSS Response
                 (SPNEGO)  ->
                                         Access-Challenge
                                         EAP GSS Request
                                       <-(SPNEGO)

                 EAP GSS Request
               <-(SPNEGO)

EAP GSS IAKERB
Response  [2]
(AS_REQ) ->

                 EAP GSS IAKERB
                 Response
                 (AS_REQ)  ->

                                         AS_REQ  ->
                                                        <- AS_REP

                                         Access-Challenge
                                         EAP GSS IAKERB Request



Aboba                         Experimental                     [Page 33]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


                                      <-(AS_REP)

                 EAP GSS IAKERB
                 Request
               <-(AS_REP)

EAP GSS IAKERB
Response [3]
(TGS_REQ) ->

                 EAP GSS IAKERB
                 Response
                 (TGS_REQ)  ->

                                         TGS_REQ  ->
                                                          <-  TGS_REP

                                         Access-Challenge
                                         EAP GSS IAKERB Request
                                       <-(TGS_REP)
                 EAP GSS IAKERB
                 Request
               <-(TGS_REP)

EAP GSS IAKERB
Response
(Empty)  ->

                 EAP GSS IAKERB
                 Response
                 (Empty)  ->
                                         Access-Accept [4]
                                      <- EAP-Success

               <- EAP-Success
AP_REQ ->
               <- AP_REP [5]

Notes:

1.   IAKERB may be requested by the EAP GSS client without the need for
     negotiation, or SPNEGO may be used.

2.   The AS_REQ requests a TGT from the KDC. It may or may not include
     PADATA. As a result, the AS_REQ may not authenticate the peer to
     the KDC, but the AS_REP authenticates the KDC to the peer.





Aboba                         Experimental                     [Page 34]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


3.   The TGS_REQ requests a ticket to the NAS service.  The ticket is
     encrypted with the NAS's key so that it can only be validated by
     the NAS.

4.   On receiving a TGS_REP from the KDC rather than a KRB_ERROR, the
     RADIUS server can conclude that the peer has successfully
     authenticated, and thus that it is appropriate to reply to the NAS
     with an Access-Accept encapsulating an EAP-Success.

5.   The IAKERB exchange ends before the AP_REQ/AP_REP exchange occurs.
     As a result, the AP_REQ/AP_REP exchange either will not occur
     (preventing mutual authentication between peer and NAS or transport
     of the session key from peer to NAS), will occur out-of-band (e.g.
     after access is granted), or will occur in a subsequent EAP GSS
     conversation (e.g. using the GSS-API Kerberos V method).

A.2 Kerberos KDC backend

In this topology, there is no RADIUS server, and the NAS functions as an
IAKERB proxy, de-capsulating EAP GSS/IAKERB frames and passing them on
to the KDC. In turn, packets from the KDC are are encapsulated in EAP
GSS/IAKERB frames and sent to the peer by the NAS.  Where IAKERB is
used, the NAS functions as an IAKERB proxy, de-capsulating EAP
GSS/IAKERB messages and passing them on to the KDC. In addition, where
the peer already has a valid TGT and ticket to the NAS, it may choose to
use the Kerberos V mechanism within EAP. Note that in the case of
802.11, the Kerberos AP_REQ/AP_REP messages are carried in messages
outside the conventional EAP exchange, such as the EAPOL-Key messages
described in [IEEE8021X] so that use of the Kerberos V mechanism within
EAP is not necessary.

In the Kerberos-only topology, messages from the KDC are encapsulated
within EAP GSS/IAKERB and sent to the peer. In this case, the NAS needs
to understand the EAP GSS, GSS-API IAKERB, as well as GSS-API Kerberos V
mechanisms.
















Aboba                         Experimental                     [Page 35]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


A successful EAP GSS/IAKERB authentication occurring in a topology with
a NAS acting as an IAKERB proxy to a Kerberos KDC will appear as
follows:

Peer                    NAS                         KDC
------              -------------                 ---------
                    EAP/Identity
                  <-Request

EAP/Identity
Response  ->

                   <-EAP GSS Start

EAP GSS IAKERB
Response [1]
(AS_REQ)  ->

                    AS_REQ ->
                                                 <-  AS_REP [2]

                    EAP GSS IAKERB Request
                  <-AS_REP)

EAP GSS IAKERB
Response [3]
(TGS_REQ)  ->

                    TGS_REQ ->

                                                   <-  TGS_REP [4]

                    EAP GSS IAKERB Request
                  <-(TGS_REP)

EAP GSS IAKERB
Response
(Empty)  ->

                  <- EAP-Success
AP_REQ [5]->
                  <- AP_REP [6]
Notes:

1.   If PADATA is not used in the AS_REQ, then the peer does not
     authenticate to the KDC.





Aboba                         Experimental                     [Page 36]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


2.   The KDC authenticates to the peer in the AS_REP.

3.   The peer authenticates to the KDC via the TGS_REQ.

4.   The KDC authenticates to the peer via the TGS_REP.  The TGS_REP
     also provides the peer with a ticket and session-key for use with
     the NAS.

5.   Up until this point, the peer has not mutually authenticated with
     the NAS, or exchanged a key with it. As a result, the peer and NAS
     need to conclude an AP_REQ/AP_REP exchange. This can occur in-band
     or out-of-band. In the AP-REQ, the peer authenticates to the NAS
     and provides it with a session key.

6.   The NAS authenticates to the peer using the AP_REP.

Appendix B - The Pseudorandom Function

Given  below is the description of the  HMAC and pseudorandom function
based on Section 5 of the TLS Protocol Version 1.0 specification
[RFC2246].

Some of operations in the key derivation require a keyed MIC; this is a
secure digest of some data protected by a secret. Forging the MIC is
infeasible without knowledge of the MIC secret. The construction we use
for this operation is known as HMAC, described in [RFC2104].

HMAC can be used with a variety of different hash algorithms. We use it
with two different algorithms: MD5 and SHA-1, denoting these as
HMAC_MD5(secret, data) and HMAC_SHA1(secret, data). Additional hash
algorithms can be defined and used to protect record data, but MD5 and
SHA-1 are hard coded into the protocol.

In addition, a construction is required to do expansion of secrets into
blocks of data for the purposes of key generation or validation. This
pseudo-random function (PRF) takes as input a secret, a seed, and an
identifying label and produces an output of arbitrary length.

In order to make the PRF as secure as possible, it uses two hash
algorithms in a way which should guarantee its security if either
algorithm remains secure.

First, we define a data expansion function, P_hash(secret, data) which
uses a single hash function to expand a secret and seed into an
arbitrary quantity of output:

P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
                       HMAC_hash(secret, A(2) + seed) +



Aboba                         Experimental                     [Page 37]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


                       HMAC_hash(secret, A(3) + seed) + ...

Where + indicates concatenation. A() is defined as:

A(0) = seed
A(i) = HMAC_hash(secret, A(i-1))

P_hash can be iterated as many times as is necessary to produce the
required quantity of data. For example, if P_SHA-1 was being used to
create 64 bytes of data, it would have to be iterated 4 times (through
A(4)), creating 80 bytes of output data; the last 16 bytes of the final
iteration would then be discarded, leaving 64 bytes of output data.

The PRF is created by splitting the secret into two halves and using one
half to generate data with P_MD5 and the other half to generate data
with P_SHA-1, then exclusive-or'ing the outputs of these two expansion
functions together.

S1 and S2 are the two halves of the secret and each is the same length.
S1 is taken from the first half of the secret, S2 from the second half.
Their length is created by rounding up the length of the overall secret
divided by two; thus, if the original secret is an odd number of bytes
long, the last byte of S1 will be the same as the first byte of S2.

L_S = length in bytes of secret;
L_S1 = L_S2 = ceil(L_S / 2);

The secret is partitioned into two halves (with the possibility of one
shared byte) as described above, S1 taking the first L_S1 bytes and S2
the last L_S2 bytes.

The PRF is then defined as the result of mixing the two pseudorandom
streams by exclusive-or'ing them together.

PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
                           P_SHA-1(S2, label + seed);

The label is an ASCII string. It should be included in the exact form it
is given without a length byte or trailing null character. For example,
the label "slithy toves"  would be processed by hashing the following
bytes:

73 6C 69 74 68 79 20 74 6F 76 65 73

Note that because MD5 produces 16 byte outputs and SHA-1 produces 20
byte outputs, the boundaries of their internal iterations will not be
aligned; to generate a 80 byte output will involve P_MD5 being iterated
through A(5), while P_SHA-1 will only iterate through A(4).



Aboba                         Experimental                     [Page 38]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to  pertain
to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; neither does it represent that it has made any
effort to identify any such rights.  Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11.  Copies of claims of
rights made available for publication and any assurances of licenses to
be made available, or the result of an attempt made to obtain a general
license or permission for the use of such proprietary rights by
implementors or users of this specification can be obtained from the
IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
which may cover technology that may be required to practice this
standard.  Please address the information to the IETF Executive
Director.

Full Copyright Statement

Copyright (C) The Internet Society (2002).  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
successors or assigns.  This document and the information contained
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
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."







Aboba                         Experimental                     [Page 39]


INTERNET-DRAFT       EAP GSS Authentication Protocol    13 February 2002


Expiration Date

This  memo is filed as <draft-aboba-pppext-eapgss-11.txt>,  and  expires
August 23, 2002.















































Aboba                         Experimental                     [Page 40]


Html markup produced by rfcmarkup 1.124, available from https://tools.ietf.org/tools/rfcmarkup/