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PPPEXT Working Group                                       Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Experimental
<draft-aboba-pppext-eapgss-04.txt>
1 July 2001

                    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 RFC2026.

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 (2001).  All Rights Reserved.

Abstract

The Extensible Authentication Protocol (EAP) provides a standard
mechanism for support of multiple authentication methods, including
public key, smart cards, Kerberos, One Time Passwords, and others. EAP
typically runs directly over the link layer without requiring IP and
therefore includes its own support for in-order delivery and re-
transmission. While EAP was originally developed for use with PPP, it is
also now in use with IEEE 802.

This document describes the EAP-GSS protocol, which enables the use of
GSS-API mechanisms within EAP. GSS-API provides for protected
negotiation of authentication methods via the SPNEGO mechanism. As a
result, any GSS-API mechanism supported by SPNEGO and providing initial
authentication can be used with EAP-GSS, including IAKERB. It is



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desirable to support GSS-API authentication methods within EAP, since
this permits developers creating GSS-API authentication methods to
leverage their development efforts. EAP-GSS includes support for
fragmentation and reassembly.

Table of Contents

1.     Introduction ..........................................    3
   1.1       Requirements language ...........................    3
   1.2       Terminology .....................................    3
2.     Protocol overview ...................... ..............    3
   2.1       EAP server as GSS-API initiator .................    4
   2.2       Peer as GSS-API initiator .......................    6
   2.3       Example topologies ..............................    7
3.     Detailed description of EAP-GSS protocol ..............   13
   3.1       EAP GSS packet format ...........................   13
   3.2       EAP GSS Request packet ..........................   14
   3.3       EAP GSS Response packet .........................   15
   3.4       Fragmentation ....... ...........................   16
   3.5       Retry behavior ..................................   19
   3.6       Identity verification ...........................   19
   3.7       Use of addresses ................................   20
   3.8       Credential reuse ................................   20
   3.9       ECP negotiation .................................   21
4.     References ............................................   21
5.     Security considerations ...............................   25
   5.1 Dictionary attacks ....................................   25
   5.2 Certificate revocation  ...............................   25
   5.3 Mutual authentication .................................   26
   5.4 Key management ........................... ............   26
6.     Acknowledgments .......................................   26
7.     Author's Addresses ....................................   26
Intellectual Property Statement ..............................   27
Full Copyright Statement .....................................   27

















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1.  Introduction

The Extensible Authentication Protocol (EAP) [5] provides a standard
mechanism for support of multiple authentication methods, including
public key [12], smart cards, Kerberos, One Time Passwords [5], and
others. EAP typically runs directly over the link layer without
requiring IP and therefore includes its own support for in-order
delivery and re-transmission. While EAP was originally developed for use
with PPP [1], it is also now in use with IEEE 802 [27].

This document describes the EAP-GSS protocol, which enables the use of
GSS-API mechanisms within EAP. GSS-API, described in [15], provides for
protected negotiation of authentication methods via the SPNEGO mechanism
[19]. As a result, any GSS-API mechanism supported by SPNEGO and
providing initial authentication can be used with EAP-GSS, including
IAKERB [18]. It is desirable to support GSS-API authentication methods
within EAP, since this permits developers creating GSS-API
authentication methods to leverage their development efforts. EAP-GSS
includes support for fragmentation and reassembly.

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 [11].

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

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

2.  Protocol overview

As described in [5], the EAP-GSS conversation will typically begin with
the authenticator and the peer negotiating EAP.  The authenticator will



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then typically send an EAP-Request/Identity packet to the peer, and the
peer will respond with an EAP-Response/Identity packet to the
authenticator, containing the peer's user-Id.

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 modes.

In the first mode, the EAP server acts as the GSS-API initiator, and the
peer acts as the GSS-API target.  In the second mode, which adds an
extra round-trip, the peer acts as the GSS-API initiator, and the EAP
server acts as the GSS-API target.  We discuss each mode in turn.

2.1.  EAP server as GSS-API initiator

As described in RFC 2284 [5], 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 method.

To initiate the conversation, the EAP-Server sends an EAP-Request packet
with EAP-Type=EAP-GSS. 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 authenticator!) 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
authenticator has been authenticated by the peer, and the
authenticator's indicated identity is provided in the src_name result,
along with an output_token to be encapsulated within an EAP-Response
packet with EAP-Type=EAP-GSS, and passed back to the EAP-Server.

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



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the peer.

The conversation (which can require as few as 2.5 round trips) appears
as follows:

Peer             Authenticator
------           -------------
                 EAP/Identity
         <-------Request

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

                  GSS_Init_sec_context(mutual_req_flag)
                  returns GSS_S_CONTINUE_NEEDED,
                  output_token

         <--------EAP Request
                  EAP-GSS
                  output_token

GSS_Accept_sec_context(input_token)
returns GSS_S_COMPLETE,
output_token

EAP Response -------->
EAP-GSS
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 [19], or if the EAP server knows the GSS-API method to be
used, but cannot initiate it (e.g. IAKERB, 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) bit
set, 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 returns GSS_S_CONTINUE_NEEDED status. The output_token is
then encapsulated within an EAP-Response packet with EAP-Type=EAP-GSS



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and sent to the authenticator.  If method negotiation is to be used,
then an initial negotiation token for the Simple and Protected GSS-API
Negotiation Mechanism (SPNEGO) [19] 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 authenticator
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 [19]. 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.





















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The conversation (which can require as few as 3.5 round trips) appears
as follows:

Authenticating Peer     Authenticator
-------------------     -------------
                        EAP-Request/
                      <- Identity
EAP-Response/
Identity (MyID) ->
                        EAP-Request/
                        EAP-Type=EAP-GSS
                      <- (GSS Start, S bit set)

GSS_Init_sec_context(mutual_req_flag)
  returns GSS_S_CONTINUE_NEEDED,
  output_token (SPNEGO)

EAP-Response/
EAP-Type=EAP-GSS
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

2.3.  Example topologies

While nominally the EAP conversation occurs between the authenticator
and the peer, the authenticator MAY act as a pass-through device, with
the EAP packets received from the peer being encapsulated for
transmission to a RADIUS server or backend security server, such as a
Kerberos KDC. In the discussion that follows, we will use the term  "EAP
server" to denote the ultimate endpoint conversing with the peer.

For use with EAP-GSS, two topologies are likely, one with a RADIUS
backend as well as a Kerberos KDC backend, and other using solely a



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Kerberos KDC backend. We discuss each of these topologies in turn.

In the other topology, EAP-GSS is used along with the GSS-API IAKERB
[18] or Kerberos V [20] mechanisms.  Where IAKERB is used, the
authenticator 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 [27] so that use of the Kerberos V
mechanism within EAP is not necessary.

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

In the examples below, conversations are provided for each topology.

2.3.1.  RADIUS backend

In the RADIUS backend topology, the authenticator functions as an EAP-
passthrough device, encapsulating EAP messages received from the peer
within RADIUS as described in [33], and passing them on to the RADIUS
server. In turn, EAP-Message attributes received from the RADIUS server
are de-capsulated by the authenticator and sent to the peer. In this
topology, the authenticator need not have knowledge of specific EAP or
GSS-API methods.























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A successful EAP-GSS/IAKERB authentication occurring in a topology with
an authenticator acting as an IAKERB proxy to a Kerberos KDC will appear
as below.

Peer             Authenticator            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
                                      <-(AS_REP)




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                 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.

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



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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
     authenticator 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 authenticator or
     transport of the session key from peer to authenticator), 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).

2.3.2.  Kerberos backend

In the solely Kerberos-based topology, EAP-GSS is used along with the
GSS-API IAKERB [18] or Kerberos V [20] mechanisms.  Where IAKERB is
used, the authenticator 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 [27] 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
authenticator needs to understand the EAP-GSS, GSS-API IAKERB, as well
as GSS-API Kerberos V mechanisms.






















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

Peer                Authenticator                    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.





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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 authenticator.

5.   Up until this point, the peer has not mutually authenticated with
     the authenticator, or exchanged a key with it. As a result, the
     peer and authenticator 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 authenticator and provides it with a session
     key.

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

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.



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Type

   14 - EAP GSS

Data

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


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      |     Flags     |      GSS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     GSS Message Length        |       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 EAP
   packet including the Code, Identifier, Length, Type, and GSS Response
   fields.

Type

   ? - EAP GSS








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Flags

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

   L = Length included
   M = More fragments
   S = EAP-GSS start
   R = Reserved

   The L bit (length included) is set to indicate the presence of the
   four octet GSS Message Length field, and MUST be set for the first
   fragment of a fragmented GSS message or set of messages. 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.

GSS Message Length

   The GSS Message Length field is four octets, and is present only if
   the L bit is set.  This field provides the total length of the GSS
   message or set of messages that is being fragmented.

GSS data

   The GSS data consists of the encapsulated GSS packet.

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      |     Flags     |      GSS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     GSS Message Length        |       GSS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

   2




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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 EAP
   packet including the Code, Identifier, Length, Type, and GSS data
   fields.

Type

   TBD - EAP GSS

Flags

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

   L = Length included
   M = More fragments
   S = EAP-GSS start
   R = Reserved

   The L bit (length included) is set to indicate the presence of the
   four octet GSS Message Length field, and MUST be set for the first
   fragment of a fragmented GSS message or set of messages. 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.

GSS Message Length

   The GSS Message Length field is four octets, and is present only if
   the L bit is set. This field provides the total length of the GSS
   message or set of messages that is being fragmented.

GSS data

   The GSS data consists of the encapsulated GSS packet.

3.4.  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 Multilink



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Maximum Received Reconstructed Unit (MRRU). As described in [2], 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
[2]. While this is desirable, there may be cases in which multi-link or
the MRRU LCP option cannot be negotiated. Also, since EAP methods must
also be usable within IEEE 802.1X [27], 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), and EAP-GSS Start (S) bits. The L flag is set
to indicate the presence of the four octet GSS Message Length field, 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 field is four octets, and 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.





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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.

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

Authenticating Peer     Authenticator
-------------------     -------------
                         EAP-Request/
                         <- Identity
EAP-Response/
Identity (MyID) ->
                         EAP-Request/
                           EAP-Type=EAP-GSS
                         <-(GSS Start, S bit set)


GSS_Init_sec_context(mutual_req_flag)
  returns GSS_S_CONTINUE_NEEDED,
  output_token (SPNEGO)

EAP-Response/
EAP-Type=EAP-GSS
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, 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



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                         <- (Fragment 3)

GSS_Init_sec_context(input_token)
  returns GSS_S_COMPLETE,
  output_token

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

3.5.  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.

3.6.  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



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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
authenticator, 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.

3.7.  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.  The IAKERB GSS-API method can
explicitly handle this, as described in [18]. However, Where Kerberos V
is negotiated [16], [20] 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.

3.8.  Credential reuse

Note that a peer with valid credentials may reuse those credentials in a
subsequent authentication. For example, a peer initially using the
IAKERB GSS-API method to obtain a TGT and a ticket to the authenticator
may subsequently reuse that ticket in an AP_REQ/AP_REP exchange that 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), while savings from out-of-band credential
reuse can be more substantial.

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 authenticator willing to accept credentials
from a previous authenticator, fast-handoff may be achieved. Credential
reuse may also prove useful during multi-link authentication.

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 [28], or the time elapsed
since the previous authentication attempt.




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If the peer attempts to reuse credentials that are not valid for the
authenticator, then no harm is done. The authenticator will respond with
an error and the peer can then 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 to
the authenticator within 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 by the
authenticator, then the peer can negotiate IAKERB on its next attempt
instead.

Note that for credential reuse to be possible while roaming, it is
necessary for authenticators to share the same key with the KDC.  If
this is not the case, then peers moving from one authenticator to
another will not be able to reuse authenticator tickets. 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, unless
the EAP authentication is remoted to a central authentication server.

3.9.  ECP negotiation

ECP, described in [6], supports unprotected cipher-suite negotiations
within PPP and is thus vulnerable to attack.  Since SPNEGO [19] supports
protected cipher-suite negotiation in the case where the negotiated
method provides authentication and integrity protection, use of SPNEGO
is preferable to ECP. Peers completing the GSS-API SPNEGO negotiation
will typically implicitly select a cipher-suite, which includes key
strength, encryption and hashing methods. As a result, a subsequent
Encryption Control Protocol (ECP) conversation [6], if it occurs, has a
predetermined result.

However, since the ECP-supported ciphersuites may not correspond to the
ciphersuites implicitly negotiated as part of SPNEGO, it may not be
possible for the ECP conversation to verify the ciphersuites implicitly
selected via SPNEGO. For example, the ECP methods defined in [9]-[10]
only support DES and 3DES transforms for confidentiality, and do not
support authentication or integrity protection. Thus, there is no
correspondence between existing ECP methods and the ciphersuites
available within GSS-API methods such as Kerberos [16]-[17].

4.  References


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

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



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[3]  Simpson, W., Editor, "PPP LCP Extensions." RFC 1570, January 1994.

[4]  Rivest, R., Dusse, S., "The MD5 Message-Digest Algorithm", RFC
     1321, April 1992.

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

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

[7]  National Bureau of Standards, "Data Encryption Standard", FIPS PUB
     46 (January 1977).

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

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

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

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

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

[13] D. Rand.  "The PPP Compression Control Protocol." RFC 1962, Novell,
     June 1996.

[14] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC
     2222, October 1997.

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

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

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

[18] 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-07.txt, June 2001.



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

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

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

[22] ISO/IEC 10038 Information technology - Telecommunications and
     information exchange between systems - Local area networks - Media
     Access Control (MAC) Bridges, (also ANSI/IEEE Std 802.1D- 1993),
     1993.

[23] ISO/IEC Final CD 15802-3 Information technology - Tele-
     communications and information exchange between systems - Local and
     metropolitan area networks - Common specifications - Part 3:Media
     Access Control (MAC) bridges, (current draft available as IEEE
     P802.1D/D15).

[24] IEEE Standards for Local and Metropolitan Area Networks: Draft
     Standard for Virtual Bridged Local Area Networks, P802.1Q/D8,
     January 1998.

[25] ISO/IEC 8802-3 Information technology - Telecommunications and
     information exchange between systems - Local and metropolitan area
     networks - Common specifications - Part 3:  Carrier Sense Multiple
     Access with Collision Detection (CSMA/CD) Access Method and
     Physical Layer Specifications, (also ANSI/IEEE Std 802.3- 1996),
     1996.

[26] IEEE Standards for Local and Metropolitan Area Networks: Demand
     Priority Access Method, Physical Layer and Repeater Specification
     For 100 Mb/s Operation, IEEE Std 802.12-1995.

[27] IEEE Standards for Local and Metropolitan Area Networks: Port based
     Network Access Control, IEEE Draft 802.1X/D11, March 2001.

[28] 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.

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





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

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

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

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

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

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

[36] 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.

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

[38] 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.

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

[40] 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.

[41] 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.







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5.  Security Considerations

5.1.  Dictionary attacks

As noted in [34]-[36], 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 [34], 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. As noted in [34],
the imposition of a password policy is likely only to decrease the
yield, but given access to sufficient exchanges, large scale password
compromise remains likely.

For this reason, when used on wireless networks, EAP-GSS SHOULD be used
only to negotiate methods thought to be invulnerable to offline
dictionary attacks against the on-the-wire protocol, such as public key
authentication techniques or those described in [37]-[40].  In
particular, Kerberos V without PKINIT [41] SHOULD NOT be used.  As noted
in [34], it has been proposed that Kerberos be augmented by utilizing
these methods as part of pre-authentication.  Such an enhancement would
be likely to address the vulnerability to dictionary attack.

5.2.  Certificate revocation

Since the EAP server is on the Internet during the EAP conversation, the
server 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



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after connecting to the Internet.

5.3.  Mutual authentication

It is highly recommended that a GSS-API method supporting mutual
authentication be selected during the SPNEGO negotiation. This addresses
vulnerabilities associated with rogue EAP servers, as well as avoiding
vulnerabilities associated with parallel one-way authentications.

5.4.  Key management

As a result of the EAP-GSS conversation, the EAP endpoints will mutually
authenticate and derive a session key for subsequent use in PPP or
802.11 WEP [28] encryption. Since the peer and EAP client reside on the
same machine, it is necessary for the EAP client module to pass the
session key to the layer 2 encryption module.

The situation may be more complex on the authenticator, which may or may
not reside on the same machine as the EAP server. In the case where the
EAP server and authenticator 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 authenticator. 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 session key negotiated between the peer and EAP server will
need to be transmitted to the authenticator.  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
authenticator and transfer the session key from the peer to the
authenticator.

6.  Acknowledgments

Thanks to Terence Spies, Paul Leach, and Mike Swift of Microsoft for
useful discussions of this problem space.

7.  Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 (425) 936-6605
EMail: bernarda@microsoft.com




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This document and translations of it may be copied and furnished to
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Expiration Date

This memo is filed as <draft-aboba-pppext-eapgss-04.txt>,  and  expires
January 15, 2002.















































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