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PPPEXT Working Group                                       Bernard Aboba
INTERNET-DRAFT                                                 Dan Simon
Category: Experimental                                         Microsoft
<draft-aboba-pppext-eapgss-08.txt>
23 October 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. The encapsulation of EAP on IEEE 802
links is described within IEEE 802.1X.

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, including IAKERB. Supporting



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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.

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 .......................    5
3.     Detailed description of EAP GSS protocol ..............    8
   3.1       EAP GSS packet format ...........................    8
   3.2       EAP GSS Request packet ..........................    9
   3.3       EAP GSS Response packet .........................   10
   3.4       Fragmentation ....... ...........................   11
   3.5       Retry behavior ..................................   14
   3.6       Identity verification ...........................   14
   3.7       Use of addresses ................................   15
4.     Key management ........................................   15
   4.1       Key derivation algorithm ........................   16
   4.2       Deriving session keys ...........................   17
5.     References ............................................   18
6.     Security considerations ...............................   22
   6.1       Dictionary attacks ..............................   22
   6.2       Certificate revocation  .........................   22
   6.3       Mutual authentication ...........................   23
   6.4       Credential reuse ................................   23
   6.5       Key transmission issues .........................   24
   6.6       Link layer ciphersuite negotiation ..............   25
7.     IANA Considerations ...................................   26
Appendix A - Example IAKERB topologies .......................   27
   A.1       RADIUS+KDC backend ..............................   28
   A.2       Kerberos KDC backend ............................   30
Appendix B - The Pseudorandom Function .......................   32
Acknowledgments ..............................................   34
Author's Addresses ...........................................   34
Intellectual Property Statement ..............................   34
Full Copyright Statement .....................................   35








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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 [21]. The
encapsulation of EAP on IEEE 802 links is described within IEEE 802.1X
[27].

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, including IAKERB [18].
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 [11].

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 session 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
          used in generation of the transient session keys for
          authentication, encryption, and IV-generation. So that the
          master session keys are to be usable with any ciphersuite,



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          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 for use with the chosen
          ciphersuite.

2.  Protocol overview

As described in [5], the EAP GSS conversation 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
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 way.

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 GSS-API 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 ().



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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, 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
the peer.

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
                  output_token

GSS_Accept_sec_context(input_token)
returns GSS_S_COMPLETE,
output_token

EAP Response -------->
EAP Type=EAP GSS



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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, 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) 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
and sent to the NAS.  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 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 [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.





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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 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 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















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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.












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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

   TBD - 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 PPP 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     NAS
-------------------     -------------
                         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
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.

3.7.  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.  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 [18]. However, where the Kerberos V protocol, described in
[16], is negotiated as a GSS-API method as described in [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.

4.  Key management

As a result of the EAP GSS conversation, the EAP endpoints will mutually
authenticate and derive a session key, which is known as the "master
key" in this specification.  Ultimately,  the master key is used to
produce the session keys for a growing number of ciphersuites.

In the most general case, authentication and encryption keys as well as
initialization vectors are required in each direction. Depending on the
negotiated ciphersuite, not all of these session keys may be used.
Within PPP, ciphersuites include DESEbis [9], 3DES [10], and MPPE [42].
For PPP DESEbis, a 56-bit encryption key is required in each direction;
for PPP 3DES, a 168-bit encryption key is needed in each direction; for
MPPE, 40-bit, 56-bit or 128-bit encryption keys can be required in each
direction, as described in [42],[43].  While these PPP ciphersuites
provide encryption, they do not provide a per-frame keyed message
integrity check (MIC).

Within 802.11, ciphersuites include WEP-40, described in [28], which
requires a 40-bit encryption key, the same in either direction; and
WEP-128, which requires a 104-bit encryption key, the same in either



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direction.  These ciphersuites also do not include a keyed MIC.
Recently, new ciphersuites have been proposed for use with 802.11 that
do provide per-frame authentication as well as encryption. These
methods, described in [44], require 128-bit authentication and
encryption keys in each direction, and are based on AES [45]-[48].

With the increase in the number of link layer ciphersuites, it is
undesirable for an EAP method to include ciphersuite-specific mechanisms
for session key derivation. This would require the specification to be
revised for operation with each new ciphersuite.

In fact, an EAP method may not even know the selected ciphersuite.  For
example, in PPP, negotiation of the ciphersuite is accomplished via ECP,
which occurs after authentication. As a result, during authentication,
the client, NAS and backend authentication server may not be able to
anticipate the negotiated ciphersuite.

As a result, it is best for an EAP method to provide master session keys
that can be used with any ciphersuite, and allow the NAS and EAP client
to derive ciphersuite-specific session keys from those master keys, once
the ciphersuite is known.

4.1.  Key derivation algorithm

EAP TLS, described in RFC 2716 [12], provides a mechanism for deriving
authentication and encryption keys as well as IVs in both directions
from the TLS master key.  Since EAP TLS cannot assume knowledge of the
negotiated ciphersuite, it provides master session keys large enough for
use with any ciphersuite, assuming that they will be suitably truncated
by the client and NAS, once the ciphersuite is known.  This technique is
therefore compatible with PPP and 802.11 ciphersuites, including both
stream and block ciphers. It is applicable to ciphersuites that provide
authentication only, encryption and authentication, or encryption only.

This specification reuses the RFC 2716 [12] key derivation technique,
with one major modification.  In EAP TLS, the TLS master key is
typically not available via an API for security reasons, and as a
result, EAP TLS utilizes the TLS PRF in the master session key
derivation. For similar reasons, EAP GSS implementations will typically
not be able to obtain access to the master key via GSS-API. However,
while GSS-API methods can call GSS_Wrap() to encrypt and GSS_GetMIC() to
generate authentication tokens based on the master key, they cannot
utilize the TLS PRF operating directly on the master key, since this is
not obtainable.  As a result, an intermediate step is required to
generate a "Pseudo-Master Key".

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



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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 given by K' =
     Extract_MIC(GSS_GetMIC("EAP GSS pseudo master key")). Here
     Extract_MIC() is a function that extracts the MIC portion of the
     authentication token returned by GSS_GetMIC().

[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).

[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 and authentication keys 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.

4.2.  Deriving session keys

The Master Send and Receive keys (Peer-to-Authenticator and
Authenticator-to-Peer encryption keys from the Peer's perspective and
Authenticator-to-Peer and Peer-to authenticator from the Authenticator's
perspective) are derived from the master key as described in the



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previous section.  40 , 56, 104 and 128-bit keys are derived using the
same algorithm defined for EAP TLS session keys in [12]. The only
difference is in the length of the keys and their effective strength: 56
and 40-bit keys are 8 octets in length, while 104 and 128-bit keys are
16 octets long. Separate keys are derived for the send and receive
directions of the session.

5.  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.

[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]  U.S. DoC/NIST, "Data encryption standard (DES)", FIPS 46-3, October
     25, 1999.

[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.





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[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-08.txt, August 2001.

[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.



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[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 Std 802.1X-2001, June 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.

[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.




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[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.

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

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

[44] 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.

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

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

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

[48] "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.

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

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






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6.  Security Considerations

6.1.  Dictionary attacks

As noted in [34]-[36], both Kerberos IV and V are vulnerable to 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 to
negotiate methods thought to be invulnerable to offline dictionary
attacks against the on-the-wire protocol. This includes public key
authentication techniques or password-based techniques described in
[37]-[40].

Kerberos V SHOULD NOT be used without extensions providing protection
against offline dictionary attacks.  As noted in [34], 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 PKINIT [41].

6.2.  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.





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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.

6.3.  Mutual authentication

It is 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.

6.4.  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.

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 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.

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.

If the peer attempts to reuse credentials that are not valid, then the
NAS will respond with an error 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 to the NAS 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 NAS, then the peer can negotiate IAKERB on its next
attempt instead.





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Note that credential reuse for the purpose of "fast handoff" has
significant limitations. For example, in order to reuse a Kerberos
ticket on a different NAS, it is necessary for NASes 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 requring communication between the NASes. Allowing
multiple NASes 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 [34]. The alternative is for the new NAS to pass the
submitted ticket and authenticator to a backend authentication server or
previous NAS for validation. Having to communicate with the backend
authentication server loses much of the benefits of "fast handoff" and
if one presumes existence of such an Inter-Access Point Protocol (IAPP)
then EAP-based "fast handoff" would not be necessary in the first place.

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 [16], 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 [29] or tunnel attributes [32] 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.

6.5.  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.



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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.

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.

6.6.  Link layer ciphersuite negotiation

SPNEGO [19] supports protected ciphersuite negotiation in the case where
the negotiated method provides authentication and integrity protection.
As a result, SPNEGO negotiations are typically more secure than than
unprotected link layer ciphersuite negotiations. For example, ECP,
described in [6], supports unprotected cipher-suite negotiations within
PPP and is thus vulnerable to attack. Similarly, 802.11, described in
[28], currently 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 link layer ciphersuite negotiation mechanisms.

Nevertheless, this presents substantial difficulties.  The available
link layer ciphersuites may not correspond to the ciphersuites
implicitly negotiated as part of SPNEGO, and therefore it may be
difficult to use SPNEGO in this way. For example, 802.11 [28] currently
only supports Wired Equivalency Privacy (WEP), a flawed cipher based on



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RC4 which lacks a keyed message integrity check, and therefore does not
support per-frame authentication and integrity protection. Similarly,
the PPP ECP methods defined in [9]-[10] support DESEbis and 3DES
transforms for confidentiality, but do not support per-frame
authentication or integrity protection. Thus, there is no correspondence
between 802.11 or PPP ciphersuites and the ciphersuites available within
GSS-API methods such as Kerberos V [16]-[17]. As a result, the use of
SPNEGO for link-layer ciphersuite negotiation is not recommended.

7.  IANA Considerations

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






































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Appendix A - Example IAKERB topologies

Where EAP GSS is used along with the GSS-API IAKERB [18] or Kerberos V
[20] mechanisms, two major topologies are possible:

RADIUS+KDC backend
     Here a RADIUS backend is used, along with a Kerberos KDC backend.
     The NAS functions as an EAP-pass-through device, encapsulating EAP
     messages received from the peer within RADIUS as described in [33],
     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 encapsulated packets from the Kerberos KDC in
     EAP GSS/IAKERB and send this 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 [27] 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.













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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



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                                      <-(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.





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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 [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 NAS 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
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.





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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 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 [49].

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 [50].

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



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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).





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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) 936-6605
EMail: bernarda@microsoft.com

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

EMail: dansimon@microsoft.com
Phone: +1 425 936 6711
Fax:   +1 425 936 7329

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to the implementation or use of the technology described in this
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The IETF invites any interested party to bring to its attention any
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standard.  Please address the information to the IETF Executive
Director.





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Full Copyright Statement

Copyright (C) The Internet Society (2001).  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,
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Standards process must be followed, or as required to translate it into
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INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Expiration Date

This  memo is filed as <draft-aboba-pppext-eapgss-08.txt>,  and  expires
May 23, 2002.
























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