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Versions: (RFC 2284) 00 01 02 03 04 05 06 07 08 09 10

PPPEXT Working Group                                            L. Blunk
INTERNET-DRAFT                                      Merit Networks, Inc.
Category: Standards Track                                  J. Vollbrecht
<draft-ietf-pppext-rfc2284bis-07.txt>           Interlink Networks, Inc.
21 October 2002                                            Bernard Aboba
Obsoletes: RFC 2284                                            Microsoft


                Extensible Authentication Protocol (EAP)

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

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

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

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

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

Copyright Notice

Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

This document defines the Extensible Authentication Protocol (EAP), an
authentication protocol which supports multiple authentication
mechanisms. EAP typically runs directly over the link layer without
requiring IP and therefore includes its own support for in-order
delivery and retransmission. Fragmentation is not supported within EAP
itself; however, individual EAP methods may support this.  While EAP was
originally developed for use with PPP, it is also now in use with IEEE
802.

This document obsoletes RFC 2284.







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Table of Contents

1.     Introduction ..........................................    3
   1.1       Specification of Requirements ...................    3
   1.2       Terminology .....................................    3
2.     Extensible Authentication Protocol (EAP) ..............    4
   2.1       EAP multiplexing model ..........................    6
3.     Media-specific issues .................................    7
   3.1       Lower layer assumptions .........................    7
   3.2       EAP usage within PPP ............................    8
   3.3       EAP usage within IEEE 802 .......................   10
4.     EAP Packet Format .....................................   10
   4.1       Request and Response ............................   11
   4.2       Success and Failure .............................   13
5.     Initial EAP Request/Response Types ....................   14
   5.1       Identity ........................................   14
   5.2       Notification ....................................   16
   5.3       Nak .............................................   16
   5.4       MD5-Challenge ...................................   18
   5.5       Vendor-specific .................................   18
6.     IANA Considerations ...................................   20
   6.1       Definition of Terms .............................   20
   6.2       Recommended Registration Policies ...............   21
7.     Security considerations ...............................   21
   7.1       Threat model ....................................   22
   7.2       Security requirements ...........................   23
   7.3       Identity protection .............................   24
   7.4       Man-in-the-middle attacks .......................   25
   7.5       Packet modification attacks .....................   26
   7.6       Denial of service attacks .......................   26
   7.7       Dictionary attacks ..............................   27
   7.8       Connection to an untrusted network ..............   27
   7.9       Negotiation attacks .............................   27
   7.10      Implementation idiosyncracies ...................   28
   7.11      Key derivation ..................................   28
   7.12      Weak ciphersuites ...............................   29
8.     Normative references ..................................   29
9.     Informative references ................................   30
ACKNOWLEDGMENTS ..............................................   31
AUTHORS' ADDRESSES ...........................................   31
Full Copyright Statement .....................................   32










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1.  Introduction

This document defines the Extensible Authentication Protocol (EAP), an
authentication protocol which supports multiple authentication
mechanisms. EAP typically runs directly over the link layer without
requiring IP and therefore includes its own support for in-order
delivery and retransmission. Fragmentation is not supported within EAP
itself; however, individual EAP methods may support this.  While EAP was
originally developed for use with PPP, it is also now in use with IEEE
802.

1.1.  Specification of Requirements

In this document, several words are used to signify the requirements of
the specification.  These words are often capitalized.  The key words
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].

1.2.  Terminology

This document frequently uses the following terms:

Authenticator
          The end of the link requiring the authentication.

Peer      The other end of the point-to-point link (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.

Silently Discard
          This means the implementation discards the packet without
          further processing.  The implementation SHOULD provide the
          capability of logging the error, including the contents of the
          silently discarded packet, and SHOULD record the event in a
          statistics counter.

Displayable Message
          This is interpreted to be a human readable string of
          characters, and MUST NOT affect operation of the protocol.
          The message encoding MUST follow the UTF-8 transformation



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          format [RFC2044].

2.  Extensible Authentication Protocol (EAP)

The Extensible Authentication Protocol (EAP)  is a general protocol for
authentication which supports multiple authentication mechanisms.  EAP
may be used on dedicated links as well as switched circuits, and wired
as well as wireless links.

To date, EAP has been implemented with hosts and routers that connect
via switched circuits or dial-up lines using PPP [RFC1661]. It has also
been implemented with switches and access points using IEEE 802
[IEEE802].  EAP encapsulation on IEEE 802 media is described in
[IEEE8021X].

One of the advantages of the EAP architecture is its flexibility.  EAP
is used to select a specific authentication mechanism, typically after
the Authenticator requests more information in order to determine the
specific authentication mechanism(s) to be used.  Rather than requiring
the Authenticator to be updated to support each new authentication
method, EAP permits the use of a backend server which actually
implements the various mechanisms while the Authenticator merely passes
through the authentication exchange.

The authentication exchange proceeds as follows:

[1]  The Authenticator sends a Request to authenticate the Peer.  The
     Request has a type field to indicate what is being requested.
     Examples of Request types include Identity,  MD5-challenge, etc.
     The MD5-challenge type corresponds closely to the CHAP
     authentication protocol [RFC1994].  Typically, the Authenticator
     will send an initial Identity Request; however, an initial Identity
     Request is not required, and MAY be bypassed. For example, the
     identity may not be required where it is requested within the EAP
     method itself, which is pre-determined; where it is determined by
     the port to which the Peer has connected (leased lines, dedicated
     switch or dial-up ports); or where the identity is obtained in
     another fashion (via calling station identity or MAC address, in
     the Name field of the MD5-Challenge Response, etc.).

[2]  The Peer sends a Response packet in reply to the Request.  As with
     the Request packet, the Response packet contains a type field which
     corresponds to the type field of the Request.

[3]  The Authenticator sends an additional Request packet, and the Peer
     replies with a Response. The sequence of Requests and Responses
     continues as long as needed.




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[4]  If the initial authentication method completes unsuccessfully, then
     the Authenticator sends a Failure packet to the Peer. If it
     completes successfully, and the Authenticator does not require
     additional authentication methods, then the Authenticator sends a
     Success  packet to the Peer.

[5]  An Authenticator MAY authenticate the Peer using a sequence of
     methods. A common example of this is an Identity request followed
     by an EAP authentication method such as an MD5-Challenge.  If
     additional authentication methods are required, the Authenticator
     MAY send a Request packet for a subsequent authentication method,
     or it MAY send another Identity request. The Peer will then respond
     with a Response packet containing a type field matching the
     Request.

[6]  The sequence of authentication methods proceeds until either an
     authentication method fails (in which case the Authenticator sends
     an Failure packet to the Peer) or the final authentication method
     completes successfully, in which case the Authenticator sends an
     Success packet to the Peer.

Whether authentication is mandatory is determined by the Authenticator
and Peer configurations. If authentication is not required by the
Authenticator, or if the identity of the Peer is verified by another
mechanism (e.g. Calling-Station-ID or MAC address), then the
Authenticator MAY send a "canned" Success message.

Where the Authenticator acts as a pass-through, it MUST determine the
outcome of the authentication solely based on the Accept/Reject
indication sent by the backend authentication server; the outcome MUST
NOT be determined by the contents of an EAP packet sent along with the
Accept/Reject indication, or the absence of such a packet.

On the Peer, the determination of whether the authentication has
succeeded or failed is based on Peer configuration, and the EAP method
that has been selected, and not merely by receipt of a Success and
Failure message. If the Peer is configured to require an EAP method
providing mutual authentication, then the Peer MUST silently discard a
Success packet sent by the Authenticator, prior to the conclusion of
mutual authentication.  Since the Success message is not protected, it
may be spoofed, or sent by a rogue Authenticator seeking to avoid having
to authenticate to the Peer. It should be understood that nothing within
the EAP specification precludes the Peer from silently discarding such a
premature Success packet, and it is the responsibility of individual EAP
methods to define the ponit at which a Peer may accept a Success from
the Authenticator.





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Advantages

   The EAP protocol can support multiple authentication mechanisms
   without having to pre-negotiate a particular one.

   Certain devices (e.g. a NAS, switch or access point) do not
   necessarily have to understand each request type and MAY be able to
   act as a pass-through agent for a backend authentication server.
   Separation of the Authenticator from the backend authentication
   server simplifies credentials management and policy decision making.
   However, support for pass-through is not required.

Disadvantages

   For use in PPP, EAP does require the addition of a new authentication
   type to PPP LCP and thus PPP implementations will need to be modified
   to use it. It also strays from the previous PPP authentication model
   of negotiating a specific authentication mechanism during LCP.
   Similarly, switch or access point implementations need to support
   [IEEE8021X] in order to use EAP.

   Where the Authenticator is separate from the backend authenticator
   server, this complicates the security analysis and, if needed, key
   distribution.

2.1.  EAP multiplexing model

Within EAP, the Type field functions much like a port number in UDP or
TCP.  With the exception of types handled by the EAP layer, it is
assumed that EAP implementations multiplex incoming EAP packets
according to their Type, and deliver them only to the EAP method
corresponding to that Type code, with some exceptions.

The Nak method is utilized for the purposes of method negotiation, and
is implemented within the EAP layer. Peers MUST respond to an EAP
Request for an unacceptable Type with a Nak Response. Authenticators
receiving an EAP Response with a Type for which the authenticator has no
outstanding Request MUST silently discard the Response.

The Notification method may be considered to reside within the EAP
layer.  While a Notification Request may be generated at the behest of
an EAP method residing on the authenticator, a Notification Response is
typically generated automatically by the peer EAP layer, so that it can
only be used as confirmation that the peer received the Notification
Request, not that it has processed it, or displayed the message to the
user. It cannot be assumed that the contents of the Notification Request
is available to another method.




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The Identity Request may also be generated at the behest of an EAP
method residing on the authenticator. However, the Identity Response can
be assumed to be stored within the EAP layer so as to be available to
methods of types other than 1 (Identity).

EAP packets with codes of Success or Failure do not include a Type, and
therefore are not delivered to an EAP method.

Given these considerations, the Success, Failure, Nak and Notification
messages MUST NOT used to carry data destined for delivery to other EAP
methods.  The architecture is illustrated in figure 1 on the next page.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           |           |             |  |           |           |             |
| EAP method| EAP method| Native      |  | EAP method| EAP method| Native      |
| Type = X  | Type = Y  | EAP         |  | Type = X  | Type = Y  | EAP         |
|           |           | Methods     |  |           |           | Methods     |
|       !   |           | (MD5)       |  |       ^   |           | (MD5)       |
|       !   |           |             |  |       !   |           |             |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       !                             |  |       !                             |
|       !   EAP                       |  |       !   EAP                       |
| (Nak, ! Success,Failure,Notif.,Id)  |  | (Nak, !  Success,Failure,Notif.,Id) |
|       !                             |  |       !                             |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       !                             |  |       !                             |
|       !  Link Layer                 |  |       !  Link Layer                 |
|       !                             |  |       !                             |
+-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                        !
        !                                        !
        +----------------------->----------------+

Figure 1. EAP Multiplexing Model

3.  Media-specific issues

EAP has been run over a variety of lower layers including PPP, wired
IEEE 802 LANs, and IEEE 802.11 wireless LANs [IEEE802.11]; UDP (L2TP
[RFC2661], PIC [PIC]) and TCP [PIC]. This section discusses EAP
dependencies on lower layers.

3.1.  Lower layer assumptions

EAP makes the following assumptions about lower layers:

[1]  Unreliable transmission. With the exception of Success and Failure,
     EAP packets are acknowledged, so that EAP does not assume that



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     lower layers are reliable. However, if lower layers exhibit a high
     loss rate, then frequent timeouts are likely to result.  Since EAP
     has its own retransmission behavior, when run over a reliable lower
     layer, it is possible for retransmission to occur both at the lower
     layer and the EAP layer.

[2]  Known MTU. EAP itself does not support fragmentation and
     reassembly. However, individual EAP methods SHOULD NOT make
     assumptions about the minimum MTU size and SHOULD be capable of
     handling fragmentation and reassembly. As a result, EAP is
     typically capable of functioning across a range of MTU sizes, as
     long as the MTU is known.

[3]  Possible duplication.  While it is desirable that lower layers
     provide for non-duplication, this is not a requirement. The
     Identifier field provides both the Peer and Authenticator with the
     ability to detect duplicates.

     Where the lower layer is reliable, it will provide the EAP layer
     with a non-duplicated stream of packets.

     In EAP, the Authenticator is responsible for retransmission.
     Although EAP Peers do not retransmit Responses, they do respond to
     duplicate Requests. Since it is possible for the Authenticator to
     retransmit before receiving a Response from the Peer,
     Authenticators can receive duplicate Responses.

[4]  Possible reordering. EAP provides its own mechanisms to detect
     reordering and so does not assume that the lower layer provides
     ordering guarantees. EAP is a "lockstep" protocol, so that the
     Authenticator MUST NOT send a new Request until a Response is
     received to an outstanding Request. Since the Peer should not
     expect a new Request until it has sent a Response, if it receives a
     new Request with a different Identifier before sending a Response
     to the outstanding Request, the new Request MUST be silently
     discarded. Similarly, if the Authenticator receives a Response with
     an Identifier that does not match the Identifier in the outstanding
     Request, the Response MUST be silently discarded.

3.2.  EAP usage within PPP

In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure the data
link during Link Establishment phase.  After the link has been
established, PPP provides for an optional Authentication phase before
proceeding to the Network-Layer Protocol phase.





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By default, authentication is not mandatory. If authentication of the
link is desired, an implementation MUST specify the Authentication-
Protocol Configuration Option during Link Establishment phase.

The server can use the identification of the connecting host or router
in the selection of options for network layer negotiations.

When implemented within PPP, EAP does not select a specific
authentication mechanism at PPP Link Control Phase, but rather postpones
this until the Authentication Phase.  This allows the Authenticator to
request more information before determining the specific authentication
mechanism.  This also permits the use of a "back-end" server which
actually implements the various mechanisms while the PPP Authenticator
merely passes through the authentication exchange.  The PPP Link
Establishment and Authentication phases, and the Authentication-Protocol
Configuration Option, are defined in The Point-to-Point Protocol (PPP)
[RFC1661].

3.2.1.  PPP Configuration Option Format

A summary of the PPP Authentication-Protocol Configuration Option format
to negotiate the EAP Authentication Protocol is shown below.  The fields
are transmitted from left to right.

Exactly one EAP packet is encapsulated in the Information field of a PPP
Data Link Layer frame where the protocol field indicates type hex C227
(PPP EAP).

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |     Authentication-Protocol   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   3

Length

   4

Authentication-Protocol

   C227 (Hex) for PPP Extensible Authentication Protocol (EAP)






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3.3.  EAP usage within IEEE 802

The encapsulation of EAP over IEEE 802 link layers is defined in
[IEEE8021X].  The IEEE 802 encapsulation of EAP does not involve PPP,
and IEEE 802.1X does not include support for link or network layer
negotiations. As a result, within IEEE 802.1X it is not possible to
negotiate non-EAP authentication mechanisms, such as PAP or CHAP
[RFC1994].

4.  EAP Packet format

A summary of the EAP 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             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+

Code

   The Code field is one octet and identifies the type of EAP packet.
   EAP Codes are assigned as follows:

         1       Request
         2       Response
         3       Success
         4       Failure

   Since EAP only defines Codes 1-4, EAP packets with other codes MUST
   be silently discarded by both Authenticators and Peers.

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 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.

Data



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   The Data field is zero or more octets.  The format of the Data field
   is determined by the Code field.

4.1.  Request and Response

Description

   The Request packet is sent by the Authenticator to the Peer.  Each
   Request has a type field which serves to indicate what is being
   requested.  The Authenticator MUST transmit an EAP packet with the
   Code field set to 1 (Request).  Additional Request packets MUST be
   sent until a valid Response packet is received, or an optional retry
   counter expires.  For IEEE 802.1X, the retry counter is effectively
   set to zero, so that retransmission never occurs, and instead the
   Peer times out and authentication is restarted.

   The contents of the data field is dependent on the Request type. The
   Peer MUST send a Response packet in reply to a Request packet.
   Responses MUST only be sent in reply to a received Request and never
   retransmitted on a timer.  The Identifier field of the Response MUST
   match that of the Request.  If a Peer receives a duplicate Request
   for which it has already sent a Response, it MUST resend its
   Response.  If a Peer receives a duplicate Request before it has sent
   a Response to the initial Request (i.e. it's waiting for user input),
   it MUST silently discard the duplicate Request.

   Retransmitted Requests MUST be sent with the same Identifier value in
   order to distinguish them from new Requests. In order to avoid
   confusion between new Requests and retransmissions, the Identifier
   value chosen for each new Request MUST be unique within the EAP
   conversation. One way to achieve this is to start the Identifier at
   an initial value and increment it for each new Request. If the
   Authenticator receives a Response with an Identifier different from
   the Identifier within the last Request, then the message MUST be
   silently discarded. Since it is possible that the Authenticator may
   retransmit prior to receiving a Response from the Peer, the
   Authenticator can receive duplicate responses.  Note that the
   Identifier field need only be unique on a per-port basis, so that
   Authenticators are not restricted to only 255 simultaneous
   authentication conversations.

      Implementation Note: Because the authentication process will often
      involve user input, some care must be taken when deciding upon
      retransmission strategies and authentication timeouts.  By default
      where EAP is run over an unreliable lower layer, the EAP
      retransmission timer (EAP_RTO) SHOULD be computed as described in
      [RFC2988]. A maximum of 3-5 retransmissions is suggested.




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      When run over a reliable lower layer (e.g. EAP over ISAKMP/TCP, as
      within [PIC]), the EAP retransmission timer SHOULD be set to an
      infinite value, so that retransmissions do not occur at the EAP
      layer.

      However, where the authentication process requires user input, the
      measured round trip times will have more to do with user
      responsiveness than network characteristics, so that dynamic
      estimation of EAP_RTO is not helpful.  Instead, the retransmission
      timers SHOULD be set so as to provide sufficient time for the user
      to respond, with longer timeouts required in certain cases, such
      as where Token Cards are involved.

      This can be accomplished by allowing the timeout value to be set
      by the EAP method and communicated to the Authenticator by the
      backend authentication server (such as via the RADIUS Session-
      Timeout attribute).  Additionally, the Peer MUST be prepared to
      silently discard received retransmissions while waiting for user
      input, so as to mitigate the ill effects of a too small
      retransmission timer.

   A summary of the Request and 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      |  Type-Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Code

   1 for Request
   2 for 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 Type-Data
   fields.  Octets outside the range of the Length field should be
   treated as Data Link Layer padding and should be ignored on



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   reception.

Type

   The Type field is one octet.  This field indicates the Type of
   Request or Response. A single Type MUST be specified for each EAP
   Request or Response.  Normally, the Type field of the Response will
   be the same as the Type of the Request.  However, there is also a Nak
   Response Type for indicating that a Request type is unacceptable to
   the peer. An initial specification of Types follows in a later
   section of this document.

Type-Data

   The Type-Data field varies with the Type of Request and the
   associated Response.

4.2.  Success and Failure

   The Success packet is sent by the Authenticator to the Peer to
   acknowledge successful completion of the authentication conversation.
   The Authenticator MUST transmit an EAP packet with the Code field set
   to 3 (Success).  If the Authenticator cannot authenticate the Peer
   (unacceptable Responses to one or more Requests) then the
   implementation MUST transmit an EAP packet with the Code field set to
   4 (Failure).  An Authenticator MAY wish to issue multiple Requests
   before sending a Failure response in order to allow for human typing
   mistakes.  Success and Failure packets MUST NOT contain additional
   data.

      Implementation Note: Because the Success and Failure packets are
      not acknowledged, the Authenticator cannot know whether they have
      been received. As a result, these packets are not retransmitted by
      the Authenticator, and if they are lost, the Peer will timeout. If
      acknowledged success and failure indications are desired, these
      MAY be implemented within individual EAP methods.

   A summary of the Success and Failure 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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code




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   3 for Success
   4 for Failure

Identifier

   The Identifier field is one octet, and aids in matching Responses
   with Requests. In order to avoid confusion between Success or Failure
   packets and retransmissions, the Identifier value chosen for a
   Success or Failure packet MUST be unique within the EAP conversation.

Length

   4

5.  Initial EAP Request/Response Types

This section defines the initial set of EAP Types used in
Request/Response exchanges.  More Types may be defined in follow-on
documents.  The Type field is one octet and identifies the structure of
an EAP Request or Response packet.  The first 3 Types are considered
special case Types.

The remaining Types define authentication exchanges.  The Nak Type is
valid only for Response packets, it MUST NOT be sent in a Request.  The
Nak Type MUST only be sent in response to a Request which uses an
authentication Type code (i.e., Type > 3).

All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 255. Follow-on RFCs MAY define
additional EAP Types.

   1       Identity
   2       Notification
   3       Nak (Response only)
   4       MD5-Challenge
 255       Vendor-specific

5.1.  Identity

Description

   The Identity Type is used to query the identity of the peer. The
   Authenticator will typically issue this as the initial Request;
   however, an Identity Request MAY also be sent after completion of
   another method, and MAY be sent multiples times within a sequence of
   method. An Identity Request MUST NOT be sent in the middle of another
   method conversation.




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   An optional displayable message MAY be included within an Identity
   Request, to prompt the Peer in the case where there is an expectation
   of interaction with a user. An Identity Response MUST be sent to an
   Identity Request. A peer MUST NOT respond to an Identity Request with
   a NAK.

   Any executing method on the authenticator can be assumed to have
   access to the Identity Response(s) returned by the peer, even though
   those messages have Type = 1 (Identity). Since the Identity Request
   is often sent automatically by the authenticator, the Identity method
   may be thought of as residing within the EAP layer and providing
   services to the method layer.

   Since Identity Requests and Responses are not protected, from a
   security perspective, it may be preferrable for protected method-
   specific Identity exchanges to be used instead.  Since sending of
   Identity Requests by authenticators is optional, implementations
   SHOULD provide a way to configure the authenticator so that Identity
   Requests are not sent. However, it is possible that a method residing
   on the authenticator may dependo on the Identity Response, and and so
   authenticator implementations SHOULD provide a way for a method to
   invoke sending of the Identity Request, in case no method-specific
   identity exchange is supported, and a claim of identity is required.

      Implementation Note:  The Peer MAY obtain the Identity via user
      input.  It is suggested that the Authenticator retry the Identity
      Request in the case of an invalid Identity or authentication
      failure to allow for potential typos on the part of the user.  It
      is suggested that the Identity Request be retried a minimum of 3
      times before terminating the authentication phase with a Failure
      reply.  The Notification Request MAY be used to indicate an
      invalid authentication attempt prior to transmitting a new
      Identity Request (optionally, the failure MAY be indicated within
      the message of the new Identity Request itself).

Type

   1

Type-Data

   This field MAY contain a displayable message in the Request,
   containing UTF-8 encoded 10646 characters [RFC2279].  The Response
   uses this field to return the Identity.  If the Identity is unknown,
   this field should be zero bytes in length.  The field MUST NOT be
   null terminated.  The length of this field is derived from the Length
   field of the Request/Response packet and hence a null is not
   required.



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5.2.  Notification

Description

   The Notification Type is optionally used to convey a displayable
   message from the authenticator to the peer. An authenticator MAY send
   a Notification Request to the peer at any time, including in the
   middle of an ongoing method conversation. The peer MUST respond to a
   Notification Request with a Notification Response; a NAK Response
   MUST NOT be sent. A Notification Response is typically generated
   automatically by the EAP layer; therefore the contents of a
   Notification Request MUST NOT be delivered to a peer method other
   than Notification.

   The Peer SHOULD display this message to the user or log it if it
   cannot be displayed. The Notification Type is intended to provide an
   acknowledged notification of some imperative nature, but it is not an
   error indication, and therefore does not change the state of the
   peer. Examples include a password with an expiration time that is
   about to expire, an OTP sequence integer which is nearing 0, an
   authentication failure warning, etc. In most circumstances,
   notification should not be required.

Type

   2

Type-Data

   The Type-Data field in the Request contains a displayable message
   greater than zero octets in length, containing UTF-8 encoded 10646
   characters [RFC2279].  The length of the message is determined by
   Length field of the Request packet.  The message MUST NOT be null
   terminated.  A Response MUST be sent in reply to the Request with a
   Type field of 2 (Notification).  The Type-Data field of the Response
   is zero octets in length.   The Response should be sent immediately
   (independent of how the message is displayed or logged).

5.3.  Nak

Description

   The Nak Type is valid only in Response messages. It is sent in reply
   to a method proposal (the initial EAP Request for a given Type) where
   the desired authentication Type is unacceptable.  Authentication
   Types are numbered 4 and above. The Response contains zero or more
   authentication Types desired by the peer. A Nak with no
   authentication Type(s) indicates that the Peer does not wish to



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   authenticate using the proposed method but is not proposing an
   alternative.

   Since the Nak Type is only valid in Responses and has very limited
   functionality, it MUST NOT be used as a general purpose error
   indication, such as for communication of error messages, or
   negotiation of parameters specific to a particular EAP method.  As a
   result, a NAK is only sent in response to a method proposal and MUST
   NOT be sent in the middle of an EAP method conversation.

Code

   2 for Response.

Identifier

   The Identifier field is one octet and aids in matching Responses with
   Requests. The Identifier field of a Nak Response MUST match the
   Identifier field of the Request packet that it is sent in response
   to.

Length

   >=5

Type

   3

Type-Data

   This field MUST contain zero or more octets indicating the desired
   authentication Types, in order of preference, with the most preferred
   method first. In order to avoid an interminable negotiation, the Peer
   MUST only include Types that it is willing to accept.
















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5.4.  MD5-Challenge

Description

   The MD5-Challenge Type is analogous to the PPP CHAP protocol
   [RFC1994] (with MD5 as the specified algorithm).  The Request
   contains a "challenge" message to the peer.  A Response MUST be sent
   in reply to the Request.  The Response MAY be either of Type 4
   (MD5-Challenge) or Type 3 (Nak). The Nak reply indicates zero or more
   of the peer's desired authentication Types.  All EAP implementations
   MUST support the MD5-Challenge mechanism.

   Note that the use of the Identifier field in the MD5-Challenge Type
   is different from that described in [RFC1994].  EAP allows for
   retransmission of MD5-Challenge Request packets while RFC 1994 states
   that both the Identifier and Challenge fields MUST change each time a
   Challenge (the CHAP equivalent of the MD5-Challenge Request packet)
   is sent.

Type

   4

Type-Data

   The contents of the Type-Data  field is summarized below.  For
   reference on the use of this fields see the PPP Challenge Handshake
   Authentication Protocol [RFC1994].

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Value-Size   |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Name ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.5.  Vendor-specific

Description

   Due to EAP's popularity, the original Method Type space, which only
   provides for 255 values, is being allocated at a pace, which if
   continued, would result in exhaustion within a few years.  Since many
   of the existing uses of EAP are vendor-specific, the Vendor-Specific
   Method Type is available to allow vendors to support their own
   extended Types not suitable for general usage. The Vendor-specific
   Type may also be used to expand the global Method Type space beyond



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   the original 255 values.

   Peers not equipped to interpret the Vendor-specific Type MUST send a
   Nak, and negotiate a more suitable authentication method.

   A summary of the Vendor-specific Type 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |               Vendor-Id                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   255 for Vendor-specific

Vendor-Id

   The Vendor-Id is 3 octets and represents the SMI Network Management
   Private Enterprise Code of the Vendor in network byte order, as
   allocated by IANA. A Vendor-Id of zero is reserved for use by the
   IETF in providing an expanded global EAP Type space.

String

   The String field is one or more octets.  The actual format of the
   information is site or application specific, and a robust
   implementation SHOULD support the field as undistinguished octets.

   The codification of the range of allowed usage of this field is
   outside the scope of this specification.

   It SHOULD be encoded as follows.  The Vendor-Specific field is
   dependent on the vendor's definition of that attribute.  An example
   encoding of the Vendor-Specific attribute using this method follows.












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Example Implementation

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |               Vendor-Id                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Vendor-Type                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Vendor-Specific...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Vendor-Type

   The Vendor-Type field is four octets and represents the vendor-
   specific Method Type. Where a Vendor-Id of zero is present, the
   Vendor-Type field provides an expanded global EAP Type space,
   beginning with EAP Type values of 256.

Vendor-Specific

   The Vendor-Specific field is dependent on the vendor's definition of
   that attribute. Where a Vendor-Id of zero is present, the Vendor-
   Specific field will be used for transporting the contents of EAP
   Methods of Types 256 or greater.

6.  IANA Considerations

This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].

There are two name spaces in EAP that require registration: Packet Codes
and Method Types.

EAP is not intended as a general-purpose protocol, and allocations
SHOULD NOT be made for purposes unrelated to authentication.

6.1.  Definition of Terms

The following terms are used here with the meanings defined in BCP 26:
"name space", "assigned value", "registration".

The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".




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6.2.  Recommended Registration Policies

For registration requests where a Designated Expert should be consulted,
the responsible IESG area director should appoint the Designated Expert.
For Designated Expert with Specification Required, the request is posted
to the EAP WG mailing list (or, if it has been disbanded, a successor
designated by the Area Director) for comment and review, and MUST
include a pointer to a public specification. Before a period of 30 days
has passed, the Designated Expert will either approve or deny the
registration request and publish a notice of the decision to the EAP WG
mailing list or its sucessor. A denial notice must be justified by an
explanation and, in the cases where it is possible, concrete suggestions
on how the request can be modified so as to become acceptable.

For registration requests requiring Expert Review, the EAP mailing list
should be consulted. If the EAP mailing list is no longer operational,
an alternative mailing list may be designated by the responsible IESG
Area Director.

Packet Codes have a range from 1 to 255, of which 1-4 have been
allocated. Because a new Packet Code has considerable impact on
interoperability, a new Packet Code requires Standards Action, and
should be allocated starting at 5.

The original EAP Method Type space has a range from 1 to 255, and is the
scarcest resource in EAP, and thus must be allocated with care.  Method
Types 1-36 have been allocated, with 20 available for re-use. Method
Types 37-191 may be allocated following Designated Expert, with
Specification Required.

Release of blocks of Method Types (more than one for a given purpose)
should require IETF Consensus.  EAP Type Values 192-254 are reserved and
allocation requires Standards Action.

Method Type 255 is allocated for Vendor-Specific extensions and the use
of that should be encouraged instead of allocation from the original
global Method Type space, for functions specific only to one vendor's
implementation of EAP, where no interoperability is deemed useful.

When used with a Vendor-Id of zero, Method Type 255 can also be used to
provide for an expanded Method Type space.  Expanded Method Type values
256-4294967295 may be allocated after Type values 1-191 have been
allocated.

7.  Security Considerations

EAP was designed for use with dialup PPP [RFC1661] and wired [IEEE802]
networks such as Ethernet [IEEE8023].  On these networks, an attacker



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would need to gain physical access to the telephone or switch
infrastructure in order to mount an attack. While such attacks have been
documented, such as in [DECEPTION], they are assumed to be rare.

However, subsequently EAP has been proposed for use on wireless
networks, and over the Internet, where physical security cannot be
assumed. On such networks, the security vulnerabilities are greater, as
are the requirements for EAP security.

This section documents the threats that exist on physically insecure
networks carrying EAP, as well as laying out the security requirements
for EAP use on those networks. We then discuss threat mitigation.

7.1.  Threat model

On physically insecure networks, it is possible for an attacker to gain
access to the physical medium. This enables a range of attacks,
including the following:

[1]  An adversary may try to discover user identities by snooping data
     packets.

[2]  An adversary may try to modify or spoof EAP packets.

[3]  An adversary may launch denial of service attacks by terminating
     EAP conversations.

[4]  An adversary might attempt to recover the passphrase by mounting an
     offline dictionary attack.

[5]  An adversary may attempt to convince the Peer to connect to an
     untrusted network.

[6]  An adversary may attempt to disrupt the EAP negotiation in order to
     weaken the authentication, gain access to user passwords or remove
     confidentiality protection.

[7]  An adversary may attempt to mount a denial of service attack by
     modify

[8]  An attacker may attempt to take advantage of weak key derivation
     techniques used within EAP methods.

[9]  An attacker may attempt to take advantage of weak ciphersuites
     subsequently used after the EAP conversation is complete.

Where EAP is used over wireless networks, an attacker needs to be within
the coverage area of the wireless medium in order to carry out these



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attacks. However, where EAP is used over the Internet, no such
restrictions apply.

7.2.  Security requirements

In order to address the threats that exist where EAP is used on a
physically insecure medium, the following requirements are imposed:

[1]  Mutual authentication. Mutual authentication of the communication
     endpoints MUST be provided in order to protect against rogue
     Authenticators.

[2]  Protected conversation. On a physically insecure network, EAP
     messages SHOULD be integrity and replay protected, authenticated
     and confidential so as to protect against downgrade attacks,
     snooping of identities, and spoofing of packets.  This includes
     protection of packets of types Identity, Nak and Notification, as
     well as packets sent within the EAP method itself, and success and
     failure indications.  Where EAP is used for ciphersuite or
     capabilities negotiation, these messages SHOULD be integrity and
     replay protected, authenticated and confidential.

[3]  Key derivation. EAP methods used on physically insecure networks
     MAY derive keys in order to enable per-packet authentication,
     integrity and replay protection as well as confidentiality.  Where
     EAP methods derive keys, the distributed keys SHOULD be master
     session keys, used only for further key derivation, independent of
     the ciphersuite. This eliminates the need for an EAP method to
     understand how to derive keys for every ciphersuite. Rather than
     inventing new key derivation techniques, well analyzed algorithms
     SHOULD be used.

[4]  Dictionary attack resistance. Where EAP is used on physically
     insecure networks resistance against dictionary attack SHOULD be
     provided.  Where password authentication is used, users are
     notoriously prone to selection of poor passwords. Without
     dictionary attack protection, it is easy for an attacker snooping
     authentication traffic to gather a large number of authentication
     exchanges, and successfully obtain a substantial fraction of the
     passwords used in those exchanges via a dictionary attack.  Given
     the steadily declining prices of computing power, successful
     dictionary attacks can now be mounted at minimal expense.

[5]  Support for fast reconnect.  On physically insecure media such as
     wireless, it is often desirable to improve scalability and minimize
     connectivity interruptions due to authentication.  Where this is
     desired, EAP methods MAY support "fast reconnect". After an initial
     authentication conversation, this enables subsequent authentication



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     conversations to take place in shortened form.

[6]  Acknowledged success and failure indications.  Where EAP is used
     over an unreliable medium, it is possible for packets to be lost.
     This can result in the Peer and Authenticator having a different
     interpretation of the state of the authentication conversation. As
     a result, where EAP is used over an unreliable medium, EAP methods
     SHOULD support acknowledged success and failure indications.

[7]  Protection against man-in-the-middle attacks. Where EAP is tunneled
     within another protocol, or where a sequence of EAP methods is used
     for authentication, the peer SHOULD demonstrate to the
     authenticator that it has acted as the peer for each method within
     the seuqence. Similarly, the authenticator SHOULD demonstrate to
     the peer that it has acted as the authenticator for each method
     within the sequence. If this is not possible, then the
     authentication sequence or tunnel may be vulnerable to a man-in-
     the-middle attack.

Since EAP methods may be used on physically insecure networks, it is
advisable to evaluate methods against the above requirements in order to
determine their suitability. To enable such an evaluation, EAP methods
desiring publication as an RFC MUST include the following declarations:

[a]  Indication of intended use. This includes a statement of whether
     the method is intended for use over a physically secure or insecure
     network, as well as a statement of the applicable media.

[b]  Indication of security claims. This includes a statement of the
     claimed security properties of the method. In particular, the
     specification MUST indicate whether the method claims to satisfy
     the requirements described above.

[c]  Description of key hierarchy. EAP methods deriving keys MUST
     describe how keys for authentication/integrity, encryption and IVs
     are to be derived from the provided keying material, and how
     cryptographic separation is maintained between keying material used
     for different purposes.

[d]  Indication of vulnerabilities. If the method is intended for use on
     a physically insecure network, yet does not satisfy the above
     requirements, the specification MUST indicate which requirements
     are not satisfied, and discuss the security implications.

7.3.  Identity protection

An Identity exchange is an optional within the EAP conversation.
Therefore, it is possible to omit the Identity exchange entirely, or to



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postpone it until later in the conversation once a protected channel has
been negotiated.

However, within networks where backend authentication proxies or relays
are present, it may be necessary to locate the appropriate backend
authentication server before the authentication conversation can
proceed.  The realm portion of the Network Access Identifier (NAI)
[RFC2486] is typically included within the Response-Identity in order to
enable the authentication exchange to be routed to the appropriate
backend authentication server. Therefore while the user-name portion of
the NAI may be omitted in the Identity-Response, where proxies or relays
are present, the realm portion may be required.

7.4.  Man-in-the-middle attacks

Where a sequence of methods is utilized for authentication or is
tunneled within another protocol that omits peer authentication, there
exists a potential vulnerability to man-in-the-middle attack.

Where a sequence of EAP methods is utilized for authentication, the peer
might not have proof that a single entity has acted as the authenticator
for all EAP methods within the sequence. For example, an authenticator
might terminate one EAP method, then forward the next method in the
sequence to another party without the peer's knowlege or consent.
Similarly, the authenticator might not have proof that a single entity
has acted as the peer for all EAP methods within the sequence.

This enables an attack by a rogue EAP authenticator tunneling EAP to a
legitimate server. Where the tunneling protocol is used for key
establishment but does not require peer authentication, an attacker
convincing a legitimate peer to connect to it will be able to tunnel EAP
packets to a legitimate server, successfully authenticating and
obtaining the key. This allows the attacker to successfully establish
itself as a man-in-the-middle, gaining access to the network, as well as
the ability to decrypt data traffic between the legitimate peer and
server.

This attack may be mitigated by the following measures:

[a]  Requiring mutual authentication within EAP tunneling mechanisms.

[b]  Requiring cryptographic binding between EAP methods executed within
     a sequence or between the EAP tunneling protocol and the tunneled
     EAP methods.

[c]  Limiting the EAP methods authorized for use without protection
     based on peer and authenticator policy.




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7.5.  Packet modification attacks

While individual EAP methods such as EAP TLS [RFC2716] may provide for
authentication, integrity and replay protection of data placed within
the Type-Data portion of EAP Request and Response packets, EAP itself
does not provide built-in support for per-packet data origin
authentication, replay or integrity protection.

This means that an attacker may inject or modify EAP packets, including
Request and Response packets of Types Identity, Notification, Nak,
MD5-Challenge as well as Success and Failure packets. An attacker may
also modify the EAP headers within EAP packets where the Type-Data field
is protected, but not the EAP headers.

In the case of PPP and IEEE 802 wired links, it is assumed that such
attacks are restricted to attackers who can gain access to the physical
link.  However, where EAP is run over wireless media such as IEEE
802.11, or over IP, such as within protocols supporting PPP or Ethernet
tunneling [RFC2661], this assumption is no longer valid and the
vulnerability to attack is much greater.

To provide protection for EAP messages sent over physically insecure
media, a variety of techniques may be used. This includes encapsulation
of EAP within ISAKMP [RFC2408], as is done in PIC [PIC], or within TLS
[RFC2246].

7.6.  Denial of service attacks

EAP headers are unprotected, as are packets of Types Identity, Nak,
Notification as well as Success and Failure. Since the Identifier is
only a single octet, and implementations typically start it at zero (0),
incrementing with each new Request, it is very easy to guess.  This
makes it easy for an attacker to deny service by spoofing Failure
packets, as well as by inserting Nak packets so as to prolong the method
negotiation. It is also possible for the attacker to attempt to replay
packets from previous conversations.

Several mechanisms are recommended for addressing this vulnerability:

[a]  Silent discard. Since only a single EAP Request can be in progress
     between an Authenticator and a Peer at a given time, if a Peer
     receives a new Request before sending a Response, the new Request
     can be silently discarded. This increases resilience against
     spoofed Requests.  Similarly, an Authenticator can silently discard
     Responses with Identifiers that do not correspond to the Identifier
     included in the last Request, or that represent duplicate
     Responses.




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[b]  Where  the  EAP conversation is protected via per-packet data
     origin authentication, integrity and replay protection, spoofing or
     data modification attacks can be detected.

7.7.  Dictionary attacks

Password authentication algorithms such as EAP-MD5, MS-CHAPv1 [RFC2433]
and Kerberos V [RFC1510] are known to be vulnerable to dictionary
attacks.  MS-CHAPv1 vulnerabilities are documented in [PPTPv1]; Kerberos
vulnerabilities are described in [KRBATTACK], [KRBLIM], and [KERB4WEAK].

In order to protect against dictionary attacks, an authentication
algorithm resistant to dictionary attack may be used. If an
authentication algorithm is used that is known to be vulnerable to
dictionary attack, then the authentication may be encrypted to obtain
additional protection.

7.8.  Connection to an untrusted network

If a one-way authentication method is negotiated, such as EAP-MD5, then
the Authenticator's identity will not be verified. While this might be
acceptable over a link that is believed to be physically secure, this
level of security is not acceptable where EAP runs over wireless media
or IP, since this leaves the Peer vulnerable to connection to an
untrusted network. As a result, where EAP is used over a physically
insecure network, it is highly desirable to use a method supporting
mutual authentication.

In EAP there is no requirement that authentication be full duplex or
that the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.  This
will, of course, depend on the specific protocols negotiated.  However,
in general, completing a single, mutual authentication is preferrable to
two one-way authentications, one in each direction.

7.9.  Negotiation attacks

In a negotiation attack, the attacker attempts to convince the Peer and
Authenticator to negotiate a less secure EAP method.

To avoid downgrading from a mutually authenticating method to one that
only authenticates the peer, where EAP is used over a physically
insecure network, it is desirable for the peer to only accept
negotiation of a mutually authenticating method.

In practice, within or associated with each backend authentication
server, it is anticipated that a particular named user will be
authenticated by a predefined method or sequence of methods, without



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leaving the user any choice. Enabling negotiation would make the user
vulnerable to attacks which negotiate the least secure method from among
a set (such as EAP-MD5 instead of a mutually authenticating method).

Instead, for each named user there SHOULD be an indication of exactly
one method or sequence of methods used to authenticate that user name.
If a user needs to make use of different authentication methods under
different circumstances, then distinct identities SHOULD be employed,
each of which identifies exactly one authentication method or sequence
of methods.

7.10.  Implementation idiosyncracies

The interaction of authentication protocols with link layer technologies
such as PPP and IEEE 802 are highly implementation dependent.

For example, upon failure of authentication, some PPP implementations do
not terminate the link, instead limiting the kind of traffic in the
Network-Layer Protocols to a filtered subset, which in turn allows the
user opportunity to update secrets or send mail to the network
administrator indicating a problem. Similarly, while in IEEE 802.1X an
authentication failure will result denied access to the controlled port,
limited traffic may be permitted on the uncontrolled port.

In EAP there is no provision for retries of failed authentication.
However, in PPP the LCP state machine can renegotiate the authentication
protocol at any time, thus allowing a new attempt. Similarly, in IEEE
802.1X the supplicant or Authenticator can re-authenticate at any time.
It is recommended that any counters used for authentication failure not
be reset until after successful authentication, or subsequent
termination of the failed link.

7.11.  Key derivation

It is possible for the EAP endpoints to mutually authenticate, negotiate
a ciphersuite, and derive session key(s) for subsequent use with per-
packet authentication, integrity protection and encryption.  Since the
Peer and EAP client reside on the same machine, the EAP client module
passes the derived session key to the link layer security module.

In the case where the backend authentication server and Authenticator
reside on different machines, there are several implications for
security:

[a]  Mutual authentication will occur between the Peer and the backend
     authentication server, not between the Peer and the Authenticator.
     This means that it is not possible for the Peer to validate the
     identity of the Authenticator.



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[b]  The session key negotiated between the Peer and backend
     authentication server will need to be transmitted to the
     Authenticator.  Therefore a mechanism needs to be provided to
     transmit the session key from the backend authentication server to
     the Authenticator that needs to use the key. The specification of
     this transit mechanism is outside the scope of this document.

This specification does not provide guidance on how EAP methods are to
derive keys. Key derivation is an art that is best practiced by
professionals; rather than inventing new key derivation algorithms,
reuse of existing algorithms such as those specified in IKE [RFC2409],
or TLS [RFC2246] is recommended.

7.12.  Weak ciphersuites

EAP authentication may be used in situations where after the initial
authentication, data packets are sent without per-packet data origin
authentication, integrity and replay protection or confidentiality.
These scenarios are inherently insecure. Without per-packet
authentication, integrity and replay protection, an attacker with access
to the media can inject packets, "flip bits" within existing packets, or
even hijack the session completely. Without per-packet confidentiality,
it is possible to snoop data packets.

On physically insecure media, EAP SHOULD be used in concert with
credible ciphersuites in order to provide defensible security. In
particular, EAP SHOULD NOT be used for authentication without subsequent
invocation of per-packet integrity protection and authentication, since
this would leave the session vulnerable to hijacking.  Due to the many
of known attacks to which it is vulnerable, Wired Equivalent Privacy
(WEP) [IEEE80211] does not qualify as a credible ciphersuite.

8.  Normative references

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

[RFC1994]      Simpson, W., "PPP Challenge Handshake Authentication
               Protocol (CHAP)", RFC 1994, August 1996.

[RFC2044]      Yergeau, F., "UTF-8, a transformation format of Unicode
               and ISO 10646", RFC 2044, October 1996.

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

[RFC2279]      Yergeau, F., "UTF-8, a transformation format of ISO
               10646", RFC 2279, January 1998.



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INTERNET-DRAFT                 RFC2284bis                21 October 2002


[RFC2434]      Alvestrand, H. and Narten, T., "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 2434,
               October 1998.

[RFC2988]      Paxson, V., Allman, M., "Computing TCP's Retransmission
               Timer", RFC 2988, November 2000.

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

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

9.  Informative references

[DECEPTION]    Slatalla, M., and  Quittner, J., "Masters of Deception."
               HarperCollins, New York, 1995.

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

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

[RFC2486]      Beadles, M., Aboba, B., "The Network Access Identifier",
               RFC 2486, January 1999.

[RFC2401]      Atkinson, R., Kent, S., "Security Architecture for the
               Internet Protocol", RFC 2401, November 1998.

[RFC2408]      Maughan, D., Schertler, M., Schneider, M., Turner, J.,
               "Internet Security Association and Key Management
               Protocol (ISAKMP)", RFC 2408, November 1998.

[RFC2433]      Zorn, G., Cobb, S., "Microsoft PPP CHAP Extensions", RFC
               2433, October 1998.

[RFC2661]      Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
               G., and Palter, B., "Layer Two Tunneling Protocol L2TP",
               RFC 2661, August 1999.

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

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



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

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

[PIC]          Sheffer, Y., Krawczyk, H., Aboba, B., "PIC, A Pre-IKE
               Credential Provisioning Protocol", Internet draft (work
               in progress), draft-ietf-ipsra-pic-05.txt, February 2002.

[PPTPv1]       Schneier, B, Mudge, "Cryptanalysis of Microsoft's Point-
               to- Point Tunneling Protocol", Proceedings of the 5th ACM
               Conference on Communications and Computer Security, ACM
               Press, November 1998.

[IEEE8023]     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.

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

Acknowledgments

This protocol derives much of its inspiration from Dave Carrel's AHA
draft as well as the PPP CHAP protocol [RFC1994].  Bill Simpson provided
much of the boilerplate used throughout this document.   Al Rubens
(Merit) also provided valuable feedback, as did Glen Zorn (Cisco) and
Ashwin Palekar (Microsoft).

Authors' Addresses

Larry J. Blunk
Merit Network, Inc.
4251 Plymouth Rd., Suite C
Ann Arbor, MI  48105




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EMail: ljb@merit.edu
Phone: 734-763-6056
FAX:   734-647-3185

John R. Vollbrecht
Interlink Networks, Inc.
775 Technology Drive, Suite 200
Ann Arbor, MI  48108
USA

Phone: +1 734 821 1205
Fax:   +1 734 821 1235
EMail: jrv@interlinknetworks.com

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

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

Full Copyright Statement

Copyright (C) The Internet Society (2002).  All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.  However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.  The limited permissions granted above are
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herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."







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Open issues

Open issues relating to this specification are tracked on the following
web site:

http://www.drizzle.com/~aboba/EAP/eapissues.html

Expiration Date

This memo is filed as <draft-ietf-pppext-rfc2284bis-07.txt>,  and
expires April 24, 2003.








































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