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Versions: (RFC 2284) 00 01 02 03 04 05 06 07 08 09 RFC 3748

EAP Working Group                                               L. Blunk
Internet-Draft                                        Merit Network, Inc
Obsoletes: 2284 (if approved)                              J. Vollbrecht
Expires: November 14, 2003                     Vollbrecht Consulting LLC
                                                                B. Aboba
                                                               Microsoft
                                                              J. Carlson
                                                                     Sun
                                                       H. Levkowetz, Ed.
                                                             ipUnplugged
                                                            May 16, 2003


                Extensible Authentication Protocol (EAP)
                   <draft-ietf-eap-rfc2284bis-03.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on November 14, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document defines the Extensible Authentication Protocol (EAP),
   an authentication framework which supports multiple authentication
   methods.  EAP typically runs directly over data link layers such as
   PPP or IEEE 802, without requiring IP.  EAP provides its own support
   for duplicate elimination and retransmission, but is reliant on lower



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   layer ordering guarantees.  Fragmentation is not supported within EAP
   itself; however, individual EAP methods may support this.

   This document obsoletes RFC 2284.  A summary of the changes between
   this document and RFC 2284 is available in Appendix B.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
         1.1   Specification of Requirements  . . . . . . . . . . . .  4
         1.2   Terminology  . . . . . . . . . . . . . . . . . . . . .  4
         1.3   Security claims terminology for EAP methods  . . . . .  5
   2.    Extensible Authentication Protocol (EAP) . . . . . . . . . .  9
         2.1   Support for sequences  . . . . . . . . . . . . . . . . 10
         2.2   EAP multiplexing model . . . . . . . . . . . . . . . . 11
   3.    Lower layer behavior . . . . . . . . . . . . . . . . . . . . 14
         3.1   Lower layer requirements . . . . . . . . . . . . . . . 14
         3.2   EAP usage within PPP . . . . . . . . . . . . . . . . . 16
               3.2.1 PPP Configuration Option Format  . . . . . . . . 16
         3.3   EAP usage within IEEE 802  . . . . . . . . . . . . . . 17
         3.4   Lower layer indications  . . . . . . . . . . . . . . . 17
   4.    EAP Packet format  . . . . . . . . . . . . . . . . . . . . . 17
         4.1   Request and Response . . . . . . . . . . . . . . . . . 18
         4.2   Success and Failure  . . . . . . . . . . . . . . . . . 21
   5.    Initial EAP Request/Response Types . . . . . . . . . . . . . 23
         5.1   Identity . . . . . . . . . . . . . . . . . . . . . . . 23
         5.2   Notification . . . . . . . . . . . . . . . . . . . . . 24
         5.3   Nak  . . . . . . . . . . . . . . . . . . . . . . . . . 25
               5.3.1 Legacy Nak . . . . . . . . . . . . . . . . . . . 25
               5.3.2 Expanded Nak . . . . . . . . . . . . . . . . . . 26
         5.4   MD5-Challenge  . . . . . . . . . . . . . . . . . . . . 28
         5.5   One-Time Password (OTP)  . . . . . . . . . . . . . . . 30
         5.6   Generic Token Card (GTC) . . . . . . . . . . . . . . . 31
         5.7   Expanded Types . . . . . . . . . . . . . . . . . . . . 32
         5.8   Experimental . . . . . . . . . . . . . . . . . . . . . 33
   6.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 34
         6.1   Packet Codes . . . . . . . . . . . . . . . . . . . . . 35
         6.2   Method Types . . . . . . . . . . . . . . . . . . . . . 35
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 35
         7.1   Threat model . . . . . . . . . . . . . . . . . . . . . 36
         7.2   Security claims  . . . . . . . . . . . . . . . . . . . 37
         7.3   Identity protection  . . . . . . . . . . . . . . . . . 38
         7.4   Man-in-the-middle attacks  . . . . . . . . . . . . . . 38
         7.5   Packet modification attacks  . . . . . . . . . . . . . 39
         7.6   Dictionary attacks . . . . . . . . . . . . . . . . . . 40
         7.7   Connection to an untrusted network . . . . . . . . . . 40
         7.8   Negotiation attacks  . . . . . . . . . . . . . . . . . 41
         7.9   Implementation idiosyncrasies  . . . . . . . . . . . . 41



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         7.10  Key derivation . . . . . . . . . . . . . . . . . . . . 41
         7.11  Weak ciphersuites  . . . . . . . . . . . . . . . . . . 42
         7.12  Link layer . . . . . . . . . . . . . . . . . . . . . . 43
         7.13  Separation of authenticator and backend authentication
         server . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
   8.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 45
         Normative References . . . . . . . . . . . . . . . . . . . . 45
         Informative References . . . . . . . . . . . . . . . . . . . 46
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
   A.    Method Specific Behavior . . . . . . . . . . . . . . . . . . 49
         A.1   Message Integrity Checks . . . . . . . . . . . . . . . 49
   B.    Changes from RFC 2284  . . . . . . . . . . . . . . . . . . . 50
   C.    Open issues  . . . . . . . . . . . . . . . . . . . . . . . . 51
         Intellectual Property and Copyright Statements . . . . . . . 53





































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1. Introduction

   This document defines the Extensible Authentication Protocol (EAP),
   an authentication framework which supports multiple authentication
   methods.  EAP typically runs directly over data link layers such as
   PPP or IEEE 802, without requiring IP.  EAP provides its own support
   for duplicate elimination and retransmission, but is reliant on lower
   layer ordering guarantees.  Fragmentation is not supported within EAP
   itself; however, individual EAP methods may support this.

   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 [IEEE-802].  EAP encapsulation on
   IEEE 802 wired media is described in [IEEE-802.1X], and encapsulation
   on IEEE wireless LANs in [IEEE-802.11i].

   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 method to be used.  Rather than
   requiring the authenticator to be updated to support each new
   authentication method, EAP permits the use of a backend
   authentication server which may implement some or all authentication
   methods, with the authenticator acting as a pass-through for some or
   all methods and peers.

   Within this document, authenticator requirements apply regardless of
   whether the authenticator is operating as a pass-through or not.
   Where the requirement is meant to apply to either the authenticator
   or backend authentication server, depending on where the EAP
   authentication is terminated, the term "EAP server" will be used.

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:






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   authenticator
             The end of the EAP link initiating EAP authentication. The
             term Authenticator is used in [IEEE-802.1X], and
             authenticator has the same meaning in this document.

   peer
             The end of the EAP Link that responds to the authenticator.
             In [IEEE-802.1X], this end is known as the Supplicant.

   backend authentication server
             A backend authentication server is an entity that provides
             an authentication service to an authenticator.  When used,
             this server typically executes EAP methods for the
             authenticator. This terminology is also used in
             [IEEE-802.1X].

   Displayable Message
             This is interpreted to be a human readable string of
             characters.  The message encoding MUST follow the UTF-8
             transformation format [RFC2279].

   EAP server
             The entity that terminates the EAP authentication method
             with the peer.  In the case where no backend authentication
             server is used the EAP server is part of the authenticator.
             In the case where the authenticator operates in
             pass-through mode, the EAP server is located on the backend
             authentication server.

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


1.3 Security claims terminology for EAP methods

   These terms are used to described the security properties of EAP
   methods:

   Mutual authentication
             This refers to an EAP method in which, within an
             interlocked exchange, the authenticator authenticates the
             peer and the peer authenticates the authenticator.  Two
             independent one-way methods, running in opposite directions
             do not provide mutual authentication as defined here.



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   Integrity protection
             This refers to providing data origin authentication and
             protection against unauthorized modification of information
             for EAP packets (including EAP Requests and Responses).
             When making this claim, a method specification MUST
             describe the EAP packets and fields within the EAP packet
             that are protected.

   Replay protection
             This refers to protection against replay of an EAP method
             or its messages, including method-specific success and
             failure indications.

   Confidentiality
             This refers to encryption of EAP messages, including EAP
             Requests and Responses, and method-specific success and
             failure indications.  A method making this claim MUST
             support identity protection (see Section 7.3).

   Key derivation
             This refers to the ability of the EAP method to derive
             exportable keying material such as the Master Session Key
             (MSK), and Extended Master Session Key (EMSK). The MSK is
             used only for further key derivation, not directly for
             protection of the EAP conversation or subsequent data.  Use
             of the EMSK is reserved.

   Key strength
             If the effective key strength is N bits, the best currently
             known methods to recover the key (with non-negligible
             probability) require an effort comparable to 2^N operations
             of a typical block cipher.

   Dictionary attack resistance
             Where password authentication is used, users are
             notoriously prone to select poor passwords.  A method may
             be said to be dictionary attack resistant if, when there is
             a weak password in the secret,  the method does not allow
             an attack more efficient than brute force.

   Fast reconnect
             The ability, in the case where a security association has
             been previously established, to create a new or refreshed
             security association in a smaller number of round-trips.

   Man-in-the-Middle resistance
             This property applies only for the use of multiple methods
             in a combination, such as in authentication sequences or



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             tunnels. It refers to the ability of the peer to
             demonstrate to the authenticator that it has acted as the
             peer for each authentication method within the
             conversation.  Similarly, the authenticator demonstrates to
             the peer that it has acted as the authenticator for each
             authentication method within the conversation.  If this is
             not possible, then there may be a vulnerability to a
             man-in-the-middle attack.

   Acknowledged result indications
             The ability of the authenticator to provide the peer with
             an indication of whether the peer has successfully
             authenticated to it, and for the peer to acknowledge
             receipt, as well as providing an indication of whether the
             authenticator has successfully authenticated to the peer.
             Since EAP Success and Failure packets are neither
             acknowledged nor integrity protected, this claim requires
             implementation of a method-specific result exchange that is
             authenticated, integrity and replay protected on a
             per-packet basis.

   Message Integrity Check (MIC)
             A keyed hash function used for authentication and integrity
             protection of data.  This is usually called a Message
             Authentication Code (MAC), but IEEE 802 specifications (and
             this document) use the acronym MIC to avoid confusion with
             Medium Access Control.

   Cryptographic binding
             The demonstration of the EAP peer to the EAP server that a
             single entity has acted as the EAP peer for all methods
             executed within a sequence or tunnel.  Binding MAY also
             imply that the EAP server demonstrates to the peer that a
             single entity has acted as the EAP server for all methods
             executed within a sequence or tunnel.  If executed
             correctly, binding serves to mitigate man-in-the-middle
             vulnerabilities.

   Cryptographic separation
             Two keys (x and y) are "cryptographically separate" if an
             adversary that knows all messages exchanged in the protocol
             cannot compute x from y or y from x without "breaking" some
             cryptographic assumption.  In particular, this definition
             allows that the adversary has the knowledge of all nonces
             sent in cleartext as well as all predictable counter values
             used in the protocol.  Breaking a cryptographic assumption
             would typically require inverting a one- way function or
             predicting the outcome of a cryptographic pseudo- random



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             number generator without knowledge of the secret state.  In
             other words, if the keys are cryptographically separate,
             there is no shortcut to compute x from y or y from x, but
             the work an adversary must do to perform this computation
             is equivalent to performing exhaustive search for the
             secret state value."

   EAP Master key (MK)
             A key derived between the EAP client and server during the
             EAP authentication process that is purely local to the EAP
             method.  The MK MUST NOT be exported from the EAP method or
             be made available to a third party.  Since derivation of
             the MK is a residue of the successful completion of the EAP
             authentication exchange, proof of MK possession may be used
             to shorten future EAP exchanges between the same EAP client
             and server, a technique known as "fast resume".

   Master Session Key (MSK)
             Keying material that is derived between the EAP client and
             server.  The MSK is used in the derivation of Transient
             Session Keys (TSKs) for the ciphersuite negotiated between
             the EAP peer and authenticator.  Where a backend
             authentication server is present, acting as an EAP server,
             it will typically transport the MSK to the authenticator.

             The MSK differs from the MK in that it not assumed to
             remain local to the EAP method, and is known by all parties
             in the EAP exchange: the peer, authenticator and the
             authentication server (if present). The MSK MAY be derived
             from the MK via a one-way function, or it may be an
             independent quantity.  However possession of the MSK MUST
             NOT provide any information useful in determining the MK.

   Extended Master Session Key (EMSK)
             Additional keying material derived between the EAP client
             and server that is exported by the EAP method.  However,
             unlike the MSK, the EMSK is known only to the EAP peer and
             EAP server and MUST NOT be provided to a third party.  The
             EMSK therefore MUST NOT be transported by the backend
             authentication server to the authenticator.  The EMSK is
             reserved for future uses that are not defined yet.  For
             example, it could be used to derive additional keying
             material for purposes such as fast handoff,
             man-in-the-middle vulnerability protection, etc.







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2. Extensible Authentication Protocol (EAP)

   The EAP 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 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 a valid 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.  EAP is a 'lock step'
       protocol, so that other than the initial Request, a new Request
       cannot be sent prior to receiving a valid Response.  The
       authenticator MUST NOT send a Success or Failure packet as a
       result of a timeout.  After a suitable number of timeouts have
       elapsed, the authenticator SHOULD end the EAP conversation.

   [4] The conversation continues until the authenticator cannot
       authenticate the peer (unacceptable Responses to one or more
       Requests), in which case the authenticator implementation MUST
       transmit an EAP Failure (Code 4).  Alternatively, the
       authentication conversation can continue until the authenticator
       determines that successful authentication has occurred, in which
       case the authenticator MUST transmit an EAP Success (Code 3).

   Since EAP is a peer-to-peer protocol, an independent and simultaneous
   authentication may take place in the reverse direction.  Both peers
   may act as authenticators and authenticatees at the same time.  For a
   discussion of security issues in peer-to-peer operation, see Section
   7.7.

   Advantages:

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



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   o  Network Access Server (NAS) devices (e.g., a switch or access
      point) do not have to understand each authentication method and
      MAY act as a pass-through agent for a backend authentication
      server.  Support for pass-through is optional.  An authenticator
      MAY authenticate local peers while at the same time acting as a
      pass-through for non-local peers and authentication methods it
      does not implement locally.

   o  Separation of the authenticator from the backend authentication
      server simplifies credentials management and policy decision
      making.

   Disadvantages:

   o  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 [IEEE-802.1X] in order to use EAP.

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


2.1 Support for sequences

   An EAP conversation MAY utilize a sequence of methods.  A common
   example of this is an Identity request followed by a single EAP
   authentication method such as an MD5-Challenge.  However the peer and
   authenticator MUST utilize only one authentication method (Type 4 or
   greater) within an EAP conversation, after which the authenticator
   MUST send a Success or Failure packet.

   Once a peer has sent a Response of the same Type as the initial
   Request, an authenticator MUST NOT send a Request of a different Type
   prior to completion of the final round of a given method (with the
   exception of a Notification-Request) and MUST NOT send a Request for
   an additional method of any Type after completion of the initial
   authentication method; a peer receiving such Requests MUST treat them
   as invalid, and silently discard them. As a result, Identity Requery
   is not supported.

   A peer MUST NOT send a Nak (legacy or expanded) in reply to a
   Request, after an initial non-Nak Response has been sent.  Since
   spoofed EAP Request packets may be sent by an attacker, an
   authenticator receiving an unexpected Nak SHOULD silently discard it



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   and log the event.

   Multiple authentication methods within an EAP conversation are not
   supported due to their vulnerability to man-in-the-middle attacks
   (see Section 7.4) and incompatibility with existing implementations.

   Where a single EAP authentication method is utilized, but other
   methods are run within it (a "tunneled" method) the prohibition
   against multiple authentication methods does not apply.  Such
   "tunneled" methods appear as a single authentication method to EAP.
   Backward compatibility can be provided, since a peer not supporting a
   "tunneled" method can reply to the initial EAP-Request with a Nak
   (legacy or expanded).  To address security vulnerabilities,
   "tunneled" methods MUST support protection against man-in-the-middle
   attacks.

2.2 EAP multiplexing model

   Conceptually, EAP implementations consist of the following
   components:

   [a] Lower layer.  The lower layer is responsible for transmitting and
       receiving EAP frames between the peer and authenticator.  EAP has
       been run over a variety of lower layers including PPP; wired IEEE
       802 LANs [IEEE-802.1X]; IEEE 802.11 wireless LANs [IEEE-802.11];
       UDP (L2TP [RFC2661] and ISAKMP [PIC]); and TCP [PIC]. Lower layer
       behavior is discussed in Section 3.

   [b] EAP layer.  The EAP layer receives and transmits EAP packets via
       the lower layer, implements duplicate detection and
       retransmission, and delivers and receives EAP messages to and
       from EAP methods.

   [c] EAP method.  EAP methods implement the authentication algorithms
       and receive and transmit EAP messages via the EAP layer.  Since
       fragmentation support is not provided by EAP itself, this is the
       responsibility of EAP methods, which are discussed in Section 5.

   The EAP multiplexing model is illustrated in Figure 1 below. Note
   that there is no requirement that an implementation conform to this
   model, as long as the on-the-wire behavior is consistent with it.










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         +-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+
         |           |           |  |           |           |
         | EAP method| EAP method|  | EAP method| EAP method|
         | Type = X  | Type = Y  |  | Type = X  | Type = Y  |
         |       V   |           |  |       ^   |           |
         +-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
         |       !               |  |       !               |
         |  EAP  ! Layer         |  |  EAP  !  Layer        |
         |       !               |  |       !               |
         +-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
         |       !               |  |       !               |
         | Lower !  Layer        |  | Lower !  Layer        |
         |       !               |  |       !               |
         +-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
                 !                          !
                 !   Peer                   ! Authenticator
                 +------------>-------------+

                    Figure 1: EAP Multiplexing Model

   Within EAP, the Type field functions much like a port number in UDP
   or TCP.  It is assumed that the EAP layer multiplexes incoming EAP
   packets according to their Type, and delivers them only to the EAP
   method corresponding to that Type code.

   Since EAP authentication methods may wish to access the Identity, the
   Identity Request and Response can be assumed to be accessible to
   authentication methods (Types 4 or greater) in addition to the
   Identity method.  The Identity Type is discussed in Section 5.1.

   A Notification Response is only 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 or Response is available to
   another method.  The Notification Type is discussed in Section 5.2.

   Nak (Type 3) or Expanded Nak (Type 254) are utilized for the purposes
   of method negotiation.  Peers respond to an initial EAP Request for
   an unacceptable Type with a Nak Response (Type 3) or Expanded Nak
   Response (Type 254). It cannot be assumed that the contents of the
   Nak Response(s) are available to another method.  The Nak Type(s) are
   discussed in Section 5.3.

   EAP packets with codes of Success or Failure do not include a Type,
   and therefore are not delivered to an EAP method.  Success and
   Failure are discussed in Section 4.2.

   Given these considerations, the Success, Failure, Nak Response(s) and



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   Notification Request/Response messages MUST NOT be used to carry data
   destined for delivery to other EAP methods.

   Where an authenticator operates as a pass-through, it forwards
   packets back and forth between the peer and a backend authentication
   server, based on the EAP layer header fields (Code, Identifier,
   Length). This includes performing validity checks on the Code,
   Identifer and Length fields, as described in Section 4.1.

   Since pass-through authenticators rely on a backend authenticator
   server to implement methods, the EAP method layer header fields
   (Type, Type-Data) are not examined as part of the forwarding
   decision.  The forwarding model is illustrated in Figure 2. Compliant
   pass-through authenticator implementations MUST by default be capable
   of forwarding packets from any EAP method.  The use of the RADIUS
   protocol for encapsulation of EAP in pass-through operation is
   described in [RFC2869bis].


              Peer     Pass-through Authenticator  Authentication
                                                       Server

         +-+-+-+-+-+-+                             +-+-+-+-+-+-+
         |           |                             |           |
         |EAP method |                             |EAP method |
         |   Layer   |                             |   Layer   |
         |     V     |                             |     ^     |
         +-+-+-!-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-!-+-+-+
         |     !     |  |           |           |  |     !     |
         |EAP  !Layer|  | EAP Layer | EAP Layer |  |EAP  !Layer|
         |     !     |  |     +-----+-----+     |  |     !     |
         |     !     |  |     !     |     !     |  |     !     |
         +-+-+-!-+-+-+  +-+-+-!-+-+-+-+-+-!-+-+-+  +-+-+-!-+-+-+
         |     !     |  |     !     |     !     |  |     !     |
         |Lower!Layer|  |Lower!Layer| AAA ! /IP |  | AAA ! /IP  |
         |     !     |  |     !     |     !     |  |     !     |
         +-+-+-!-+-+-+  +-+-+-!-+-+-+-+-+-!-+-+-+  +-+-+-!-+-+-+
               !              !           !              !
               !              !           !              !
               +-------->-----+           +------->------+

                  Figure 2: Pass-through Authenticator

   For sessions in which 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



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   an encapsulated EAP packet.

3. Lower layer behavior

3.1 Lower layer requirements

   EAP makes the following assumptions about lower layers:

   [1] Unreliable transport.  In EAP, the authenticator retransmits
       Requests that have not yet received Responses, so that EAP does
       not assume that lower layers are reliable.  Since EAP defines it
       own retransmission behavior, when run over a reliable lower
       layer, it is possible (though undesirable) for retransmission to
       occur both in the lower layer and the EAP layer.

       Note that EAP Success and Failure packets are not retransmitted.
       Without a reliable lower layer, and a non-negligible error rate,
       these packets can be lost, resulting in timeouts. It is therefore
       desirable for implementations to improve their resilience to loss
       of EAP Success or Failure packets, as described in Section 4.2.

   [2] Lower layer error detection.  While EAP does not assume that the
       lower layer is reliable, it does rely on lower layer error
       detection (e.g., CRC, Checksum, MIC, etc.). EAP methods may not
       include a MIC, or if they do, it may not be computed over all the
       fields in the EAP packet, such as the Code, Identifier, Length or
       Type fields.  As a result, without lower layer error detection,
       undetected errors could creep into the EAP layer or EAP method
       layer header fields, resulting in authentication failures.

       For example, EAP TLS [RFC2716], which computes its MIC over the
       Type-Data field only, regards MIC validation failures as a fatal
       error.  Without lower layer error detection, this method and
       others like it will not perform reliably.

   [3] Lower layer security.  EAP assumes that lower layers either
       provide physical security (e.g., wired PPP or IEEE 802 links) or
       support per-packet authentication, integrity and replay
       protection.  EAP SHOULD NOT be used on physically insecure links
       (e.g., wireless or the Internet) where subsequent data is not
       protected by per-packet authentication, integrity and replay
       protection.

       After EAP authentication is complete, the peer will typically
       transmit data to the network via the authenticator.  In order to
       provide assurance that the peer transmitting data is the same
       entity that successfully completed EAP authentication, the lower
       layer needs to bind per-packet integrity, authentication and



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       replay protection to the original EAP authentication, using keys
       derived during EAP authentication.  Alternatively, the lower
       layer needs to be physically secure.  Otherwise it is possible
       for subsequent data traffic to be hijacked, or replayed.

       As a result of these considerations, EAP SHOULD be used only when
       lower layers provide physical security for data (e.g., wired PPP
       or IEEE 802 links), or for insecure links, where per-packet
       authentication, integrity and replay protection is provided.

       Where keying material for the lower layer ciphersuite is itself
       provided by EAP, ciphersuite negotiation and key activation is
       controlled by the lower layer.  In PPP, ciphersuites are
       negotiated within ECP so that it is not possible to use keys
       derived from EAP authentication until the completion of ECP.
       Therefore an initial EAP exchange cannot protected by a PPP
       ciphersuite, although EAP re-authentication can be protected.

       In IEEE 802 media, key activation also typically occurs after
       completion of EAP authentication.  Therefore an initial EAP
       exchange typically cannot be protected by lower layer
       ciphersuite, although an EAP re-authentication or
       pre-authentication exchange can be protected.

   [4] MTU known a-priori.  The EAP layer does not support fragmentation
       and reassembly.  However, EAP methods SHOULD be capable of
       handling fragmentation and reassembly.  As a result, EAP is
       capable of functioning across a range of MTU sizes, as long as
       the MTU is known a-priori.  EAP does not support path MTU
       discovery.

   [5] Possible duplication.  Where the lower layer is reliable, it will
       provide the EAP layer with a non-duplicated stream of packets.
       However, 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.

   [6] Ordering guarantees.  EAP does not require the Identifier to be
       monotonically increasing, and so is reliant on lower layer
       ordering guarantees for correct operation.  EAP was originally
       defined to run on PPP, and [RFC1661] Section 1 has an ordering
       requirement:

       "The Point-to-Point Protocol is designed for simple links which
       transport packets between two peers.  These links provide
       full-duplex simultaneous bi-directional operation, and are
       assumed to deliver packets in order."



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       Lower layer transports for EAP MUST preserve ordering between a
       source and destination, at a given priority level (the ordering
       guarantee provided by [IEEE-802]).


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.

   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.

   If the identity of the peer has been established in the
   Authentication phase, the server can use that identity in the
   selection of options for the following 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
   "backend" 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 EAP 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type

      3

   Length

      4

   Authentication Protocol

      C227 (Hex) for Extensible Authentication Protocol (EAP)


3.3 EAP usage within IEEE 802

   The encapsulation of EAP over IEEE 802 is defined in [IEEE-802.1X].
   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].

3.4 Lower layer indications

   The reliability and security of lower layer indications is dependent
   on the lower layer.  Since EAP is media independent, the presence or
   absence of lower layer security is not taken into account in the
   processing of EAP messages.

   Lower layer failure indications provided to EAP by the lower layer
   MUST be processed and will cause an EAP exchange in progress to be
   aborted.  However, lower layer success indications MUST NOT affect
   EAP message processing; an EAP implementation cannot conclude that
   authentication has succeeded based on those indications. This ensures
   that an attacker spoofing lower layer indications can at best succeed
   in a denial of service attack.

   A discussion of some reliability and security issues with lower layer
   indications in PPP, IEEE 802 wired networks and IEEE 802.11 wireless
   LANs can be found in the Security Considerations, Section 7.12.

4. EAP Packet format

   A summary of the EAP packet format is shown below.  The fields are
   transmitted from left to right.






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    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

      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 (Code field set to 1) is sent by the
      authenticator to the peer.  Each Request has a Type field which
      serves to indicate what is being requested.  Additional Request
      packets MUST be sent until a valid Response packet is received, or
      an optional retry counter expires.



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      Retransmitted Requests MUST be sent with the same Identifier value
      in order to distinguish them from new Requests. The contents of
      the data field are dependent on the Request Type.  The peer MUST
      send a Response packet in reply to a valid Request packet.
      Responses MUST only be sent in reply to a valid Request and never
      retransmitted on a timer.

      The Identifier field of the Response MUST match that of the
      currently outstanding Request.  An authenticator receiving a
      Response whose Identifier value does not match that of the
      currently outstanding Request MUST silently discard the Response.
      The Type field of a Response MUST either match that of the
      Request, or correspond to a legacy or Expanded Nak (see Section
      5.3). An EAP server receiving a Response not meeting this
      requirement MUST silently discard it.

         Implementation Note: The authenticator is responsible for
         retransmitting Request messages.  If the Request message is
         obtained from elsewhere (such as from a backend authentication
         server), then the authenticator will need to save a copy of the
         Request in order to accomplish this.  The peer is responsible
         for detecting and handling duplicate Request messages before
         processing them in any way, including passing them on to an
         outside party.  The authenticator is also responsible for
         discarding Response messages with a non-matching Identifier
         value before acting on them in any way, including passing them
         on to the backend authentication server for verification.
         Since the authenticator can retransmit before receiving a
         Response from the peer, the authenticator can receive multiple
         Responses, each with a matching Identifier. Until a new Request
         is received by the authenticator, the Identifier value is not
         updated, so that the authenticator forwards Responses to the
         backend authentication server, one at a time.

         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 SHOULD be computed as described in
         [RFC2988]. This includes use of Karn's algorithm to filter RTT
         estimates resulting from retransmissions.  A maximum of 3-5
         retransmissions is suggested.

         When run over a reliable lower layer (e.g., EAP over ISAKMP/
         TCP, as within [PIC]), the authenticator retransmission timer
         SHOULD be set to an infinite value, so that retransmissions do
         not occur at the EAP layer.  Note that in this case the peer
         may still maintain a timeout value so as to avoid waiting



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         indefinitely for a Request.

         Where the authentication process requires user input, the
         measured round trip times are largely determined by user
         responsiveness rather than network characteristics, so that RTO
         estimation is not helpful.  Instead, the retransmission timer
         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 (see Section 5.6) are involved.

         In order to provide the EAP authenticator with guidance as to
         the appropriate timeout value, a hint can be communicated to
         the authenticator by the backend authentication server (such as
         via the RADIUS Session-Timeout attribute).

   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.  The Identifier field MUST be
      the same if a Request packet is retransmitted due to a timeout
      while waiting for a Response.  Any new (non-retransmission)
      Requests MUST modify the Identifier field.  In order to avoid
      confusion between new Requests and retransmissions, the Identifier
      value chosen for each new Request need only be different from the
      previous Request, but need not be unique within the conversation.
      One way to achieve this is to start the Identifier at an initial
      value and increment it for each new Request.  Initializing the
      first Identifier with a random number rather than starting from
      zero is recommended, since it makes sequence attacks somewhat
      harder.





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      Since the Identifier space is unique to each session,
      authenticators are not restricted to only 256 simultaneous
      authentication conversations.  Similarly, with re-authentication,
      an EAP conversation might continue over a long period of time, and
      is not limited to only 256 roundtrips.

      If a peer receives a valid duplicate Request for which it has
      already sent a Response, it MUST resend its original Response.  If
      a peer receives a duplicate Request before it has sent a Response,
      but after it has determined the initial Request to be valid (i.e.,
      it is waiting for user input), it MUST silently discard the
      duplicate Request.  An EAP message may be found invalid for a
      variety of reasons: failed lower layer CRC or checksum, malformed
      EAP packet, EAP method MIC failure, etc.

   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
      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 are
      also Nak Response Types for indicating that a Request Type is
      unacceptable to the peer (see Section 5.3). An initial
      specification of Types follows in Section 5 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 after
   completion of an EAP authentication method (Type 4 or greater), to
   indicate that the peer has authenticated successfully to the
   authenticator.  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 after unsuccessful completion of the EAP method in



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   progress, 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.

   EAP Success or Failure packets MUST NOT be sent by an EAP server
   prior to completion of the final round of a given method.  A peer EAP
   implementation receiving a Success or Failure packet prior to
   completion of the method in progress MUST silently discard it.  By
   default, an EAP peer MUST silently discard a "canned" EAP Success
   message (an EAP Success message sent immediately upon connection).
   This ensures that a rogue authenticator will not be able to bypass
   mutual authentication by sending an EAP Success prior to conclusion
   of the EAP method conversation.

      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.  If acknowledged result indications are
      desired, these MAY be implemented within individual EAP methods.
      Since only a single EAP authentication method is supported within
      an EAP conversation, a peer that successfully authenticates the
      authenticator MAY, in the event that an EAP Success is not
      received, conclude that the EAP Success packet was lost and enable
      the link.

   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

      3 for Success
      4 for Failure

   Identifier

      The Identifier field is one octet and aids in matching replies to
      Responses.  The Identifier field MUST match the Identifier field
      of the Response packet that it is sent in response to.




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   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.  Nak (Type 3) or
   Expanded Nak (Type 254) are valid only for Response packets, they
   MUST NOT be sent in a Request.

   All EAP implementations MUST support Types 1-4, which are defined in
   this document, and SHOULD support Type 254.  Implementations MAY
   support other Types defined here or in future RFCs.


          1       Identity
          2       Notification
          3       Nak (Response only)
          4       MD5-Challenge
          5       One Time Password (OTP)
          6       Generic Token Card (GTC)
        254       Expanded Types
        255       Experimental use

   EAP methods MAY support authentication based on shared secrets.  If
   the shared secret is a passphrase entered by the user,
   implementations MAY support entering passphrases with non-ASCII
   characters.  In this case, the input should be processed using an
   appropriate stringprep [RFC3454] profile, and encoded in octets using
   UTF-8 encoding [RFC2279]. A possible registered stringprep profile is
   specified in [SASLPREP].

5.1 Identity

   Description

      The Identity Type is used to query the identity of the peer.
      Generally, the authenticator will issue this as the initial
      Request.  An optional displayable message MAY be included to
      prompt the peer in the case where there is an expectation of
      interaction with a user.  A Response of Type 1 (Identity) SHOULD



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      be sent in Response to a Request with a Type of 1 (Identity).

      Since Identity Requests and Responses are not protected, from a
      privacy perspective, it may be preferable for protected
      method-specific Identity exchanges to be used instead. Where the
      peer is configured to only accept authentication methods
      supporting protected identity exchanges, the peer MAY provide an
      abbreviated Identity Response (such as omitting the peer-name
      portion of the NAI [RFC2486]). For further discussion of identity
      protection, see Section 7.3.

         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 ISO 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.


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 when there is
      no outstanding Request, prior to completion of an EAP
      authentication method.  The peer MUST respond to a Notification
      Request with a Notification Response unless the EAP authentication



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      method specification prohibits the use of Notification message.
      In any case, a Nak Response MUST NOT be sent in response to a
      Notification Request.

      An EAP method MAY indicate within its specification that
      Notification messages must not be sent during that method. In this
      case, the peer MUST silently discard Notification Requests from
      the point where an initial Request for that Type is answered with
      a Response of the same Type.

      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 ISO
      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

5.3.1 Legacy Nak

   Description

      The legacy Nak Type is valid only in Response messages.  It is
      sent in reply to a Request where the desired authentication Type
      is unacceptable.  Authentication Types are numbered 4 and above.
      The Response contains one or more authentication Types desired by
      the Peer.  Type zero (0) is used to indicate that the sender has
      no viable alternatives.



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      Since the legacy Nak Type is valid only 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.

   Code

      2 for Response.

   Identifier

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

   Length

      >=6

   Type

      3

   Type-Data

      Where a peer receives a Request for an unacceptable authentication
      Type (4-253,255), or a peer lacking support for Expanded Types
      receives a Request for Type 254, a Nak Response (Type 3) MUST be
      sent.  The Type-Data field of the Nak Response (Type 3) MUST
      contain one or more octets indicating the desired authentication
      Type(s), one octet per Type, or the value zero (0) to indicate no
      proposed alternative.  A peer supporting Expanded Types that
      receives a Request for an unacceptable authentication Type (4-253,
      255) MAY include the value 254 in the Nak Response (Type 3)  in
      order to indicate the desire for an Expanded authentication Type.
      If the authenticator can accommodate this preference, it will
      respond with an Expanded Type Request (Type 254).


5.3.2 Expanded Nak

   Description

      The Expanded Nak Type is valid only in Response messages.  It MUST
      be sent only in reply to a Request of Type 254 (Expanded Type)
      where the authentication Type is unacceptable.  The Expanded Nak
      Type uses the Expanded Type format itself, and the Response



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      contains one or more authentication Types desired by the peer, all
      in Expanded Type format.  Type zero (0) is used to indicate that
      the sender has no viable alternatives.  The general format of the
      Expanded Type is described in Section 5.7.

      Since the Expanded Nak Type is valid only 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.

   Code

      2 for Response.

   Identifier

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

   Length

      >=40

   Type

      254

   Vendor-Id

      0 (IETF)

   Vendor-Type

      3 (Nak)

   Vendor-Data

      The Expanded Nak Type is only sent when the Request contains an
      Expanded Type (254) as defined in Section 5.7. The Vendor-Data
      field of the Nak Response MUST contain one or more authentication
      Types (4 or greater), all in expanded format, 8 octets per Type,
      or the value zero (0), also in Expanded Type format, to indicate
      no proposed alternative.  The desired authentication Types may
      include a mixture of Vendor-Specific and IETF Types.  For example,
      an Expanded Nak Response indicating a preference for OTP (Type 5),



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      and an MIT (Vendor-Id=20) Expanded Type  of 6 would appear as
      follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     2         |  Identifier   |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type=254    |                0 (IETF)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                3 (Nak)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type=254    |                0 (IETF)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                5 (OTP)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type=254    |                20 (MIT)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                6                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      An Expanded Nak Response indicating a no desired alternative would
      appear as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     2         |  Identifier   |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type=254    |                0 (IETF)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                3 (Nak)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type=254    |                0 (IETF)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                0 (No alternative)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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), Nak (Type 3) or Expanded Nak (Type 254). The



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      Nak reply indicates the peer's desired authentication Type(s).
      EAP peer and EAP server implementations MUST support the
      MD5-Challenge mechanism.  An authenticator that supports only
      pass-through MUST allow communication with a backend
      authentication server that is capable of supporting MD5-Challenge,
      although the EAP authenticator implementation need not support
      MD5-Challenge itself.  However, if the EAP authenticator can be
      configured to authenticate peers locally (e.g., not operate in
      pass-through), then the requirement for support of the
      MD5-Challenge mechanism applies.

      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
      [RFC1994] 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.

      Note: [RFC1994] treats the shared secret as an octet string, and
      does not specify how it is entered into the system (or if it is
      handled by the user at all). EAP MD5-Challenge implementations MAY
      support entering passphrases with non-ASCII characters.  See
      Section 5 for instructions how the input should be processed and
      encoded into octets.

   Type

      4

   Type-Data

      The contents of the Type-Data  field is summarized below.  For
      reference on the use of these 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 ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+









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   Security Claims (see Section 7.2):

      Intended use:           Wired networks, including PPP, PPPOE, and
                              IEEE 802 wired media.  Use over the
                              Internet or with wireless media only when
                              protected.
      Mechanism:              Password or pre-shared key.
      Mutual authentication:  No
      Integrity protection:   No
      Replay protection:      No
      Confidentiality:        No
      Key Derivation:         No
      Key strength:           N/A
      Dictionary attack prot: No
      Key hierarchy:          N/A
      Fast reconnect:         No
      MiTM resistance:        N/A
      Acknowledged S/F:       No


5.5 One-Time Password (OTP)

   Description

      The One-Time Password system is defined in "A One-Time Password
      System" [RFC2289] and "OTP Extended Responses" [RFC2243].  The
      Request contains an OTP challenge in the format described in
      [RFC2289].  A Response MUST be sent in reply to the Request.  The
      Response MUST be of Type 5 (OTP), Nak (Type 3) or Expanded Nak
      (Type 254).  The Nak Response indicates the peer's desired
      authentication Type(s).

   Type

      5

   Type-Data

      The Type-Data field contains the OTP "challenge" as a displayable
      message in the Request.  In the Response, this field is used for
      the 6 words from the OTP dictionary [RFC2289].  The messages MUST
      NOT be null terminated.  The length of the field is derived from
      the Length field of the Request/Reply packet.

      Note: [RFC2289] does not specify how the secret pass-phrase is
      entered by the user, or how the pass-phrase is converted into
      octets.  EAP OTP implementations MAY support entering passphrases
      with non-ASCII characters.  See Section 5 for instructions how the



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      input should be processed and encoded into octets.

   Security Claims (see Section 7.2):

      Intended use:           Wired networks, including PPP, PPPOE, and
                              IEEE 802 wired media.  Use over the
                              Internet or with wireless media only when
                              protected.
      Mechanism:              One-Time Password
      Mutual authentication:  No
      Integrity protection:   No
      Replay protection:      No
      Confidentiality:        No
      Key Derivation:         No
      Key strength:           N/A
      Dictionary attack prot: No
      Key hierarchy:          N/A
      Fast reconnect:         No
      MiTM resistance:        N/A
      Acknowledged S/F:       No


5.6 Generic Token Card (GTC)

   Description

      The Generic Token Card Type is defined for use with various Token
      Card implementations which require user input.  The Request
      contains a displayable message and the Response contains the Token
      Card information necessary for authentication. Typically, this
      would be information read by a user from the Token card device and
      entered as ASCII text.  A Response MUST be sent in reply to the
      Request.  The Response MUST be of Type 6 (GTC), Nak (Type 3) or
      Expanded Nak (Type 254).  The Nak Response indicates the peer's
      desired authentication Type(s).

   Type

      6

   Type-Data

      The Type-Data field in the Request contains a displayable message
      greater than zero octets in length.  The length of the message is
      determined by the 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 6 (Generic Token Card).  The
      Response contains data from the Token Card required for



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      authentication.  The length of the data is determined by the
      Length field of the Response packet.

      EAP GTC implementations MAY support entering a response with
      non-ASCII characters.  See Section 5 for instructions how the
      input should be processed and encoded into octets.

   Security Claims (see Section 7.2):

      Intended use:           Wired networks, including PPP, PPPOE, and
                              IEEE 802 wired media.  Use over the
                              Internet or with wireless media only when
                              protected.
      Mechanism:              Hardware token.
      Mutual authentication:  No
      Integrity protection:   No
      Replay protection:      No
      Confidentiality:        No
      Key Derivation:         No
      Key strength:           N/A
      Dictionary attack prot: No
      Key hierarchy:          N/A
      Fast reconnect:         No
      MiTM resistance:        N/A
      Acknowledged S/F:       No


5.7 Expanded Types

   Description

      Since many of the existing uses of EAP are vendor-specific, the
      Expanded method Type is available to allow vendors to support
      their own Expanded Types not suitable for general usage.

      The Expanded Type is also used to expand the global Method Type
      space beyond the original 255 values.  A Vendor-Id of 0 maps the
      original 255 possible Types onto a namespace of 2^32-1 possible
      Types, allowing for virtually unlimited expansion. (Type 0 is only
      used in a Nak Response, to indicate no acceptable alternative)

      An implementation that supports the Expanded attribute MUST treat
      EAP Types that are less than 256 equivalently whether they appear
      as a single octet or as the 32-bit Vendor-Type within a Expanded
      Type where Vendor-Id is 0.  Peers not equipped to interpret the
      Expanded Type MUST send a Nak as described in Section 5.3.1, and
      negotiate a more suitable authentication method.




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      A summary of the Expanded 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                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Vendor-Type                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Vendor data...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      254 for Expanded Type

   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.

   Vendor-Type

      The Vendor-Type field is four octets and represents the
      vendor-specific method Type.

      If the Vendor-Id is zero, the Vendor-Type field is an extension
      and superset of the existing namespace for EAP Types.  The first
      256 Types are reserved for compatibility with single-octet EAP
      Types that have already been assigned or may be assigned in the
      future.  Thus, EAP Types from 0 through 255 are semantically
      identical whether they appear as single octet EAP Types or as
      Vendor-Types when Vendor-Id is zero.

   Vendor-Data

      The Vendor-Data field is defined by the vendor.  Where a Vendor-Id
      of zero is present, the Vendor-Data field will be used for
      transporting the contents of EAP methods of Types defined by the
      IETF.


5.8 Experimental





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   Description

      The Experimental Type has no fixed format or content.  It is
      intended for use when experimenting with new EAP Types.  This Type
      is intended for experimental and testing purposes.  No guarantee
      is made for interoperability between peers using this Type, as
      outlined in [IANA-EXP].

   Type

      255

   Type-Data

      Undefined


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.

   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".

   For registration requests where a Designated Expert should be
   consulted, the responsible IESG area director should appoint the
   Designated Expert. The intention is that any allocation will be
   accompanied by a published RFC.  But in order to allow for the
   allocation of values prior to the RFC being approved for publication,
   the Designated Expert can approve allocations once it seems clear
   that an RFC will be published.  The Designated expert will post a
   request to the EAP WG mailing list (or a successor designated by the
   Area Director) for comment and review, including an Internet-Draft.
   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 successor, as well



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   as informing IANA.  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.

6.1 Packet Codes

   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.

6.2 Method Types

   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 1-41 have been allocated, with 20 available for re-use.
   Method Types 42-191 may be allocated on the advice of a Designated
   Expert, with Specification Required.

   Allocation of blocks of method Types (more than one for a given
   purpose) should require IETF Consensus.  EAP Type Values 192-253 are
   reserved and allocation requires Standards Action.

   Method Type 254 is allocated for the Expanded Type.  Where the
   Vendor-Id field is non-zero, the Expanded Type is used 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 254 can also be used to provide for an expanded
   IETF method Type space.  Method Type values 256-4294967295 may be
   allocated after Type values 1-191 have been allocated.

   Method Type 255 is allocated for Experimental use, such as testing of
   new EAP methods before a permanent Type code is allocated.

7. Security Considerations

   EAP was designed for use with dialup PPP [RFC1661] and was later
   adapted for use in wired IEEE 802 networks [IEEE-802] in
   [IEEE-802.1X].  On these networks, an attacker 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.



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   This section defines the threat model and security terms and
   describes the security claims section required in EAP method
   specifications.  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 attacker may try to discover user identities by snooping
       authentication traffic.

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

   [3] An attacker may launch denial of service attacks by spoofing
       lower layer indications or Success/Failure packets; by replaying
       EAP packets; or by  generating packets with overlapping
       Identifiers.

   [4] An attacker may attempt to recover the pass-phrase by mounting an
       offline dictionary attack.

   [5] An attacker may attempt to convince the peer to connect to an
       untrusted network, by mounting a man-in-the-middle attack.

   [6] An attacker may attempt to disrupt the EAP negotiation in order
       cause a weak authentication method to be selected.

   [7] An attacker may attempt to recover keys by taking advantage of
       weak key derivation techniques used within EAP methods.

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

   [9] An attacker may attempt to perform downgrading attacks on lower
       layer ciphersuite negotiation in order to ensure that a weaker
       ciphersuite is used subsequently to EAP authentication.

   Where EAP is used over wired networks, an attacker typically requires
   access to the physical infrastructure in order to carry out these
   attacks.  However, where EAP is used over wireless networks, EAP
   packets may be forwarded by authenticators (e.g., pre-authentication)
   so that the attacker need not be within the coverage area of an
   authenticator in order to carry out an attack on it or its peers.
   Where EAP is used over the Internet, attacks may be carried out at an
   even greater distance.




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7.2 Security claims

   In order to clearly articulate the security provided by an EAP
   method, EAP method specifications MUST include a Security Claims
   section including the following declarations:

   [a] 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 lower layers.

   [b] Mechanism.  This is a statement of the authentication technology:
       certificates, pre-shared keys, passwords, token cards, etc.

   [c] Security claims.  This is a statement of the claimed security
       properties of the method, using terms defined in Section 1.2:
       mutual authentication, integrity protection, replay protection,
       confidentiality, key derivation, dictionary attack resistance,
       fast reconnect, man-in-the-middle resistance, acknowledged result
       indications.  The Security Claims section of an EAP method
       specification SHOULD provide justification for the claims that
       are made.  This can be accomplished by including a proof in an
       Appendix, or including a reference to a proof.

   [d] Key strength.  If the method derives keys, then the effective key
       strength MUST be estimated.  This estimate is meant for potential
       users of the method to determine if the keys produced are strong
       enough for the intended application.

       The effective key strength SHOULD be stated as number of bits,
       defined as follows: If the effective key strength is N bits, the
       best currently known methods to recover the key (with
       non-negligible probability) require an effort comparable to 2^N
       operations of a typical block cipher.  The statement SHOULD be
       accompanied by a short rationale, explaining how this number was
       arrived at.  This explanation SHOULD include the parameters
       required to achieve the stated key strength based on current
       knowledge of the algorithms.

       (Note: Although it is difficult to define what "comparable
       effort" and "typical block cipher" exactly mean, reasonable
       approximations are sufficient here.  Refer to e.g. [SILVERMAN]
       for more discussion.)

       The key strength depends on the methods used to derive the keys.
       For instance, if keys are derived from a shared secret (such as a
       password or a long-term secret), and possibly some public
       information such as nonces, the effective key strength is limited
       by the strength of the long-term secret (assuming that the



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       derivation procedure is computationally simple). To take another
       example, when using public key algorithms, the strength of the
       symmetric key depends on the strength of the public keys used.

   [e] Description of key hierarchy.  EAP methods deriving keys MUST
       either provide a reference to a key hierarchy specification, or
       describe how Master Session Keys (MSKs) and Extended Master
       Session Keys (EMSKs) are to be derived.

   [f] Indication of vulnerabilities.  In addition to the security
       claims that are made, the specification MUST indicate which of
       the security claims detailed in Section 1.2 are NOT being made.


7.3 Identity protection

   An Identity exchange is optional within the EAP conversation.
   Therefore, it is possible to omit the Identity exchange entirely, or
   to use a method-specific identity exchange once a protected channel
   has been established.

   However, where roaming is supported as described in [RFC2607], 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 Identity-Response in order to enable the
   authentication exchange to be routed to the appropriate backend
   authentication server.  Therefore while the peer-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 EAP is tunneled within another protocol that omits peer
   authentication, there exists a potential vulnerability to
   man-in-the-middle attack.

   As noted in Section 2.1, EAP does not permit sequences of
   authentication methods.  Were a sequence of EAP authentication
   methods to be permitted, 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 knowledge 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



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   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 the EAP tunneling
       protocol and the tunneled EAP methods.  Where cryptographic
       binding is supported, a mechanism is also needed to protect
       against downgrade attacks that would bypass it.

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

   [d] Avoiding the use of tunnels when a single, strong method is
       available.


7.5 Packet modification attacks

   While EAP methods may support per-packet data origin authentication,
   integrity and replay protection, support is not provided within the
   EAP layer.

   Since the Identifier is only a single octet, it is easy to guess,
   allowing an attacker to successfully inject or replay EAP packets. An
   attacker may also modify EAP headers within EAP packets where the
   header is unprotected.  This could cause packets to be
   inappropriately discarded or misinterpreted.

   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 physically insecure
   lower layers such as wireless (802.11 or cellular) or the Internet
   (such as within protocols supporting PPP, EAP or Ethernet Tunneling),
   this assumption is no longer valid and the vulnerability to attack is
   greater.

   To protect EAP messages sent over physically insecure lower layers,
   methods providing mutual authentication and key derivation, as well
   as per-packet origin authentication, integrity and replay protection



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   SHOULD be used.  Method-specific MICs may be used to provide
   protection.  Since EAP messages of Types Identity, Notification, and
   Nak do not include their own MIC, it may be desirable for the EAP
   method MIC to cover information contained within these messages, as
   well as the header of each EAP message.  For details, see Appendix A.

   To provide protection, EAP also may be encapsulated within a
   protected channel created by protocols such as ISAKMP [RFC2408] as is
   done in [PIC] or within TLS [RFC2246]. However, as noted in Section
   7.4, EAP tunneling may result in a man-in-the-middle vulnerability.

7.6 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].

   Where EAP runs over lower layers which are not physically secure, in
   order to protect against dictionary attacks, an authentication
   algorithm resistant to dictionary attack (as defined in Section 7.2)
   SHOULD be used.

   If an authentication algorithm is used that is known to be vulnerable
   to dictionary attack, then the conversation may be tunneled within a
   protected channel, in order to provide additional protection.
   However, as noted in Section 7.4, EAP tunneling may result in a
   man-in-the-middle vulnerability, and therefore dictionary attack
   resistant methods are preferred.

7.7 Connection to an untrusted network

   With EAP methods supporting one-way authentication, such as EAP-MD5,
   the peer does not authenticate the authenticator.  Where the lower
   layer is not physically secure (such as where EAP runs over wireless
   media or the Internet), the peer is vulnerable to a rogue
   authenticator.  As a result, where the lower layer is not physically
   secure, a method supporting mutual authentication is RECOMMENDED.

   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 unitary mutual
   authentication is preferable to two one-way authentications, one in
   each direction.  This is because separate authentications that are
   not bound cryptographically so as to demonstrate they are part of the



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   same session are subject to man-in-the-middle attacks, as discussed
   in Section 7.4.

7.8 Negotiation attacks

   In a negotiation attack, the attacker attempts to convince the peer
   and authenticator to negotiate a less secure EAP method.  EAP does
   not provide protection for Nak Response packets, although it is
   possible for a method to include coverage of Nak Responses within a
   method-specific MIC.

   Within or associated with each authenticator, it is not anticipated
   that a particular named peer will support a choice of methods.  This
   would make the peer vulnerable to attacks that negotiate the least
   secure method from among a set.  Instead, for each named peer there
   SHOULD be an indication of exactly one method used to authenticate
   that peer name.  If a peer 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.

7.9 Implementation idiosyncrasies

   The interaction of EAP with lower layers 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 traffic in Network-Layer
   Protocols to a filtered subset, which in turn allows the peer the
   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 in 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.

7.10 Key derivation

   It is possible for the peer and EAP server to mutually authenticate,
   and derive keys.  In order to provide keying material for use in a
   subsequently negotiated ciphersuite, an EAP method supporting key



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   derivation MUST export a Master Session Key (MSK) of at least 64
   octets, and an Extended Master Session Key (EMSK) of at least 64
   octets.

   The MSK and EMSK are not used directly to protect data; however, they
   are of sufficient size to enable subsequent derivation of Transient
   Session Keys (TSKs) for use with the selected ciphersuite.  Each
   ciphersuite is responsible for specifying how to derive the TSKs from
   the MSK. The EAP method is also responsible for the derivation of
   Transient EAP Keys (TEKs) used for protection of the EAP conversation
   itself.

   EAP methods provide Master Session Keys and not Transient Session
   Keys so as to allow EAP methods to be ciphersuite and media
   independent.  Depending on the lower layer, EAP methods may run
   before or after ciphersuite negotiation, so that the selected
   ciphersuite may not be known to the EAP method.  By providing keying
   material usable with any ciphersuite, EAP methods can used with a
   wide range of ciphersuites and media.

   Non-overlapping substrings of the MSK MUST be cryptographically
   separate from each other.  This is required because some existing
   ciphersuites form TSKs by simply splitting the MSK to pieces of
   appropriate length.  Likewise, non-overlapping substrings of EMSK
   MUST be cryptographically separate from each other, and from
   substrings of the MSK.

   The EMSK is reserved for future use and MUST remain on the EAP peer
   and EAP server where it is derived; it MUST NOT be transported to, or
   shared with, additional parties, or used to derive any other keys.
   (This restriction will be relaxed in a future document that specifies
   how the EMSK can be used.)

   This specification does not provide detailed guidance on how EAP
   methods are to derive the MSK, EMSK and TEKs, or how the TSKs are to
   be derived from the MSK.  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.

   Further details on EAP Key Derivation are provided within [KEYFRAME].

7.11 Weak ciphersuites

   If after the initial EAP authentication, data packets are sent
   without per-packet authentication, integrity and replay protection,
   an attacker with access to the media can inject packets, "flip bits"
   within existing packets, replay packets, or even hijack the session



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   completely.  Without per-packet confidentiality, it is possible to
   snoop data packets.

   As a result, as noted in Section 3.1, where EAP is used over a
   physically insecure lower layer, per-packet authentication, integrity
   and replay protection SHOULD be used, and per-packet confidentiality
   is also recommended.

   Additionally, if the lower layer performs ciphersuite negotiation, it
   should be understood that EAP does not provide by itself integrity
   protection of that negotiation.  Therefore, in order to avoid
   downgrading attacks which would lead to weaker ciphersuites being
   used, clients implementing lower layer ciphersuite negotiation SHOULD
   protect against negotiation downgrading.

   This can be done by enabling users to configure which are the
   acceptable ciphersuites as a matter of security policy; or, the
   ciphersuite negotiation MAY be authenticated using keying material
   derived from the EAP authentication and a MIC algorithm agreed upon
   in advance by lower-layer peers.

7.12 Link layer

   There exists a number of reliability and security issues with link
   layer indications in PPP, IEEE 802 wired networks and IEEE 802.11
   wireless LANs:

   [a] PPP.  In PPP, link layer indications such as LCP-Terminate (a
       link failure indication) and NCP (a link success indication) are
       not authenticated or integrity protected.  They can therefore be
       spoofed by an attacker with access to the physical medium.

   [b] IEEE 802 wired networks.  On wired networks, IEEE 802.1X messages
       are sent to a non-forwardable multicast MAC address.  As a
       result, while the IEEE 802.1X EAPOL-Start and EAPOL-Logoff frames
       are not authenticated or integrity protected, only an attacker
       with access to the physical link can spoof these messages.

   [c] IEEE 802.11 wireless LANs.  In IEEE 802.11, link layer
       indications include Disassociate and Deauthenticate frames (link
       failure indications), and Association and Reassociation Response
       frames (link success indications). These messages are not
       authenticated or integrity protected, and although they are not
       forwardable, they are spoofable by an attacker within range.

       In IEEE 802.11, IEEE 802.1X data frames may be sent as Class 3
       unicast data frames, and are therefore forwardable.  This implies
       that while EAPOL-Start and EAPOL-Logoff messages may be



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       authenticated and integrity protected, they can be spoofed by an
       authenticated attacker far from the target when
       "pre-authentication" is enabled.

       In IEEE 802.11 a "link down" indication is an unreliable
       indication of link failure, since wireless signal strength can
       come and go and may be influenced by radio frequency interference
       generated by an attacker.  To avoid unnecessary resets, it is
       advisable to damp these indications, rather than passing them
       directly to the EAP.  Since EAP supports retransmission, it is
       robust against transient connectivity losses.


7.13 Separation of authenticator and backend authentication server

   It is possible for the EAP peer and authenticator to mutually
   authenticate, and derive a Master Session Key (MSK) for a ciphersuite
   used to protect subsequent data traffic.  This does not present an
   issue on the peer, since the peer and EAP client reside on the same
   machine; all that is required is for the EAP client module to derive
   and pass a Transient Session Key (TSK) to the ciphersuite module.

   However, in the case where the authenticator and authentication
   server reside on different machines, there are several implications
   for security.

   [a] Authentication will occur between the peer and the 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 that it is speaking to, using EAP alone.

   [b] As discussed in [RFC2869bis], the authenticator is dependent on
       the AAA protocol in order to know the outcome of an
       authentication conversation, and does not look at the
       encapsulated EAP packet (if one is present) to determine the
       outcome.  In practice this means that the AAA protocol spoken
       between the authenticator and authentication server MUST support
       per-packet authentication, integrity and replay protection.

   [c] Where EAP is used over lower layers which are not physically
       secure, subsequent to completion of the EAP conversation, a
       subsequent protocol SHOULD be run between the peer and
       authentication in order to mutually authenticate the peer and
       authenticator; guarantee liveness of the TSKs; provide protected
       ciphersuite and capabilities negotiation; and provide for
       synchronized key usage.





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   [d] An EAP Master Session Key (MSK) negotiated between the peer and
       authentication server MAY be transmitted to the authenticator.
       Therefore a mechanism needs to be provided to transmit the MSK
       from the authentication server to the authenticator that needs
       it.  The specification of the key transport and wrapping
       mechanism is outside the scope of this document.


8. Acknowledgments

   This protocol derives much of its inspiration from Dave Carrel's AHA
   draft as well as the PPP CHAP protocol [RFC1994]. Valuable feedback
   was provided by Yoshihiro Ohba of Toshiba America Research, Jari
   Arkko of Ericsson, Sachin Seth of Microsoft, Glen Zorn of Cisco
   Systems, Jesse Walker of Intel, Bill Arbaugh, Nick Petroni and Bryan
   Payne of the University of Maryland, Steve Bellovin of AT&T Research,
   Paul Funk of Funk Software, Pasi Eronen of Nokia, Joseph Salowey of
   Cisco and Paul Congdon of HP and members of the EAP working group.

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.

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

   [RFC2243]  Metz, C., "OTP Extended Responses", RFC 2243, November
              1997.

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

   [RFC2289]  Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time
              Password System", RFC 2289, February 1998.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

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

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



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   [IEEE-802]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Overview and
              Architecture", IEEE Standard 802, 1990.

   [IEEE-802.1X]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Port-Based Network Access
              Control", IEEE Standard 802.1X, September 2001.

Informative References

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

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

   [RFC2246]  Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
              and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
              January 1999.

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

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

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

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

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

   [RFC2607]  Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
              Implementation in Roaming", RFC 2607, June 1999.

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

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



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   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

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

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

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

   [PIC]      Aboba, B., Krawczyk, H. and Y. Sheffer, "PIC, A Pre-IKE
              Credential Provisioning Protocol", draft-ietf-ipsra-pic-06
              (work in progress), October 2002.

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

   [IEEE-802.3]
              Institute of Electrical and Electronics Engineers,
              "Information technology - Telecommunications and
              Information Exchange between Systems - Local and
              Metropolitan Area Networks - Specific requirements - Part
              3: Carrier sense multiple access with collision detection
              (CSMA/CD) Access Method and Physical Layer
              Specifications", IEEE Standard 802.3, September 1998.

   [IEEE-802.11]
              Institute of Electrical and Electronics Engineers,
              "Information Technology - Telecommunications and
              Information Exchange between Systems - Local and
              Metropolitan Area Network - Specific Requirements - Part
              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications", IEEE Standard 802.11, 1999.

   [SILVERMAN]
              Silverman, Robert D., "A Cost-Based Security Analysis of
              Symmetric and Asymmetric Key Lengths", RSA Laboratories



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              Bulletin 13, April 2000 (Revised November 2001), http://
              www.rsasecurity.com/rsalabs/bulletins/bulletin13.html.

   [RFC2869bis]
              Aboba, B. and P. Calhoun, "RADIUS Support For Extensible
              Authentication Protocol (EAP)",
              draft-aboba-radius-rfc2869bis-21 (work in progress), May
              2003.

   [IANA-EXP]
              Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful",
              draft-narten-iana-experimental-allocations-03 (work in
              progress), December 2002.

   [KEYFRAME]
              Aboba, B. and D. Simon, "EAP Keying Framework",
              draft-aboba-pppext-key-problem-06 (work in progress),
              March 2003.

   [SASLPREP]
              Zeilenga, K., "SASLprep: Stringprep profile for user names
              and passwords", draft-ietf-sasl-saslprep-01 (work in
              progress), May 2003.

   [IEEE-802.11i]
              Institute of Electrical and Electronics Engineers,
              "Unapproved Draft Supplement to Standard for
              Telecommunications and Information Exchange Between
              Systems - LAN/MAN Specific Requirements - Part 11:
              Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications: Specification for Enhanced
              Security", IEEE Draft 802.11i (work in progress), 2003.


Authors' Addresses

   Larry J. Blunk
   Merit Network, Inc
   4251 Plymouth Rd., Suite 2000
   Ann Arbor, MI  48105-2785
   USA

   Phone: +1 734-647-9563
   Fax:   +1 734-647-3185
   EMail: ljb@merit.edu





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   John R. Vollbrecht
   Vollbrecht Consulting LLC
   9682 Alice Hill Drive
   Dexter, MI  48130
   USA

   Phone:
   EMail: jrv@umich.edu


   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 706 6605
   Fax:   +1 425 936 6605
   EMail: bernarda@microsoft.com


   James Carlson
   Sun Microsystems, Inc
   1 Network Drive
   Burlington, MA  01803-2757
   USA

   Phone: +1 781 442 2084
   Fax:   +1 781 442 1677
   EMail: james.d.carlson@sun.com


   Henrik Levkowetz
   ipUnplugged AB
   Arenavagen 33
   Stockholm  S-121 28
   SWEDEN

   Phone: +46 8 725 9513
   EMail: henrik@levkowetz.com

Appendix A. Method Specific Behavior

A.1 Message Integrity Checks

   Today, EAP methods commonly define message integrity checks (MICs)
   that cover more than one EAP packet.  For example, EAP-TLS [RFC2716]
   defines a MIC over a TLS record that could be split into multiple



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   fragments; within the FINISHED message, the MIC is computed over
   previous messages.  Where the MIC covers more than one EAP packet, a
   MIC validation failure is typically considered a fatal error.

   If a per-packet MIC is employed within an EAP method, then peers,
   authentication servers, and authenticators not operating in
   pass-through mode MUST validate the MIC. MIC validation failures
   SHOULD be logged. Whether a MIC validation failure is considered a
   fatal error or not is determined by the EAP method specification.

   Within EAP-TLS [RFC2716] a MIC validation failure is treated as a
   fatal error, since that is what is specified in TLS [RFC2246].
   However, it is also possible to develop EAP methods that support
   per-packet MICs, and respond to verification failures by silently
   discarding the offending packet.

   In this document, descriptions of EAP message handling assume that
   per-packet MIC validation, where it occurs, is effectively performed
   as though it occurs before sending any responses or changing the
   state of the host which received the packet.

Appendix B. Changes from RFC 2284

   This section lists the major changes between [RFC2284] and this
   document.  Minor changes, including style, grammar, spelling and
   editorial changes are not mentioned here.

   o  The Terminology section (Section 1.2) has been expanded, defining
      more concepts and giving more exact definitions.

   o  In Section 2, it is explicitly specified that more than one
      exchange of Request and Response packets may occur as part of the
      EAP authentication exchange.  How this may and may not be used is
      specified in detail in Section 2.1.

   o  Also in Section 2, some requirements on the authenticator when
      acting in pass-through mode has been made explicit.

   o  An EAP multiplexing model (Section 2.2) has been added, to
      illustrate a typical implementation of EAP.  There is no
      requirement that an implementation conforms to this model, as long
      as the on-the-wire behavior is consistent with it.

   o  As EAP is now in use with a variety of lower layers, not just PPP
      for which it was first designed, Section 3 on lower layer behavior
      has been added.

   o  In the description of the EAP Request and Response interaction



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      (Section 4.1), it has been more exactly specified when packets
      should be silently discarded, and also the behavior on receiving
      duplicate requests. The implementation notes in this section has
      been substantially expanded.

   o  In Section 4.2, it has been clarified that Success and Failure
      packets must not contain additional data, and the implementation
      note has been expanded.  A subsection giving requirements on
      processing of success and failure packets has been added.

   o  Section 5 on EAP Request/Response Types lists two new Type values:
      the Expanded Type (Section 5.7), which is used to expand the Type
      value number space, and the Experimental Type. In the Expanded
      Type number space, the new Expanded Nak (Section 5.3.2) Type has
      been added.  Clarifications have been made in the description of
      most of the existing Types.  Security claims summaries have been
      added for authentication methods.

   o  In Section 5.5, support for OTP Extended Responses [RFC2243] has
      been added to EAP OTP.

   o  An IANA Considerations section (Section 6) has been added, giving
      registration policies for the numbering spaces defined for EAP.

   o  The Security Considerations (Section 7) have been greatly
      expanded, aiming at giving a much more comprehensive coverage of
      possible threats and other security considerations.

   o  Appendix A has been added on method-specific behavior, providing
      guidance on how EAP method-specific integrity checks should be
      processed.  Where possible, it is desirable for a method-specific
      MIC to be computed over the entire EAP packet, including the EAP
      layer header (Code, Identifier, Length) and EAP method layer
      header (Type, Type-Data).


Appendix C. Open issues

   (This section should be removed by the RFC editor before publication)

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

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

   The current working documents for this draft are available at this
   web site:




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   http://www.levkowetz.com/pub/ietf/drafts/eap/


















































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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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   pertain to the implementation or use of the technology described in
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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