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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 5296

Network Working Group                                       V. Narayanan
Internet-Draft                                                L. Dondeti
Intended status: Standards Track                          Qualcomm, Inc.
Expires: May 16, 2008                                  November 13, 2007


        EAP Extensions for EAP Re-authentication Protocol (ERP)
                        draft-ietf-hokey-erx-07

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   This Internet-Draft will expire on May 16, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The extensible authentication protocol (EAP) is a generic framework
   supporting multiple types of authentication methods.  In systems
   where EAP is used for authentication, it is desirable to not repeat
   the entire EAP exchange with another authenticator.  This document
   specifies extensions to EAP and EAP keying hierarchy to support an
   EAP method-independent protocol for efficient re-authentication
   between the peer and the server through an authenticator.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  ERP Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  ERP With the Home AAA Server . . . . . . . . . . . . . . .  5
     3.2.  ERP with a Local ER Server . . . . . . . . . . . . . . . .  7
   4.  ER Key Hierarchy . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Key Derivations and Properties . . . . . . . . . . . . . .  9
       4.1.1.  rRK Derivation . . . . . . . . . . . . . . . . . . . .  9
       4.1.2.  rRK Properties . . . . . . . . . . . . . . . . . . . . 10
       4.1.3.  rIK Derivation . . . . . . . . . . . . . . . . . . . . 11
       4.1.4.  rIK Properties . . . . . . . . . . . . . . . . . . . . 11
       4.1.5.  rIK Usage  . . . . . . . . . . . . . . . . . . . . . . 12
       4.1.6.  rMSK Derivation  . . . . . . . . . . . . . . . . . . . 12
       4.1.7.  rMSK Properties  . . . . . . . . . . . . . . . . . . . 13
   5.  Protocol Description . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  ERP Bootstrapping  . . . . . . . . . . . . . . . . . . . . 14
       5.1.1.  ERP Bootstrapping with a Local ER Server . . . . . . . 16
     5.2.  EAP Re-auth Protocol . . . . . . . . . . . . . . . . . . . 17
       5.2.1.  Failure Handling . . . . . . . . . . . . . . . . . . . 19
     5.3.  New EAP Messages . . . . . . . . . . . . . . . . . . . . . 20
       5.3.1.  EAP-Initiate/Re-auth-Start Packet  . . . . . . . . . . 21
       5.3.2.  EAP-Initiate/Re-auth Packet  . . . . . . . . . . . . . 23
       5.3.3.  EAP Finish/Re-auth Packet  . . . . . . . . . . . . . . 26
       5.3.4.  TV and TLV Attributes  . . . . . . . . . . . . . . . . 28
     5.4.  Replay Protection  . . . . . . . . . . . . . . . . . . . . 29
     5.5.  Channel Binding  . . . . . . . . . . . . . . . . . . . . . 30
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 35
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Example ERP Exchange  . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
   Intellectual Property and Copyright Statements . . . . . . . . . . 39














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1.  Introduction

   The extensible authentication protocol (EAP) is a generic framework
   for transport of methods that authenticate two parties; the
   authentication is either one-way or mutual.  The primary purpose is
   network access control, and a key-generating method is recommended to
   enforce access control.  The EAP keying hierarchy defines two keys
   that are derived at the top level: the Master Session Key (MSK) and
   the Extended MSK (EMSK).  In the most common deployment scenario, a
   peer and a server authenticate each other through a third party known
   as the authenticator.  The authenticator or an entity controlled by
   the authenticator enforces access control.  After successful
   authentication, the server transports the MSK to the authenticator;
   the authenticator and the peer derive transient session keys (TSK)
   using the MSK as the authentication key or a key derivation key and
   use the TSK for per-packet access enforcement.

   When a peer moves from one authenticator to another, it is desirable
   to avoid a full EAP authentication.  The full EAP exchange with
   another run of the EAP method can take several round trips and
   significant time to complete, causing delays in handover times.  Some
   EAP methods specify the use of state from the initial authentication
   to optimize re-authentications by reducing the computational
   overhead, but method-specific re-authentication takes at least 2
   round trips in most cases (e.g., [8]).  It is also important to note
   that many methods do not offer support for re-authentication.  Thus,
   it is beneficial to have efficient re-authentication support in EAP
   rather than in individual methods.

   Key sharing across authenticators is sometimes used as a practical
   solution to lower handover times.  In that case, compromise of an
   authenticator results in compromise of keying material established
   via other authenticators.

   Other solutions for fast re-authentication exist in the literature
   [9] [10].

   In conclusion, there is a need to design an efficient EAP re-
   authentication mechanism that allows a fresh key to be established
   between the peer and an authenticator without having to execute the
   EAP method again.  The EAP re-authentication problem statement is
   described in detail in [11].

   This document specifies EAP Re-authentication Extensions (ERX) for
   efficient re-authentication using EAP.  The protocol that uses these
   extensions itself is referred to as the EAP re-authentication
   Protocol (ERP).  It supports EAP method independent re-authentication
   for a peer that has valid, unexpired key material from a previously



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   performed EAP authentication.  The protocol and the key hierarchy
   required for EAP re-authentication is described in this document.


2.  Terminology

   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 RFC 2119 [1].

   This document uses the basic EAP terminology [2] and EMSK keying
   hierarchy terminology [3].  In addition, this document uses the
   following terms:

      ER peer - An EAP peer that supports the EAP re-authentication
      protocol.  All references to "peer" in this document imply an ER
      peer, unless specifically noted otherwise.

      ER Authenticator - An entity that supports the authenticator
      functionality for EAP re-authentication described in this
      document.  All references to "authenticator" in this document
      imply an ER authenticator, unless specifically noted otherwise.

      ER Server - An entity that performs the server portion of ERP
      described here.  This entity may or may not be an EAP server.  All
      references to "server" in this document imply an ER server, unless
      specifically noted otherwise.

      ERX: EAP re-authentication extensions.

      ERP: EAP re-authentication Protocol that uses the re-
      authentication extensions.

      rRK - re-authentication root Key, derived from the EMSK or DSRK.

      rIK - re-authentication Integrity Key, derived from the rRK.

      rMSK - re-authentication MSK.  This is a per-authenticator key,
      derived from the rRK.

      Domain - Refers to a "key management domain" as defined in [3].
      For simplicity, it is referred to as "domain" in this document.
      The terms "home domain" and "local domain" are used to
      differentiate between the originating key management domain that
      performs the full EAP exchange with the peer and the local domain
      to which a peer may be attached to at a given time.





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3.  ERP Overview

   Figure 1 shows the protocol exchange.  The first time the peer
   attaches to an authenticator, it performs a full EAP exchange with
   the EAP server; as a result an MSK is distributed to the
   authenticator.  The MSK is then used by the authenticator and the
   peer to generate TSKs as needed.  At the time of the initial EAP
   exchange, the peer and the server derive a re-authentication Root Key
   (rRK).  The rRK may be derived from the EMSK or from a Domain
   Specific Root Key (DSRK).  The rRK is only available to the peer and
   the ER server and is never handed out to any other entity.  Further,
   a re-authentication Integrity Key (rIK) is derived from the rRK; the
   peer uses the rIK to provide proof of possession while performing an
   ERP exchange at a later time.  The rIK is also never handed out to
   any entity and is only available to the peer and server.

   When the ER server is in the home domain, the peer and the server use
   the rIK and rRK derived from the EMSK and when the ER server is not
   in the home domain, they use the DS-rIK and DS-rRK corresponding to
   the local domain.  The realm in the rIKname-NAI or the Peer-ID
   reflects the ER server's domain.

3.1.  ERP With the Home AAA Server




























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   Peer               Authenticator                   Server
   ====               =============                   ======

    <--- EAP-Request/ ------
            Identity

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

    <------------ EAP Method exchange------------------->

                              <----MSK, EAP-Success------

    <---EAP-Success---------


        Peer               Authenticator                   Server
        ====               =============                   ======

    [<-- EAP-Request/ ------
        Identity]
    [<-- EAP Initiate/ ------
        Reauth-Start]


    ---- EAP Initiate/ ----> ----EAP Initiate/ ---------->
          Re-auth/                  Re-auth/
         [Bootstrap]              [Bootstrap]

    <--- EAP Finish/ ------> <---rMSK,EAP Finish/---------
          Re-auth/                   Re-auth/
        [Bootstrap]               [Bootstrap]


                          Figure 1: ERP Exchange

   When the peer subsequently identifies a target authenticator that
   supports EAP re-authentication, it performs an ERP exchange, as shown
   in Figure 1 as well; the exchange itself may happen when the peer
   attaches to a new authenticator supporting EAP re-authentication, or
   prior to attachment.  The peer initiates ERP by itself; it may also
   do so in response to an EAP-Request/Identity or EAP-Initiate/
   Re-auth-Start message from the new authenticator.  The EAP-Initiate/
   Re-auth-Start message allows the authenticator to initiate the ERP
   exchange.  It is plausible that the authenticator does not know
   whether the peer supports ERP and whether it has performed a full EAP
   authentication through another authenticator and hence the
   authenticator initiation of the ERP exchange may require the



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   authenticator to send both the EAP-Request/Identity and EAP-Initiate/
   Re-auth-Start messages.

   We introduce two new codes to EAP: EAP-Initiate and EAP-Finish.  The
   peer sends an EAP-Initiate/Re-auth message that includes the Peer-ID
   or a temporary NAI based on the rIKname, and a sequence number for
   replay protection.  The Peer-ID used here is the same as that
   exported by the EAP method, when it is available.  The EAP-Initiate/
   Re-auth message is integrity protected with the rIK.  The message is
   routed using the NAI in the rIKname-NAI [4], field and if that is not
   present, it is routed using the NAI in the Peer-ID.  The server uses
   the rIKname or the Peer-ID in that order to lookup the rIK.  The
   server, after verifying proof of possession of the rIK, and freshness
   of the message, derives a re-authentication MSK (rMSK) from the rRK
   using the sequence number as an input to the key derivation.

   In response to the EAP-Initiate/Re-auth message, the server sends an
   EAP-Finish/Re-auth message; this message is integrity protected with
   the rIK.  The server transports the rMSK along with this message to
   the authenticator.  The rMSK is transported in a manner similar to
   that of the MSK along with the EAP-Success message in a full EAP
   exchange.  Ongoing work in [12] describes an additional key
   distribution protocol that can be used to transport the rRK from an
   EAP server to one of many different ER servers that share a AAA trust
   relationship with the EAP server.

   In an ERP bootstrap exchange, the peer may request the rRK lifetime
   to be sent to it.  If so, the ER server sends the lifetime along with
   the EAP-Finish/Re-auth message.

   The peer verifies the replay protection and the origin of the
   message.  It then uses the sequence number in the EAP-Finish/Re-auth
   message to compute the rMSK.  The lower-layer security association
   protocol is ready to be triggered after this point.

3.2.  ERP with a Local ER Server

   The defined ER extensions allow executing the ERP with a local ER
   server that may be topologically closer to the authenticator.  The
   local ER server may be collocated with a local AAA server.  The peer
   may learn about the presence of a local ER server in the network and
   the local domain (or ER server) name either via the lower layer or by
   means of ERP bootstrapping.  Figure 2 shows the full EAP and
   subsequent local ERP exchange with a local ER server.







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   Peer           Authenticator       Local Server       Home Server
   ====           =============       ============       ===========

    <-- EAP-Request/ -----
          Identity

    --- EAP Response/ --->
          Identity         --EAP Response/-->
                               Identity       --EAP Response/Identity->
                                             [DSRK Req, Domain Identity]

    <------------------------ EAP Method exchange------------------->

                                           <---MSK, DSRK, EAP-Success--

                       <---MSK, EAP-Success--

      <---EAP-Success---


        Peer               Authenticator                Local Server
        ====               =============                ============

    [<-- EAP-Request/ ------
        Identity]
    [<-- EAP Initiate/ ------
        Re-auth-Start]


    ---- EAP Initiate/ ----> ----EAP Initiate/ ---------->
          Re-auth/                  Re-auth/


    <--- EAP Finish/ ------ <---rMSK,EAP Finish/---------
          Re-auth/                   Re-auth/



                       Figure 2: Local ERP Exchange

   As shown in Figure 2, the local ER server may be present in the path
   of the full EAP exchange (e.g., this may be one of the AAA entities,
   such as AAA proxies, in the path between the authenticator and the
   home EAP server of the peer).  In that case, at the end of a full
   authentication exchange, the DSRK may be provided to the local ER
   server.  Alternatively, the DSRK can be obtained at the time of an
   ERP bootstrap exchange with the home server.  The DSRK is computed as
   specified in [3].  The local ER server then computes a DS-rRK and a



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   DS-rIK (and the appropriate key names) from the DSRK as defined in
   Section 4.1.1 and Section 4.1.3 below.  The peer also derives the
   DSRK, followed by the DS-rRK and the DS-rIK (and the appropriate key
   names) following the EAP or ERP bootstrap exchange.

   Subsequently, when the peer attaches to an authenticator within the
   local domain, it may perform an ERP exchange with the local ER server
   to obtain an rMSK for the new authenticator.


4.  ER Key Hierarchy

   We define a key hierarchy for ER, rooted at the rRK, and derived as a
   result of a full EAP exchange.  The rRK may be derived from an EMSK
   or DSRK as specified in this document.  For the purpose of rRK
   derivation, this document derives a Usage Specific Root Key (USRK) or
   a Domain Specific USRK (DS-USRK) in accordance with [3] for re-
   authentication.  The USRK designated for re-authentication is the re-
   authentication root key (rRK).  A DS-USRK designated for re-
   authentication is the DS-rRK available to a local ER server in a
   particular domain.  For simplicity, the keys are referred to without
   the DS label in the rest of the document.  However, the scope of the
   various keys are limited to just the respective domains they are
   derived for, in the case of the domain specific keys.  Based on the
   ER server with which the peer performs the ERP exchange, it knows the
   corresponding keys that must be used.

   The rRK is used to derive a rIK and one or more rMSKs.  The figure
   below shows the key hierarchy with the rRK, rIK and rMSKs.


             rRK
              |
     +--------+--------+
     |        |        |
    rIK     rMSK1 ...rMSKn


                 Figure 3: Re-authentication Key Hierarchy

4.1.  Key Derivations and Properties

4.1.1.  rRK Derivation

   The rRK may be derived from the EMSK or DSRK.  This section provides
   the relevant key derivations for that purpose.

   The rRK is derived as specified in [3].



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   rRK = prf+ (K, S), where,

      K = EMSK or K = DSRK and

      S = rRK Label + "\0" + NULL + length

   The rRK Label is an IANA-assigned 8-bit ASCII string "EAP Re-
   authentication Root Key@ietf.org" assigned from the Key Label name
   space in accordance with [3].  This document specifies IANA
   registration for the rRK label above.

   The prf+ operation is as defined in [3].

   Along with the rRK, a unique rRK name is derived to identify the rRK.

   The rRKname is derived as follows.

   rRKname = SHA-256-64 (NameDerivationKey, rRK Label)

   where the SHA-256-64 operation is as defined in [3].

   NameDerivationKey = EAP Session-ID, when K used in rRK derivation is
   the EMSK,

   NameDerivationKey = DSRK Name, when K used in rRK derivation is the
   DSRK.

   An rRK derived from the DSRK is referred to as a DS-rRK in the rest
   of the document.  All the key derivation and properties specified in
   this section remain the same.

4.1.2.  rRK Properties

   The rRK has the following properties.  These properties apply to the
   rRK regardless of the parent key used to derive it.

   o  The length of the rRK MUST be equal to the length of the parent
      key used to derive it.

   o  The rRK is to be used only as a root key for re-authentication and
      never used to directly protect any data.

   o  The rRK is only used for derivation of rIK and rMSK as specified
      in this document.

   o  The rRK MUST remain on the peer and the server that derived it and
      MUST NOT be transported to any other entity.




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   o  The lifetime of the rRK is never greater than that of its parent
      key.  The rRK is expired when the parent key expires and MUST be
      removed from use at that time.

4.1.3.  rIK Derivation

   The re-authentication Integrity Key (rIK) is used for integrity
   protecting the ERP exchange.  This serves as the proof of possession
   of valid keying material from a previous full EAP exchange by the
   peer to the server.

   The rIK is derived as follows.

   rIK = prf+ (K, S ) where,

      K = rRK and

      S = rIK Label + "\0" + cryptosuite + length

   The rIK Label is the 8-bit ASCII string "Re-authentication Integrity
   Key@ietf.org" and the length refers to the length of the rIK in
   octets.

   The PRF used MAY be the same as that used by the EAP method - using
   the PRF from the EAP method provides algorithm agility.  Otherwise,
   the default PRF used is HMAC-SHA256.  The PRF is specified as part of
   the ERP message exchange.

   The cryptosuite and length of the rIK are part of the input to the
   key derivation function to ensure cryptographic separation of keys if
   different rIKs of different lengths for use with different MAC
   algorithms are derived from the same rRK.  See Section 5.3.2 for
   cryptosuite specification.

   The rIKname is derived as follows.

   rIKname = SHA256-64(rRKname, rIK Label)

   An rIK derived from a DS-rRK is referred to as a DS-rIK in the rest
   of the document.  All the key derivation and properties specified in
   this section remain the same.

4.1.4.  rIK Properties

   The rIK has the following properties.

   o  The length of the rIK MUST be equal to the length of the rRK.




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   o  The rIK is only used for authentication of the ERP exchange as
      specified in this document.

   o  The rIK MUST NOT be used to derive any other keys.

   o  The rIK must remain on the peer and the server and MUST NOT be
      transported to any other entity.

   o  The rIK is cryptographically separate from any other keys derived
      from the rRK.

   o  The lifetime of the rIK is never greater than that of its parent
      key.  The rIK MUST be expired when the EMSK expires and MUST be
      removed from use at that time.

4.1.5.  rIK Usage

   The rIK is the key whose possession is demonstrated by the peer and
   the ERP server to the other party.  The peer demonstrates possession
   of the rIK by computing the integrity checksum over the EAP-Initiate/
   Re-auth message.  When the peer uses the rIK for the first time, it
   can choose the integrity algorithm to use with the rIK.  The peer and
   the server MUST use the same integrity algorithm with a given rIK for
   all ERP messages protected with that key.  The peer and the server
   store the algorithm information after the first use and the same
   algorithm for all subsequent uses of that rIK.

   If the server's policy does not allow the use of the cryptosuite
   selected by the peer, the server may reject the EAP-Initiate/Re-auth
   message and send a list of acceptable cryptosuites in the EAP-Finish/
   Re-auth message.

   The rIK length may be different from the key length required by an
   integrity algorithm.  In case of hash-based MAC algorithms, the key
   is first hashed to the required key length as specified in [5].  In
   case of cipher-based MAC algorithms, if the required key length is
   less than 32 octets, the rIK is hashed using HMAC-SHA256 and the most
   significant k octets of the output are used where k is the key length
   required by the algorithm.  If the required key length is more than
   32 octets, the most significant k octets of the rIK are used by the
   cipher-based MAC algorithm.

4.1.6.  rMSK Derivation

   The rMSK is derived at the peer and server and delivered to the
   authenticator.  The rMSK is derived following an EAP Re-auth protocol
   exchange.




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   The rMSK is derived as follows.

   rMSK = prf+ (K, S ) where,

      K = rRK and

      S = rMSK label + "\0" + SEQ + length

   The rMSK label is the 8-bit ASCII string "Re-authentication Master
   Session Key@ietf.org" and the length refers to the length of the rMSK
   in octets.

   The SEQ is the sequence number sent by the peer in the EAP-Initiate/
   Re-auth message.

   The PRF is specified as part of the ERP message exchange.  The
   default PRF used is HMAC-SHA256.

   The rMSK name is derived as follows:

   rMSK_name = HMAC-SHA256-64 (rMSK, "rMSK Name")

   An rMSK derived from a DS-rRK is referred to as a DS-rIK in the rest
   of the document.  All the key derivation and properties specified in
   this section remain the same.

4.1.7.  rMSK Properties

   The rMSK has the following properties:

   o  The length of the rMSK MUST be equal to the length of the rRK.

   o  The rMSK is delivered to the authenticator and is used for the
      same purposes that an MSK is used at an authenticator.

   o  The rMSK is cryptographically separate from any other keys derived
      from the rRK.

   o  The lifetime of the rMSK is less than or equal to that of the rRK.
      It MUST NOT be greater than the lifetime of the rRK.

   o  If a new rRK is derived, subsequent rMSKs MUST be derived from the
      new rRK.  Previously delivered rMSKs MAY still be used until the
      expiry of the lifetime.

   o  A given rMSK MUST NOT be shared by multiple authenticators.





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5.  Protocol Description

   ERP allows a peer and server to verify proof of possession of keying
   material from an earlier EAP method run and establish a security
   association between the peer and an authenticator.  The authenticator
   acts as a pass-through entity for the Re-auth protocol in a manner
   similar to that described in RFC 3748 [2].  ERP is a single round-
   trip exchange between the peer and the server; it is independent of
   the lower layer and the EAP method used during the full EAP exchange.

5.1.  ERP Bootstrapping

   When the peer requires the local domain identity to use ERP in the
   local domain, or when it moves to a new domain and needs to have a
   new DSRK delivered to the local ER server and wants to obtain the
   domain identity for domain-specific key derivation, it can use the
   bootstrapping process with the home domain ER server.

   We identify two types of bootstrapping for ERP: explicit and implicit
   bootstrapping.  In implicit bootstrapping, the domain specific keys
   are delivered to the local ER server during the EAP exchange.  The
   peer learns the domain identity through out-of-band means.  When the
   domain identity is available to the peer during or after the full EAP
   authentication, it attempts to use ERP when it associates with a new
   authenticator.

   For explicit bootstrapping, the peer initiates the EAP Re-auth
   exchange with the bootstrapping flag turned on immediately after the
   full EAP authentication finishes.  The following steps summarize the
   process:

   o  The peer sends the EAP-Initiate/Re-auth message with the
      bootstrapping flag turned on.  It is RECOMMENDED that the
      authenticator hold on to the state (e.g., called station id in
      RADIUS) that allows all messages of a full EAP conversation to be
      routed to the same server.  The EAP-Initiate/Re-auth message
      contains one or more TLVs containing identification information to
      assist the authenticator further in routing the message to the
      appropriate ER server -- in this case to the ER server that holds
      the EMSK, rRK, and rIK.

      *  It is mandatory to send the rIKname either by itself, or as
         part of an NAI (see Section 5.3.2).  The authenticator may use
         the NAI to route the EAP-Initiate/Re-auth Bootstrap message.

      *  When rIKname-NAI is not available, the Peer-ID SHOULD be
         included.  The Peer-ID may be in the form of a pseudonym for
         identity privacy.



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      *  If an NAI is not available as part of the peer-name or the
         rIKname, an authenticator routes the ERP packets to the default
         ER server in the network.  The default ER server may be the
         authenticator itself.  When neither an NAI nor a default ER
         server are available to an authenticator, it drops the ERP
         packets silently.

   o  In addition to the identities, the message contains a sequence
      number for replay protection, a cryptosuite, and an integrity
      checksum.  The cryptosuite indicates the authentication algorithm
      and the PRF.  The integrity checksum indicates that the message
      originated at the claimed entity, the peer indicated by the
      Peer-ID, or the rIKname.

   o  The peer may additionally set the lifetime flag to request that
      the rRK lifetime be sent to it.

   o  When an ERP-capable authenticator receives EAP-Initiate/Re-auth
      message from a peer, it looks for local EAP forwarding state
      corresponding to the peer's lower-layer address and forwards the
      message accordingly.  This forwarding is similar to that of
      messages of an EAP conversation.  It is RECOMMENDED that an ERP-
      capable authenticator store that forwarding information for a
      finite amount of time after the EAP-Success message has been sent
      to the peer.

      *  In the absence of forwarding state, the authenticator parses
         the EAP-Initiate/Re-auth message to locate the rIKname, and if
         the rIKname is in the NAI form, uses that domain identity to
         forward the message.

      *  Otherwise, it finds the Peer-ID and uses the realm portion of
         the Peer-ID to route the EAP message to the appropriate server.

      *  In the absence of an NAI, the authenticator routes packets to
         the default ER server in the local domain.  If no such
         information is available, the authenticator silently drops the
         packets.

   o  Upon receipt of an EAP-Initiate/Re-auth message, the server
      verifies whether the message is fresh or a replay by evaluating
      whether the received sequence number is equal to or greater than
      the expected sequence number for that rIK.  The server then
      verifies to ensure that the cryptosuite used by the peer is
      acceptable.  Next, it verifies the origin authentication of the
      message by looking up the rIK.  If any of the checks fail, the
      server sends an EAP-Finish/Re-auth message with the Result flag
      set to '1'.  Please refer to Section 5.2.1 for details on failure



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      handling.  This error MUST NOT have any correlation to any EAP-
      Success message that may have been received by the authenticator
      and the peer earlier.  If the EAP-Initiate/Re-auth message is
      well-formed and valid, the server prepares the EAP-Finish/Re-auth
      message.  The bootstrap flag is set to indicate that this is a
      bootstrapping exchange.  The message contains the following
      fields:

      *  A sequence number for replay protection.

      *  The rIKname so that the peer can correctly identify the rIK to
         verify the integrity and origin authentication of the EAP-
         Finish/Re-auth message.

      *  If the lifetime flag was set in the EAP-Initiate/Re-auth
         message, the ER server SHOULD include the rRK lifetime in the
         EAP-Finish/Re-auth message.  The server may have a local policy
         to maintain and enforce lifetime unilaterally.  In such cases,
         the server need not respond to the peer's request for the
         lifetime.

      *  An authentication tag to prove that the EAP-Finish/Re-auth
         message originates at a server that possesses the rIK
         corresponding to the rIKname.

   o  In addition, the rMSK is sent along with the EAP-Finish/Re-auth
      message, in a AAA attribute [13].

   Since the ER bootstrapping exchange is typically done immediately
   following the full EAP exchange, it is feasible that the process is
   completed through the same entity that served as the EAP
   authenticator for the full EAP exchange.  In this case, the lower
   layer may already have derived the TSKs based on the MSK received
   earlier.  The lower layer may then choose to ignore the rMSK that was
   received with the ER bootstrapping exchange.  Alternatively, the
   lower layer may choose to generate a TSK from the rMSK.  However, the
   bootstrapping exchange may be carried out via a new authenticator, in
   which case, the rMSK received is used by the lower layer.

5.1.1.  ERP Bootstrapping with a Local ER Server

   When a local ER server is present, it may be in the path of the full
   EAP exchange performed by the peer.  In this case, the local ER
   server SHOULD include a request for DSRK and its domain or server
   name along with the AAA message encapsulating the first EAP Response
   message sent by the peer.  If the EAP exchange is successful, the
   server sends a DSRK (for the local ER server) along with the EAP-
   Success message.  The local ER server MUST extract the DSRK, if



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   present, before forwarding the EAP-Success message to the peer, as
   specified in [13] or [14].  Note that the MSK (also present along
   with the EAP Success message) is still extracted by the authenticator
   as usual.

   If the peer performs an ERP bootstrapping exchange when a local ER
   server is present, the local ER server MUST include the DSRK request
   and its domain identity in the AAA message encapsulating the EAP-
   Initiate/Re-auth message sent by the peer.  If the exchange is
   successful, the home ER server MUST include a DSRK along with the
   EAP-Finish Re-auth message.  The local ER server MUST extract the
   DSRK, if present, before forwarding the EAP-Finish/Re-auth message to
   the peer.

   When the server receives an EAP-Initiate/Re-auth message with the
   bootstrap flag set along with a DSRK request, it SHOULD return the
   domain identity to which the DSRK was sent, in the EAP-Finish/Re-auth
   message.  The other processing rules for the ERP bootstrapping
   exchange specified in Section 5.1 apply as well.

   When the peer receives an EAP-Finish/Re-auth message with the
   bootstrap flag set, if a local domain identity is present, it MUST
   use that to derive the appropriate DSRK, DS-rRK and DS-rIK.  If not,
   the peer SHOULD derive the domain-specific keys using the domain
   identity it learned via the lower layer.  If the peer has no
   available domain identity, it must assume that there is no local ER
   server available.

   The procedures for encapsulating ERP and obtaining relevant keys
   using RADIUS and Diameter are specified in [13] and [14]
   respectively.

5.2.  EAP Re-auth Protocol

   When a peer that has an active rRK and rIK associates with a new
   authenticator that supports ERP, it may perform an ERP exchange with
   that authenticator.  ERP is typically a peer-initiated exchange,
   consisting of an EAP-Initiate/Re-auth and an EAP-Finish/Re-auth
   message.  The ERP exchange may be performed with a local ER server
   (when one is present) or with the original EAP server.

   It is plausible for the network to trigger the EAP re-authentication
   process however.  When an ERP capable authenticator sends an EAP-
   Request/Identity the peer may in response initiate the EAP re-
   authentication exchange.  Additionally, an ERP-capable authenticator
   may also send an EAP-Initiate/Re-auth-Start message to indicate
   support for ERP.  The peer may or may not wait for these messages to
   arrive to initiate the EAP-Initiate/Re-auth message.



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   The EAP-Initiate/Re-auth-Start message is sent by an ERP-capable
   authenticator.  The authenticator may retransmit it a few times until
   it receives an EAP-Initiate/Re-auth message in response from the
   peer.  The EAP-Initiate/Re-auth message from the peer may have
   originated before the peer receives either an EAP-Request/Identity or
   an EAP-Initiate/Re-auth-Start message from the authenticator.  Hence
   the Identifier value in the EAP-Initiate/Re-auth message is
   independent of the Identifier value in the EAP-Initiate/Re-auth Start
   or the EAP-Request/Identity messages.

   Operational Considerations at the Peer:

   ERP requires that the peer maintain retransmission timers for
   reliable transport of EAP re-authentication messages.  The
   reliability considerations of Section 4.3 of RFC 3748 apply with the
   peer as the retransmitting entity.

   The EAP Re-auth protocol has the following steps:

      The peer sends an EAP-Initiate/Re-auth message including one or
      more identity TLVs: the rIKname or rIKname-NAI, and optionally the
      Peer-ID; also included are the peer's rIK sequence number, and a
      cryptosuite indicating the cryptographic algorithms used.  The
      message is integrity protected with the rIK.  When the peer is
      performing ERP with a local ER server, it MUST use the
      corresponding DS-rIK it shares with the local ER server.  The peer
      sets the lifetime flag to request the rRK lifetime from the
      server.  It may learn this to know when to trigger an EAP method
      exchange.

      If rIKname-NAI is present, the authenticator MUST use that NAI to
      route the message.  If the rIKname-NAI is not present, the
      authenticator MUST use the NAI in the Peer-ID to forward the
      message via AAA.  If neither are available, the authenticator MUST
      forward the ERP messages to the local ER server.  If none of these
      rules apply, the authenticator MUST drop the packets silently.

      The server uses the rIKname to lookup the rIK.  It first verifies
      whether the sequence number is equal to or greater than the
      expected sequence number.  The server then verifies to ensure that
      the cryptosuite used by the peer is acceptable.  The server then
      proceeds to verify the integrity of the message using the rIK,
      thereby verifying proof of possession of that key by the peer.  If
      any of these verifications fail, the server sends an EAP-Finish/
      Re-auth message with the Result flag set to '1' (Failure).  Please
      refer to Section 5.2.1 for details on failure handling.
      Otherwise, it computes an rMSK from the rRK using the sequence
      number as the additional input to the key derivation.



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      The server then sends an EAP-Finish/Re-auth message containing the
      rIK sequence number and the rIKname.  The sequence number MUST be
      same as the received sequence number.  The local copy of the
      sequence number is incremented by 1.  The EAP-Finish/Re-auth
      message is also integrity protected with the rIK.  The server may
      include the server-id with this message.

      If the lifetime flag was set in the EAP-Initiate/Re-auth message,
      the ER server SHOULD include the rRK lifetime in the EAP-Finish/
      Re-auth message.

      The server transports the rMSK along with this message to the
      authenticator.  The rMSK is transported in a manner similar to the
      MSK transport along with the EAP-Success message in a regular EAP
      exchange.

      The peer looks up the sequence number to verify whether it is
      expecting a EAP-Finish/Re-auth message with that sequence number.
      It then looks up the rIKname and verifies the integrity of the
      message.  This also verifies the proof of possession of the rIK at
      the server.  If the verifications fail, the peer logs an error and
      stops the process; otherwise, it proceeds to the next step.

      The peer uses the sequence number to compute the rMSK.

      The lower-layer security association protocol can be triggered at
      this point.

5.2.1.  Failure Handling

   If the processing of the EAP-Initiate/Re-auth message results in a
   failure, the ER server MUST send an EAP Finish Re-auth message with
   the Result flag set to '1'.  If the server has a valid rIK for the
   peer, it MUST integrity protect the EAP-Finish/Re-auth failure
   message.  If the failure is due to an unacceptable cryptosuite, the
   server SHOULD send a list of acceptable cryptosuites (in a TLV of
   Type 5) along with the EAP-Finish/Re-auth message.  In this case, the
   server MUST indicate the cryptosuite used to protect the EAP-Finish/
   Re-auth message in the cryptosuite.  The rIK used with the EAP-
   Finish/Re-auth message in this case MUST be computed as specified in
   Section 4.1.3 using the new cryptosuite.  If the server does not have
   a valid rIK for the peer, the EAP-Finish/Re-auth message indicating a
   failure will be unauthenticated; the server MAY include a list of
   acceptable cryptosuites in the message.

   The peer, upon receiving an EAP-Finish/Re-auth message with the
   Result flag set to '1', MUST verify the sequence number and the
   Authentication Tag to determine the validity of the message.  If the



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   peer supports the cryptosuite, it MUST verify the integrity of the
   received EAP-Finish/Re-auth message.  If the EAP-Finish message
   contains a TLV of Type 5, the peer SHOULD retry the ERP exchange with
   a cryptosuite picked from the list included by the server.  The peer
   MUST use the appropriate rIK for the subsequent ERP exchange, by
   computing it with the corresponding cryptosuite, as specified in
   Section 4.1.3.  If the PRF in the chosen cryptosuite is different
   from the PRF originally used by the peer, it MUST derive a new DSRK
   (if required), rRK and rIK before proceeding with the subsequent ERP
   exchange.

   If the peer cannot verify the integrity of the received message, it
   MAY choose to retry the ERP exchange with one of the cryptosuites in
   the TLV of Type 5, after a failure has been clearly determined
   following the procedure in the next paragraph.

   If the replay or integrity checks fail, the failure message may have
   been sent by an attacker.  It may also imply that the server and peer
   do not support the same cryptosuites; however, the peer cannot
   determine if that is the case.  Hence, the peer SHOULD continue the
   ERP exchange per the retransmission timers before declaring a
   failure.

5.3.  New EAP Messages

   Two new EAP Codes are defined for the purpose of ERP: EAP-Initiate
   and EAP-Finish.  The packet format for these messages follows the EAP
   packet format defined in RFC3748 [2].


   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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


                           Figure 4: EAP Packet

      Code

         5 Initiate

         6 Finish





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         Two new code values are defined for the purpose of ERP.  The
         code values themselves are TBD based on IANA assignment.

      Identifier

         The Identifier field is one octet.  The Identifier field MUST
         be the same if an EAP-Initiate packet is retransmitted due to a
         timeout while waiting for a Finish message.  Any new (non-
         retransmission) Initiate message MUST use a new Identifier
         field.

         The Identifier field of the Finish message MUST match that of
         the currently outstanding Initiate message.  A Peer or
         Authenticator receiving a Finish message whose Identifier value
         does not match that of the currently outstanding Initiate
         message MUST silently discard the packet.

         In order to avoid confusion between new EAP-Initiate messages
         and retransmissions, the peer must choose a an Identifier value
         that is different from the previous EAP-Initiate message,
         especially if that exchange has not finished.  It is
         RECOMMENDED that the authenticator clear EAP Re-auth state
         after 300 seconds.

      Type

         This field indicates that this is an ERP exchange.  Two type
         values are defined in this document for this purpose - Re-auth-
         Start (assigned Type 1), Re-auth (assigned Type 2).

      Type-Data

         The Type-Data field varies with the Type of re-authentication
         packet.

5.3.1.  EAP-Initiate/Re-auth-Start Packet

   The EAP-Initiate/Re-auth-Start packet contains the parameters shown
   in Figure 5 :












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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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Reserved    |     1 or more TVs or TLVs     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 5: EAP-Initiate/Re-auth-Start Packet

      Type = 1.

      Reserved, MUST be zero.  Set to zero on transmission and ignored
      on reception.

      One or more TVs or TLVs are used to convey information to the
      peer; for instance the authenticator may send domain identity to
      the peer.

      TVs or TLVs: In the TV payloads, there is a 1-octet type payload
      and a value with type-specific length.  In the TLV payloads, there
      is a 1-octet type payload and a 1-octet length payload.  The
      length field indicates the length of the value expressed in number
      of octets.

         Domain-Identity: This is a TLV payload.  The Type is 4.  The
         domain identity is to be used as the realm in an NAI [4].

5.3.1.1.  Authenticator Operation

   The authenticator optionally sends the EAP-Initiate/Re-auth-Start
   message to indicate support for ERP to the peer and to initiate ERP
   if the peer has already performed full EAP authentication and has
   unexpired key material.  The authenticator may include the domain
   identity to allow the peer to learn it without lower-layer support or
   the ERP bootstrapping exchange.

   The authenticator may re-transmit the EAP-Initiate/Re-auth-Start
   message a few times for reliable transport.

5.3.1.2.  Peer Operation

   The peer may send the EAP-Initiate/Re-auth message in response to the
   EAP-Initiate/Re-auth-Start message from the authenticator.  If the
   peer does not recognize the Initiate code value, it silently discards
   the message.




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   If the EAP-Initiate/Re-auth-Start message contains the domain
   identity, and if the peer does not already have the domain
   information, the peer uses the domain identity to compute the DSRK
   and uses the corresponding DS-rIK to send an EAP-Initiate/Re-auth
   message in response.

5.3.2.  EAP-Initiate/Re-auth Packet

   The EAP-Initiate/Re-auth packet contains the parameters shown in
   Figure 6 :


   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      |R|B|L| Reserved|             SEQ               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 1 or more TVs or TLVs                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | cryptosuite  |        Authentication Tag                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 6: EAP-Initiate/Re-auth Packet

      Type = 2.

      Flags



         'R' - The R flag is set to 0 and ignored upon reception.

         'B' - The B flag is used as the bootstrapping flag.  If the
         flag is turned on, the message is a bootstrap message.

         'L' - The L flag is used to request the rRK lifetime from the
         server.

         The rest of the 5 bits are set to 0 and ignored on reception.

      SEQ: A 16-bit sequence number is used for replay protection.  The
      SEQ number field is initialized to zero every time a new rRK is
      derived.





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      TVs or TLVs: In the TV payloads, there is a 1-octet type payload
      and a value with type-specific length.  In the TLV payloads, there
      is a 1-octet type payload and a 1-octet length payload.  The
      length field indicates the length of the value expressed in number
      of octets.

         rIKname: This is carried in a TV payload.  The Type is 1 and
         the value is a 64-bit field computed as specified in Section
         Section 4.1.3 and is used to identify the rIK with which the
         ERP messages are protected.

         rIKname-NAI: This is carried in a TLV payload.  The Type is 2.
         The NAI is variable in length, not exceeding 253 octets.  The
         rIKname is the username part of the NAI and is encoded in
         hexadecimal values.  If the rIK is derived from the EMSK, the
         realm part of the NAI is the home domain identity and if the
         rIK is derived from a DSRK, the realm part of the NAI is the
         domain identity used in the derivation of the DSRK.  The NAI
         syntax follows [6].

         Peer-ID: This is a TLV payload.  The Type is 3.  The Peer-ID is
         the NAI of the peer, and is variable in length, not exceeding
         253 octets.

         Authenticator Identifier: This is a TLV payload.  The Type is
         TBD (see Section 5.5 for additional discussion).  The server
         sends the Authenticator Identifier so that the peer can verify
         the identity seen at the lower layer, if channel binding is to
         be supported.

      Cryptosuite: This field indicates the integrity algorithm and the
      PRF used for ERP.  Key lengths and output lengths are either
      indicated or are obvious from the cryptosuite name.  We specify
      some cryptosuites below, in the format Integrity-algorithm_PRF-
      name:

      *  0 RESERVED

      *  1 HMAC-SHA256-64_HMAC-SHA256

      *  2 HMAC-SHA256-128_HMAC-SHA256

      *  3 HMAC-SHA256-256_HMAC-SHA256

      HMAC-SHA256-128_HMAC-SHA256 is mandatory to support.






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      Authentication Tag: This field contains the integrity checksum
      over the ERP packet, excluding the authentication tag field
      itself.  The length of the field is indicated by the Cryptosuite.

5.3.2.1.  Peer Operation

   When an ER capable peer receives an EAP-Request/Identity message from
   an Authenticator, it checks to see if it has valid EAP state from a
   previous EAP authentication.  If the peer has state from a previous
   authentication, and if it knows that the Authenticator is ER capable,
   it sends an EAP-Initiate/Re-auth message instead of an EAP-Response/
   Identity message.  The peer may, upon attachment to an authenticator
   send an EAP-Initiate/Re-auth message in an unsolicited manner.

5.3.2.2.  Authenticator Operation

   If the Authenticator does not recognize the EAP-Initiate Code, it
   silently discards the EAP-Initiate/Re-auth message.

   The Authenticator then parses the message to find the rIKname and
   Peer-ID TLVs.

   If rIKname-NAI is present, the authenticator MUST use that NAI to
   route the message.  If the rIKname-NAI is not present, the
   authenticator MUST use the NAI in the Peer-ID to forward the message
   via AAA.  If neither are available, the authenticator MUST forward
   the ERP messages to the local ER server.  If none of these rules
   apply, the authenticator MUST drop the packets silently.

   The Authenticator sends the ERP messages just as it forwards other
   EAP messages to the EAP server.

5.3.2.3.  Server Operation

   The server uses the following steps in processing EAP re-
   authentication messages:

      The server uses the rIKname to lookup the rIK.  It first verifies
      whether the sequence number is equal to or greater than the
      expected sequence number.  The server then proceeds to verify the
      integrity of the message using the rIK, thereby verifying proof of
      possession of that key by the peer.  If the verifications fail,
      the server sends an EAP-Finish/Re-auth message with a Failure
      indication.  Otherwise, it computes an rMSK from the rRK using the
      sequence number.






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5.3.3.  EAP Finish/Re-auth Packet

   The EAP-Finish/Re-auth packet contains the parameters shown in
   Figure 7 :


   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      |R|B|L| Reserved |             SEQ               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 1 or more TVs or TLVs                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | cryptosuite  |        Authentication Tag                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 7: EAP Finish/Re-auth Packet

      Type = 2.

      Flags



         'R' - The R flag is used as the Result flag - when set to 0, it
         indicates success and when set to '1', it indicates a failure.

         'B' - The B flag is used as the bootstrapping flag.  If the
         flag is turned on, the message is a bootstrap message.

         'L' - The L flag is used to indicate the presence of the rRK
         lifetime TLV.

         The rest of the 5 bits are set to 0 and ignored on reception.

      SEQ: A 16-bit sequence number is used for replay protection.  The
      SEQ number field is initialized to zero every time a new rRK is
      derived.

      TVs or TLVs: In the TV payloads, there is a 1-octet type payload
      and a value with type-specific length.  In the TLV payloads, there
      is a 1-octet type payload and a 1-octet length payload.  The
      length field indicates the length of the value expressed in number
      of octets.




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         rIKname: This is carried in a TV payload.  The Type is 1 and
         the value is a 64-bit field computed as specified in Section
         Section 4.1.3 and is used to identify the rIK with which the
         ERP messages are protected.

         rIKname-NAI: This is carried in a TLV payload.  The Type is 2.
         The NAI is variable in length, not exceeding 253 octets.  The
         rIKname (username part of the NAI) is encoded in hexadecimal
         values.  If the rIK is derived from the EMSK, the realm part of
         the NAI is the home domain identity and if the rIK is derived
         from a DSRK, the realm part of the NAI is the domain identity
         used in the derivation of the DSRK.

         List of cryptosuites: This is a TLV payload.  The Type is 5.
         The value field contains a list of cryptosuites, each of size 1
         octet.  The cryptosuite values are as specified in Figure 6.

         Authenticator Identifier: This is a TLV payload.  The Type is
         TBD (see Section 5.5 for additional discussion).  The server
         sends the Authenticator Identifier so that the peer can verify
         the identity seen at the lower layer, if channel binding is to
         be supported.

      Cryptosuite: This field indicates the integrity algorithm and the
      PRF used for ERP.  Key lengths and output lengths are either
      indicated or are obvious from the cryptosuite name.

      Authentication Tag: This field contains the integrity checksum
      over the ERP packet, excluding the authentication tag field
      itself.  The length of the field is indicated by the Cryptosuite.

5.3.3.1.  Server Operation

   The server then sends an EAP-Finish/Re-auth message containing the
   rIK sequence number, and the rIKname; this message is also integrity
   protected with the rIK.  The server may include one or more server-
   ids with this message.  The server-id is for the peer to use to send
   future ERP messages.

   The server transports the rMSK along with this message to the
   authenticator.  The rMSK is transported in a manner similar to the
   MSK transport along with the EAP-Success message in a regular EAP
   exchange.

5.3.3.2.  Authenticator Operation

   The Authenticator Operation is similar to that in processing an EAP-
   Success message.  It extracts the rMSK just as it does an MSK from a



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   AAA message containing an EAP-Success packet.  The procedures for
   RADIUS and Diameter are defined in [13] and [14] respectively.

5.3.3.3.  Peer Operation

   The peer uses the following steps in processing an EAP-Finish/Re-auth
   message:

      The peer first checks if the identifier in the EAP-Finish/Re-auth
      message is the expected value.

      The peer then checks to see if the sequence number in the received
      message is the same as the sequence number in the EAP-Initiate/
      Re-auth message; otherwise it logs an error.

      Next, it uses the rIKname to lookup the appropriate rIK and
      verifies the integrity of the message.  If the verification
      succeeds, it proceeds to the next step; otherwise, it logs an
      error.

      The peer then uses the sequence number to compute the rMSK.

      The lower-layer security association protocol can be triggered at
      this point.

5.3.4.  TV and TLV Attributes

   The TV attributes that may be present in the EAP-Initiate or EAP-
   Finish messages are of the following format:


   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      |              Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                       Figure 8: TV Attribute Format

   The TLV attributes that may be present in the EAP-Initiate or EAP-
   Finish messages are of the following format:









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


                      Figure 9: TLV Attribute Format

   The following Types are defined in this document:

      '1' - rIKname: TV Payload

      '2' - rIKname-NAI: This is a TLV payload

      '3' - Peer-ID: This is a TLV payload

      '4' - Domain Identity: This is a TLV payload

      '5' - cryptosuite list: This is a TLV payload

      The TLV type range of 128-191 is reserved to carry channel binding
      information in the EAP-Initiate and Finish/Re-auth messages.
      Below are the current assignments (all of them are TLVs):

         '128' - Called-Station-Id

         '129' - Calling-Station-Id

         '130' - NAS-Identifier

         '131' - NAS-IP-Address

         '132' - NAS-IPv6-Address

   The length field indicates the length of the value part of the
   attribute in octets.

5.4.  Replay Protection

   For replay protection, ERP uses sequence numbers.  The sequence
   number is maintained per rIK and is initialized to zero in both
   directions.  In the first EAP-Initiate/Re-auth message, the peer uses
   the sequence number zero or higher.  Note that the when the sequence
   number rotates, the rIK MUST be changed.  The server expects a
   sequence number of zero or higher.  When the server receives an EAP-
   Initiate/Re-auth message, it uses the same sequence number in the
   EAP-Finish Re-auth message.  It increments the expected sequence



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   number by 1.  The server accepts sequence numbers greater than or
   equal to the expected sequence number.

   If the peer sends an EAP-Initiate/Re-auth message, but does not
   receive a response, it retransmits the request (with no changes to
   the message itself) a pre-configured number of times before giving
   up.  However, it is plausible that the server itself may have
   responded to the message and it was lost in transit.  Thus the peer
   MUST increment the sequence number and use the new sequence number to
   send subsequent EAP re-authentication messages.  The peer SHOULD
   increment the sequence number by 1; however, it may choose to
   increment by a larger number.  When the sequence number rotates, the
   peer MUST run full EAP authentication.

5.5.  Channel Binding

   ERP provides a protected facility to carry channel binding (CB)
   information, according to the guidelines in Section 7.15 of [2].  The
   TLV type range of 128-191 is reserved to carry CB information in the
   EAP-Initiate/Re-auth and EAP-Finish/Re-auth messages.  Called-
   Station-Id, Calling-Station-Id, NAS-Identifier, NAS-IP-Address, and
   NAS-IPv6-Address are some examples of channel binding information
   listed in RFC 3748 and they are assigned values 128-132.  Other
   values may be added in future versions of this draft and the rest are
   IANA managed based on IETF Consensus [7].


6.  Security Considerations

   This section provides an analysis of the protocol in accordance with
   the AAA key management requirements specified in [15].

      Cryptographic algorithm independence

         The EAP Re-auth protocol satisfies this requirement.  The
         algorithm chosen by the peer for the PRF used in key derivation
         as well as for the MAC generation is indicated in the EAP re-
         authentication Response message.  If the chosen algorithms are
         unacceptable, the EAP server returns an EAP-Failure message in
         response.  Only when the specified algorithms are acceptable,
         the server proceeds with derivation of keys and verification of
         the proof of possession of relevant keying material by the
         peer.  A full blown negotiation of algorithms cannot be
         provided in a single round trip protocol.  Hence, while the
         protocol provides algorithm agility, it does not provide true
         negotiation.





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      Strong, fresh session keys

         ERP results in the derivation of strong, fresh keys that are
         unique for the given session.  An rMSK is always derived on-
         demand when the peer requires a key with a new authenticator.
         The derivation ensures that the compromise of one rMSK does not
         result in the compromise of a different rMSK at any time.

      Limit key scope

         The scope of all the keys derived by ERP are well defined.  The
         rRK and rIK are never shared with any entity and always remain
         on the peer and the server.  The rMSK is provided only to the
         authenticator through which the peer performs the ERP exchange.
         No other authenticator is authorized to use that rMSK.

      Replay detection mechanism

         For replay protection of ERP messages, a sequence number
         associated with the rIK is used.  The sequence number is
         maintained by the peer and the server, and initialized to zero
         when the rIK is generated.  The peer increments the sequence
         number by one after it sends an ERP message.  The server
         increments the sequence number when it receives and responds to
         the message.

      Authenticate all parties

         The EAP Re-auth protocol provides mutual authentication of the
         peer and the server.  Both parties need to possess the keying
         material that resulted from a previous EAP exchange in order to
         successfully derive the required keys.  Also, both the EAP re-
         authentication Response and the EAP re-authentication
         Information messages are integrity protected so that the peer
         and the server can verify each other.  When the ERP exchange is
         executed with a local ER server, the peer and the local server
         mutually authenticate each other via that exchange in the same
         manner.  The peer and the authenticator authenticate each other
         in the secure association protocol executed by the lower layer
         just as in the case of a regular EAP exchange.

      Peer and authenticator authorization

         The peer and authenticator demonstrate possession of the same
         key material without disclosing it, as part of the lower layer
         secure association protocol.  Channel binding with ERP may be
         used to verify consistency of the identities exchanged, when
         the identities used in the lower layer differ from that



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         exchanged within the AAA protocol.

      Keying material confidentiality

         The peer and the server derive the keys independently using
         parameters known to each entity.  The rMSK is sent to the
         authenticator via the AAA protocol.  It is RECOMMENDED that the
         AAA protocol be protected using IPsec or TLS so that the key
         can be sent encrypted to the authenticator.

      Confirm cryptosuite selection

         Crypto algorithms for integrity and key derivation in the
         context of ERP MAY be the same as that used by the EAP method.
         In that case, the EAP method is responsible for confirming the
         cryptosuite selection.  Furthermore, the cryptosuite is
         included in the ERP exchange by the peer and confirmed by the
         server.  The protocol allows the server to reject the
         cryptosuite selected by the peer and provide alternatives.
         When a suitable rIK is not available for the peer, the
         alternatives may be sent in an unprotected fashion.  The peer
         is allowed to retry the exchange using one of the allowed
         cryptosuites.  However, any enroute modifications of the list
         sent by the server in this case will go undetected.  If the
         server does have an rIK available for the peer, the list will
         be provided in a protected manner and this issue does not
         apply.

      Uniquely named keys

         All keys produced within the ERP context are uniquely named
         using key name derivations specified in this documnet.  Also,
         the key names do not reveal any part of the keying material.

      Prevent the domino effect

         The compromise of one peer does not result in the compromise of
         keying material held by any other peer in the system.  Also,
         the rMSK is meant for a single authenticator and is not shared
         with any other authenticator.  Hence, the compromise of one
         authenticator does not lead to the compromise of sessions or
         keys held by any other authenticator in the system.  Hence, the
         EAP Re-auth protocol allows prevention of the domino effect by
         appropriately defining key scopes.

      Bind key to its context





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         All the keys derived for ERP are bound to the appropriate
         context using appropriate key labels.  Lifetime of a child key
         is less than or equal to that of its parent key as specified in
         RFC 4962 [15].  The key usage, lifetime and the parties that
         have access to the keys are specified.

      Confidentiality of identity

         Deployments where privacy is a concern may find the use of
         rIKname-NAI to route ERP messages serves their privacy
         requirements.  Note that it is plausible to associate multiple
         runs of ERP messages since the rIKname is not changed as part
         of the ERP protocol.  There was no consensus for that
         requirement at the time of development of this specification.
         If the rIKname is not used and the Peer-ID is used instead, the
         ERP exchange will reveal the Peer-ID over the wire.

      Authorization restriction

         All the keys derived are limited in lifetime by that of the
         parent key or by server policy.  Any domain specific keys are
         further restricted for use only in the domain for which the
         keys are derived.  All the keys specified in this document are
         meant for use in ERP only.  Any other restrictions of session
         keys may be imposed by the specific lower layer and is out of
         scope for this specification.

   A denial of service attack on the peer may be possible when using the
   EAP Initiate/Re-auth message.  An attacker may send a bogus EAP-
   Initiate/Re-auth message, which may be carried by the authenticator
   in a RADIUS-Access-Request to the server; in response to that the
   server may send an EAP-Finish/Re-auth with Failure indication in a
   RADIUS Access-Reject message.  Note that such attacks may be
   plausible with the EAP-Start capability of IEEE 802.11 and other
   similar facilities in other link layers and where the peer can
   initiate EAP authentication; an attacker may use such messages to
   start an EAP method run, which fails and may result in the server
   sending a RADIUS Access-Reject message and thus resulting in the link
   layer connections being terminated.

   To prevent such DoS attacks, an ERP failure should not result in
   deletion of any authorization state established by a full EAP
   exchange.  Alternately, the lower layers and AAA protocols may define
   mechanisms to allow two link layer SAs derived from different EAP
   keying materials for the same peer to exist so that smooth migration
   from the current link layer SA to the new one is possible during
   rekey.  These mechanisms prevent the link layer connections from
   being terminated when a re-authentication procedure fails due to the



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   bogus EAP-Initiate/Re-auth message.


7.  IANA Considerations

   This document requires IANA registration of two new EAP Codes:

   o  5 (Initiate)

   o  6 (Finish)

   These values are in accordance with [2].

   This document also requires IANA registration of two new Types
   related to Initiate and one for Finish message :

   o  1 (Re-auth-Start, applies to Initiate Code only),

   o  2 (Re-auth, applies to Initiate and Finish Codes).

   Additional type values are IANA managed and assigned based on IETF
   Consensus.

   Next, a number of type values corresponding to the TLVs within EAP-
   Initiate and EAP-Finish messages.  Those are as follows:

   o  rIKname: TV Payload.  The Type is 1

   o  rIKname-NAI: This is a TLV payload.  The Type is 2.

   o  Peer-ID: This is a TLV payload.  The Type is 3.

   o  Domain Identity: This is a TLV payload.  The Type is 4.

   o  The TLV type range of 128-191 is reserved to carry CB information
      in the EAP-Initiate/Re-auth and EAP-Finish/Re-auth messages.
      Below are the current assignments (all of them are TLVs):

      *  Called-Station-Id: 128

      *  Calling-Station-Id: 129

      *  NAS-Identifier: 130

      *  NAS-IP-Address: 131

      *  NAS-IPv6-Address: 132




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      Other values may be added in future versions of this draft and the
      rest are IANA managed based on IETF Consensus.

   o  192-255 is reserved for Experimental/Private use.

   Further, this document registers a USRK label with a value "EAP re-
   authentication Root Key" in accordance with [3].

   We specify some cryptosuites below, in the format Integrity-
   algorithm_PRF-name:

   o  0 RESERVED

   o  1 HMAC-SHA256-64_HMAC-SHA256

   o  2 HMAC-SHA256-128_HMAC-SHA256

   o  3 HMAC-SHA256-256_HMAC-SHA256

   o  4-191 IANA managed and assigned based on IETF consensus

   o  192-255 is reserved for Experimental/Private use.


8.  Acknowledgments

   In writing this draft, we benefited from discussing the problem space
   and the protocol itself with a number of folks including, Bernard
   Aboba, Jari Arkko, Sam Hartman, Russ Housley, Joe Salowey, Jesse
   Walker, Charles Clancy, Michaela Vanderveen, Kedar Gaonkar, Parag
   Agashe, Pasi Eronen, Dan Harkins, Yoshi Ohba, Glen Zorn and other
   participants of the HOKEY working group.  The credit for the idea to
   use EAP-Initiate/Re-auth-Start goes to Charles Clancy and the
   multiple link layer SAs idea to mitigate the DoS attack goes to Yoshi
   Ohba.  Many thanks to Pasi Eronen for the suggestion to use
   hexadecimal encoding for rIKname when sent as part of rIKname-NAI
   field.


9.  References

9.1.  Normative References

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

   [2]   Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
         Levkowetz, "Extensible Authentication Protocol (EAP)",



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         RFC 3748, June 2004.

   [3]   Salowey, J., "Specification for the Derivation of Root Keys
         from an Extended Master  Session Key (EMSK)",
         draft-ietf-hokey-emsk-hierarchy-01 (work in progress),
         June 2007.

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

   [5]   Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [6]   Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network
         Access Identifier", RFC 4282, December 2005.

   [7]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs",
         draft-narten-iana-considerations-rfc2434bis-08 (work in
         progress), October 2007.

9.2.  Informative References

   [8]   Arkko, J. and H. Haverinen, "Extensible Authentication Protocol
         Method for 3rd Generation Authentication and Key Agreement
         (EAP-AKA)", RFC 4187, January 2006.

   [9]   Lopez, R., Skarmeta, A., Bournelle, J., Laurent-Maknavicus, M.,
         and J. Combes, "Improved EAP keying framework for a secure
         mobility access service", IWCMC '06 Proceedings of the 2006
         international conference on Wireless  communications and mobile
         computing, New York, NY, USA, 2006.

   [10]  Arbaugh, W. and B. Aboba, "Experimental Handoff Extension to
         RADIUS", draft-irtf-aaaarch-handoff-04 (work in progress),
         November 2003.

   [11]  Clancy, C., Nakhjiri, M., Narayanan, V., and L. Dondeti,
         "Handover Key Management and Re-authentication Problem
         Statement", draft-ietf-hokey-reauth-ps-07 (work in progress),
         November 2007.

   [12]  Nakhjiri, M. and Y. Ohba, "Derivation, delivery and management
         of EAP based keys for handover and  re-authentication",
         draft-ietf-hokey-key-mgm-01 (work in progress), November 2007.

   [13]  Gaonkar, K., Dondeti, L., Narayanan, V., and G. Zorn, "RADIUS
         Support for EAP Re-authentication Protocol",



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         draft-gaonkar-radext-erp-attrs-01 (work in progress),
         November 2007.

   [14]  Dondeti, L., "Diameter Support for EAP Re-authentication
         Protocol", draft-dondeti-dime-erp-diameter-00 (work in
         progress), September 2007.

   [15]  Housley, R. and B. Aboba, "Guidance for Authentication,
         Authorization, and Accounting (AAA) Key Management", BCP 132,
         RFC 4962, July 2007.


Appendix A.  Example ERP Exchange


0. Authenticator --> Peer:  [EAP-Request/Identity]

1. Peer --> Authenticator:  EAP Initiate/Re-auth(SEQ, rIKname,[Peer-ID],
                             cryptosuite,Auth-tag*)

1a. Authenticator --> Re-auth-Server: AAA-Request{Authenticator-Id,
                            EAP Initiate/Re-auth(SEQ,rIKname,[Peer-ID],
                             cryptosuite,Auth-tag*)

2. ER-Server --> Authenticator:  AAA-Response{rMSK,
                            EAP-Finish/Re-auth(SEQ,rIKname,
                            cryptosuite,[CB-Info],Auth-tag*)

2b. Authenticator --> Peer: EAP-Finish/Re-auth(SEQ,rIKname,
                             cryptosuite,[CB-Info],Auth-tag*)

* Auth-tag computation is over the entire EAP Initiate/Finish message;
  the code values for Initiate and Finish are different and thus
  reflection attacks are mitigated.



                          Figure 10: ERP Exchange













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Authors' Addresses

   Vidya Narayanan
   Qualcomm, Inc.
   5775 Morehouse Dr
   San Diego, CA
   USA

   Phone: +1 858-845-2483
   Email: vidyan@qualcomm.com


   Lakshminath Dondeti
   Qualcomm, Inc.
   5775 Morehouse Dr
   San Diego, CA
   USA

   Phone: +1 858-845-1267
   Email: ldondeti@qualcomm.com































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Full Copyright Statement

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