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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4764

   Internet Draft                                       Florent Bersani
   File: draft-bersani-eap-psk-00.txt                France Telecom R&D
   Expires: May 2004                                       January 2004

                           The EAP PSK Protocol


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
   groups may also distribute working documents as Internet-Drafts.

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

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

   Copyright Notice

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

Abstract

   This document specifies an Extensible Authentication Protocol (EAP)
   method for authentication and session key distribution using a pre-
   shared key (PSK). The PSK is used by a unique underlying
   cryptographic primitive, a block cipher, which is instantiated with
   AES-128.
   EAP-PSK performs mutual authentication and session key derivation. It
   provides identity protection and shall provide fast reconnect in
   future versions. It provides a secure communication channel within
   EAP in case the authentication is successful. This secure channel can
   be used to allow for instance protected result indications.
   EAP-PSK is intended to be easy to deploy and well-suited for
   authentication over insecure networks.







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Table of Contents

   1. Introduction...................................................4
      1.1 Design goals for EAP-PSK...................................4
      1.2 Caveats....................................................4
      1.3 Specification of requirements..............................5
      1.4 Terminology................................................5
      1.5 Conventions................................................5
      1.6 Related work...............................................6
   2. Protocol Overview..............................................7
      2.1 Cryptographic design of EAP-PSK............................7
          2.1.1 The authentication............................. .....7
          2.1.2 The key derivation................................ ..8
          2.1.3 The protected channel............................. ..8
      2.2 EAP-PSK Key hierarchy......................................9
          2.2.1 The PSK.......................................... ...9
          2.2.2 The TEK.......................................... ...9
          2.2.3 The MSK.............................................10
          2.2.4 The EMSK............................................10
          2.2.5 The IV..............................................10
      2.3 EAP-PSK message flow......................................10
          2.3.1 EAP-PSK basic message flow..........................10
          2.3.2 EAP-PSK advanced message flows......................11
      2.4 Retry Behavior............................................12
      2.5 Fragmentation.............................................12
   3. EAP-PSK message format........................................12
      3.1 Expected attributes by message............................12
      3.2 Table of attributes Type field............................13
      3.3 Format of different attributes............................13
          3.3.1 AT-MAC..............................................13
          3.3.2 AT-NOT..............................................14
          3.3.3 AT-NTID.............................................14
          3.3.4 AT-PCHANNEL.........................................15
          3.3.5 AT-PIDREQ...........................................16
          3.3.6 AT-PIDRES...........................................17
          3.3.7 AT-RAND.............................................17
   4. IANA considerations...........................................18
   5. Security considerations.......................................18
      5.1 Identity Protection.......................................18
      5.2 Mutual Authentication.....................................19
      5.3 Key Derivation............................................19
      5.4 Dictionary Attacks........................................20
      5.5 Protected channel.........................................20
      5.6 Negotiation attacks.......................................20
      5.7 Fast reconnect............................................20
      5.8 Man-in-the-middle Attacks.................................20
      5.9 Generating Random Numbers.................................21
   6. Security claims...............................................21
   7. Acknowledgements..............................................21
   8. References....................................................22
   9. Authors' Addresses............................................24
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   10. Full Copyright Statement.....................................24
   Annex A: Work still to be done on this document..................24
      1. Editorial..................................................24
      2. Security...................................................25
      3. Technical..................................................25
   Annex B: Guidance for PSK generation from a password.............26













































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1. Introduction

1.1 Design goals for EAP-PSK

   The Extensible Authentication Protocol, [EAP], provides a standard
   mechanism for support of additional authentication methods within
   [PPP]. EAP is also used within IEEE 802 networks through the [IEEE
   802.1X] framework.

   EAP supports many authentication mechanisms usually called EAP
   methods. This document specifies an EAP method that uses a pre-shared
   key (PSK).

   Design goals for this method were:
     1. Simplicity: It should be easy to implement and to deploy without
        any pre-existing infrastructure.
     2. Wide applicability: It should be possible to use this method to
        authenticate over any network. In particular, it should be
        suitable for [IEEE 802.11] wireless LANs and comply to [IEEE
        802REQ]
     3. Security: It should be conservative in its cryptographic design
        and enjoy security proofs
     4. Extensibility: It should be possible to complement this method
        with the required extensions as their need appears
     5. Patent-avoidance: It should be free of any IPR claims and its
        specification should be released to the public domain

   Thus EAP-PSK relies on a single cryptographic primitive, [AES], and
   performs mutual authentication and session key derivation. It also
   provides identity protection and shall provide fast reconnect in
   future versions. It provides a secure communication channel within
   EAP in case the authentication is successful. This secure channel can
   be used to allow for instance protected result indications. It uses a
   Type-Length-Value design to ensure that it will be easy to extend.

1.2 Caveats

   Since PSK are of frequent use in security protocols, because a PSK
   simply means a cryptographic key in the symmetric setting, attention
   should be paid not to confuse EAP-PSK with any other protocols that
   may also refer to a PSK, for instance [WPA] when used in its PSK
   mode.

   EAP-PSKÆs PSK should also not be confused with the PSKs possibly used
   by other protocols relying on PSKs: EAP-PSKÆs PSK should be
   cryptographically separated from any other PSK or else the security
   of EAP-PSK might be voided.

   The generation of the PSK used by EAP-PSK is outside of the scope of
   this document. The PSK SHOULD be generated by a good source of
   randomness (see [RFC 1750]). In particular, a PSK should not be
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   confused with a password. However, in case one wants to generate the
   PSK from a password, although this is strongly discouraged, guidance
   to do so is provided in annex A.

   The definition of the repository of the PSK used by EAP-PSK is also
   outside of the scope of the document. In particular, nothing prevents
   from storing the PSK on a tamper-resistant device such as a smart
   card rather than having it memorized or written down on a sheet of
   paper.

1.3 Specification of requirements

   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 2219].

1.4 Terminology

   This document frequently uses the following terms:

   TBC

   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.].]

   EAP Authenticator
             The end of the EAP link initiating the EAP authentication
             methods. [Note: This terminology is also used in
             [IEEE 802.1X.], and has the same meaning in this
             document].

   EAP Peer (or simply Peer)
             The end of the EAP Link that responds to the
             authenticator. [Note: In [IEEE 802.1X], this end is
             known as the Supplicant.]

   EAP Server (or simply Server)
             The entity that terminates the EAP authentication with the
             peer. In the case where there is no Backend Authentication
             Server, this term refers to the EAP Authenticator. Where
             the EAP Authenticator operates in pass-through, it refers
             to the Backend Authentication Server.

1.5 Conventions

   All numbers involved in cryptographic calculations are considered in
   network-byte order.
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   || denotes concatenation of strings.

   [String] denotes MAC of String calculated as specified by the
   context.

   ** denotes integer exponentiation.

   ôiö denotes the unsigned binary representation on 128 bits of the
   integer I in network byte order. Therefore this notation only makes
   sense when i is between 0 and 2**128-1.

1.6 Related work

   There exist other EAP methods which also somehow rely on PSKs:

   [EAP-SIM] and [EAP-AKA] are two of them, which do not directly
   compete with EAP-PSK since they are designed to take advantage of the
   GSM and UMTS infrastructure. There are therefore not easy to deploy
   in case one does not dispose of such infrastructure.

   EAP-MD5, described in [EAP], is another one, which has however been
   deprecated for security reasons: it is not safe to use it over
   insecure networks.

   EAP-OTP, described in [EAP], and other One-Time Password methods such
   as [EAP-SecurID] may also rely on PSKs but they also do not directly
   compete with EAP-PSK since they require additional elements (e.g. the
   One-Time password generator and server) on the client and on the
   server side.

   [LEAP] (Lightweight EAP) also known as EAP-Cisco Wireless is quite
   similar to EAP-PSK. However it is a proprietary protocol that has
   been shown to bear cryptographic weaknesses (see for instance
   [LEAPVUL]).

   [EAP-SKE] is quite similar to EAP-PSK. This method is still work in
   progress and has put emphasis on network efficiency in roaming
   situations. Work could be done to merge EAP-PSK and EAP-SKE.

   [EAP-Archie] is very similar to EAP-PSK. This method is still work in
   progress and has very much inspired this work. EAP-PSK makes
   amendments to the cryptographic parts specified in EAP-Archie and
   provides the protected channel and the TLV approach as new features.
   Work could be done to merge EAP-PSK and EAP-Archie.

   [EAP-SRP] is quite similar to EAP-PSK except that it uses both
   symmetric and asymmetric cryptography and that it is subject to IPR
   claims by Stanford university.


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2. Protocol Overview

   EAP-PSK is a native EAP method, that is a stand-alone version of EAP-
   PSK outside of EAP is not defined.

2.1 Cryptographic design of EAP-PSK

   EAP-PSK rely on a single cryptographic primitive, a block cipher,
   which is instantiated with AES-128. This instantiation has been
   chosen because:
     1. AES-128 is standardized and its implementation are widely
        available
     2. AES-128 has been carefully reviewed by the community and is
        believed to be secure

   For a description of AES-128, please refer to [AES].

   However, in case it should be needed, new instantiations of EAP-PSK
   could easily be proposed as it does not intricately depend on the
   chosen block cipher.

   EAP-PSK uses three cryptographic parts:
     1. An authentication protocol to mutually authenticate the
        communicating parties, that follows the design presented in
        [EAKD] under the name MAP1 instantiated with [OMAC] and [AES]
     2. A key derivation protocol to derive keying material according to
        the EAP Key Management Framework [EKMF], that uses the modified
        counter mode presented in [SOBMMO]
     3. An authenticated encryption protocol with associated data to
        provide a protected channel for both mutually authenticated
        parties to communicate securely within the method, that uses the
        [EAX] mode of operation

2.1.1 The authentication

   The authentication protocol used by EAP-PSK is the Mutual
   Authentication Protocol 1.

   The Mutual Authentication Protocol 1 (MAP1) is described in [EAKD].

   It consists of a one and half round trip exchange:










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          B                                                          A
          |                                                          |
          |                           RA                             |
          |<---------------------------------------------------------|
          |                                                          |
          |                      [B||A||RA|RB]                       |
          |--------------------------------------------------------->|
          |                                                          |
          |                        [A||RB]                           |
          |<---------------------------------------------------------|

   where:

     1. RA and RB are random numbers chosen respectively by A and B
     2. A and B are A and B respective identities

   EAP-PSK instantiates this protocol with:

     1. RA and RB 128 bit random numbers chosen respectively by A and B
     2. A and B are A and BÆs respective permanent full NAIs
     3. The MAC algorithm used in [] is OMAC1 with AES-128 using KCK and
        producing a tag length of 128 bits

2.1.2 The key derivation

   The key derivation is realized using the modified counter mode.

   The modified counter mode is described in [SOBMMO].

   EAP-PSK instantiates this modified counter mode with all rotation
   values (the ris following [SOBMMO] Figure 3 notation) taken equal to
   zero (no rotations) and the counter values (the cis following
   [SOBMMO] Figure 3 notation) taken respectively equal to the the first
   t integers (that is ci=i, starting with c1).

   The parameter t is taken equal to 4 across all key derivations.

   The underlying block cipher used by this counter mode is AES-128.

   The input block to the different key derivations (see next section
   for EAP-PSKÆs key hierarchy) is taken to be:

     1. [B||A||RA|RB||ö1ö] for the TEK derivation
     2. [B||A||RA|RB||ö2ö] for the MSK derivation
     3. [B||A||RA|RB||ö3ö] for the EMSK derivation

2.1.3 The protected channel

   To provide a protected channel within EAP-PSK in case of a successful
   authentication, EAP-PSK uses the EAX mode of operation described in
   [EAX].
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   EAX is instantiated with AES-128 as the underlying block cipher keyed
   with the 128 first bits of the TEK.

   EAX is instantiated within EAP with a tag length of 128 bits.

   The nonce N used by EAX (following [EAX] Figure 3 notation) is a
   counter starting with ô0ö and incremented by the one at each
   subsequent EAP-PSK message (except retransmissions of course) within
   one EAP-PSK dialog. Thus, N can be considered a replay counter.

   The message M used by EAX (following [EAX] Figure 3 notation)
   consists of the message that one party wishes to send to the other
   over the protected channel.

   The header H used by EAX (following [EAX] Figure 3 notation) is
   currently unused and is taken to be the Type field of the AT-PCHANNEL
   attribute within which M is encapsulated (see EAP-PSK message format
   section).


2.2 EAP-PSK Key hierarchy

   This section instantiates the EAP Key hierarchy described in [EKMF]
   for EAP-PSK.

2.2.1 The PSK

   EAP-PSK uses a long-lived 256-bit secret shared between the EAP Peer
   and the EAP Server called the PSK.

   The PSK has an internal structure. It consists of one 128-bit subkey
   and a 128-bit subkey, respectively called the key-confirmation key
   (KCK) and the key-derivation key (KDK). The protocol uses the KCK to
   mutually authenticate the EAP Peer and the EAP Server. The protocol
   uses the KDK to derive keying material between the EAP Peer and the
   EAP Server.

   EAP-PSK assumes that the PSK is known only to the EAP Peer and EAP
   Server, and the security properties of the protocol may be
   compromised if it has wider distribution.

   The protocol also assumes the EAP Server and EAP Peer identify the
   correct PSK to use with the other by their respective [NAI]s.

2.2.2 The TEK

   EAP-PSK allows for TEK derivation from the random values exchanged
   during authentication and the KDK.


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   The TEK is a 128 bit key that MAY be used to set a protected channel
   for both mutually authenticated parties to communicate securely
   within the method.

2.2.3 The MSK

   EAP-PSK allows for MSK derivation from the random values exchanged
   during authentication and the KDK.

2.2.4 The EMSK

   EAP-PSK allows for EMSK derivation from the random values exchanged
   during authentication and the KDK.

2.2.5 The IV

   EAP-PSK does not derive any IV.

2.3 EAP-PSK message flow

2.3.1 EAP-PSK basic message flow

   Basically, EAP-PSK is comprised of four messages:
     1. A first message sent by the server to the peer which starts the
        mutual authentication procedure and essentially consists of a
        random value chosen by the server
     2. A second message sent by the peer to the server which contains a
        random value chosen by the peer and an authentication tag over
        both random values as well as the peer and serverÆs permanent
        full NAIs that proves the identity of the peer to the server
     3. A third message sent by the server to the peer that contains an
        authentication tag calculated over the random value chosen by
        the peer and the serverÆs permanent full NAI that proves the
        identity of the server to the peer. This message may also
        contain data encapsulated in a protected channel that has just
        been set up as a result of the authentication procedure
     4. A fourth message sent by the peer to the server that may also
        contain data encapsulated in a protected channel that has just
        been set up as a result of the authentication procedure












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        Peer                                                      Server
          |                                                          |
          |                                   EAP-PSK/AT-RandS       |
          |<---------------------------------------------------------|
          |                                                          |
          | EAP-PSK/AT-RandP, AT-MAC                                 |
          |--------------------------------------------------------->|
          |                                                          |
          |                        EAP-PSK/AT-MAC, AT-PCHANNEL       |
          |<---------------------------------------------------------|
          |                                                          |
          | EAP-PKS/AT-PCHANNEL                                      |
          |--------------------------------------------------------->|
          |                                                          |

   This basic message flow could be comprised of only three messages,
   were it not the request/response nature of EAP that prevents the
   third message to be the last one. We take advantage of this situation
   by mandating the setup of a protected channel over which result
   indications must be sent.

2.3.2 EAP-PSK advanced message flows

   EAP-PSK provides different advanced features that may lead to
   different message flows:

     1. Identity protection. This feature might add an additional
        roundtrip to the basic message flow when used. This additional
        round trip is inserted before the first message in the basic
        flow when the peer and the server fall out of synchronization on
        the pseudonym the peer MUST use to protect its identity. Hence,
        when the server does not recognize the identity provided by the
        peer in response to EAP-Request/Identity, the server sends a
        message to the peer asking it to disclose its permanent
        identity. The peer MAY respond to this message if his security
        policy allows him to do so by sending his permanent identity.
        The basic message flow then proceeds.
     2. Fast reconnect. TBC
     3. Conversation over the protected channel. This feature might add
        some round trips to the basic message flow when used. These
        additional round trips are inserted after the fourth message in
        the basic message flow. They consist in exchanging data between
        the peer and the server over the protected channel that has been
        set thanks to the authentication. This protected data exchange
        might for instance be of some use if the peerÆs account is pre
        paid and his charged on a per packet or temporal basis: in case
        the peer wants to top it up, he can do so, e.g. by making a
        financial transaction with the server. This protected data
        exchange might also be used to check the identity of the claimed
        NAS that the peer has connected to.

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2.4 Retry Behavior

   EAP-PSK complies to the EAP retry behavior described in [EAP], that
   is: the EAP Server is responsible for retry behavior. This means that
   if the EAP Server does not receive a reply from the peer, it MUST
   resend the EAP-Request for which it has not yet received an EAP-
   Response. However, the EAP Peer MUST NOT resend EAP-Response messages
   without first being prompted by the EAP Server.

   As a result, it is possible that an EAP Peer will receive duplicate
   EAP-Request messages, and may send duplicate EAP-Responses. Both the
   EAP Peer and the EAP Server should be engineered to handle this
   possibility.

2.5 Fragmentation

   EAP-PSK does not support fragmentation or reassembly.

   Therefore it is to be used either over networks which MTU is enough
   to convey encapsulated EAP-Archie packets without fragmentation or
   encapsulated in other protocols which take care of fragmentation and
   reassembly.

3. EAP-PSK message format

3.1 Expected attributes by message

   TBC

   The first message received by the peer is either comprised of AT-
   PIDREQ or AT-RAND.

   In case it consists of AT-PIDREQ, the peer MAY then send a message
   containing AT-PID or abort the conversation according to its identity
   protection policy. If the peer send AT-PID, the conversation should
   then proceed normally that is to say the server sends AT-RAND to the
   peer f it has recognized its identity or abort the conversation if
   not.

   In case it consists of AT-RAND, the peer MUST send a response
   containing its own AT-RAND along with AT-MAC calculated over the
   fields specified in section 2.

   The server then checks the validity of AT-MAC sent by the peer. If it
   is a valid MAC, then the server sends a message containing its own
   AT-MAC calculated over the fields specified in section 2 and AT-
   PCHANNEL with at least an AT-Notification encapsulated to indicate to
   the peer the success or failure of the authentication and
   authorization. If it is not valid, the server MUST abort the
   conversation.

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   The peer then checks the validity of the AT-MAC sent by the server.
   If it is a valid MAC then the peer decapsulate the data contained in
   AT-PCHANNEL. If it is not valid, the peer MUST discard the message
   without further examination and abort the conversation.

   The peer and the server MAY exchange further messages each containing
   only an AT-PCHANNEL attribute. TBC.


3.2 Table of attributes Type field

   TBC

   Attribute Name              Type Field Value
   AT-RAND                            1
   AT-PIDREQ                          2
   AT-PIDRES                          3
   AT-MAC                             4
   AT-PCHANNEL                        5
   AT-NTID                            6

3.3 Format of different attributes

   This section presents the respective formats of the different
   attributes listed in alphabetical order.

   Within the different attribute formats, reserved bytes are specified.
   These reserved bytes are essentially here for word alignment on 32
   bit boundaries. They are set to zero when sending and ignored on
   reception.

3.3.1 AT-MAC

   The AT-MAC attribute is used for authentication within EAP-PSK.

   The value field of the AT-MAC attribute contains two reserved bytes
   followed by a keyed message authentication code (MAC). The MAC is
   calculated over message-specific data.

   The contents of the message-specific data that are MACed are
   specified separately for each EAP/PSK message in Section 2.

   The format of the AT-MAC attribute is shown below.








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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     AT-MAC    | Length = 5    |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                           MAC                                 |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   When the AT-MAC attribute is expected to be included in an EAP-PSK
   message, the recipient MUST process the AT-MAC attribute before
   looking at any other attributes.

   If the message authentication code is absent or invalid, then the
   recipient MUST ignore all other attributes in the message and operate
   as specified in Section TBC.


3.3.2 AT-NOT


   The format of the AT_NOT attribute is shown below.

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AT-NOT     |  Length = 1   |      Notification Code        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The value field of this attribute contains a two-byte notification.
   code.

   The different notification codes remain TBC.

3.3.3 AT-NTID

   The format of the AT-NTID attribute is shown below.











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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AT-NTID    |    Length     | Actual Identity Length        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                          Identity                             :
      :   .                                                           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The use of the AT-NTID is defined in Section 2.

   The value field of this attribute begins with 2-byte actual identity
   length, which specifies the length of the identity in bytes. This
   field is followed by the subscriber identity of the indicated actual
   length.

   The identity is the peer next temporary identity (i.e. pseudonym)
   chosen by the server.

   The identity does not include any terminating null characters.
   Because the length of the attribute must be a multiple of 4 bytes,
   the sender pads the identity with zero bytes when necessary.

3.3.4 AT-PCHANNEL

   AT-PCHANNEL_is used to transmit information between the peer and
   server over a protected channel, that is to say a channel that
   provides confidentiality, data origin authentication and replay
   protection.

   The format of the AT-PCHANNEL attribute is shown below.


















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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  AT-PCHANNEL  |     Length    |          Reserved             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                           Nonce                               |
      |                                                               |
      |                                                               |
      |---------------------------------------------------------------|
      |                                                               |
      |                            Tag                                |
      |                                                               |
      |                                                               |
      |---------------------------------------------------------------|
      |                                                               |
      |                            Payload                            |
      :                                                               :
      :                                                               :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The value field of the AT-PCHANNEL attribute consists of one reserved
   bytes followed by a 4 byte nonce, a 4 byte tag and a variable length
   payload.

   The payload consists in the ciphertext resulting from the encryption
   in the EAX mode of operation of the information that the peer and the
   server wish to exchange over the protected channel under Nonce and
   the first 128 bits of the derived TEK (see Section 2.).

   The information that the peer and the server may exchange over the
   protected channel consists of a concatenation of EAP-PSK attributes
   in the TLV format.

3.3.5 AT-PIDREQ

   The format of the AT-PIDREQ attribute is shown below.


      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AT-PIDREQ   | Length = 1    |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The use of the AT-PIDREQ is defined in section 2.


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3.3.6 AT-PIDRES

   The format of the AT-PIDRES attribute is shown below.

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AT-PIDRES   |    Length     | Actual Identity Length        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                          Identity                             :
      :   .                                                           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The use of the AT-PIDRES is defined in Section 2.

   The value field of this attribute begins with 2-byte actual identity
   length, which specifies the length of the identity in bytes. This
   field is followed by the subscriber identity of the indicated actual
   length.

   The identity is the peer permanent identity that is to the say the
   peerÆs permanent NAI.

   The identity does not include any terminating null characters.
   Because the length of the attribute must be a multiple of 4 bytes,
   the sender pads the identity with zero bytes when necessary.

3.3.7 AT-RAND

   The format of the AT_RAND attribute is shown below.

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    AT_RAND    | Length = 5    |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                             RAND                              |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The value field of the AT-RAND attribute contains two reserved bytes
   followed by a 16 byte random number generated either by the peer or
   the server freshly for this EAP-PSK authentication exchange. The
   random numbers are used as a mean to authenticate and indirectly as a
   seed value for the new keying material.

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   The server MUST choose a fresh RAND value and send it to the peer at
   the beginning of the EAP-PSK exchange. The peer MUST also choose a
   fresh RAND value and send it to the server at the beginning of the
   EAP-PSK exchange.

   The randomness of the RAND values are critical for the security of
   EAP-PSK.

4. IANA considerations

   This document introduces one new IANA consideration.

   It requires IANA to allocate a new EAP Type for EAP-PSK.

5. Security considerations

   The EAP base protocol [EAP] highlights several attacks that are
   possible against the EAP protocol as there is no inherent security
   mechanisms provided. This section discusses the claimed security
   properties of EAP SIM as well as vulnerabilities and security
   recommendations.

5.1 Identity Protection

   EAP-PSK includes optional identity privacy support that protects the
   privacy of the peer identity against passive eavesdropping.

   The peer and the server SHOULD store the pseudonym in a non-volatile
   memory so that it can be maintained across reboots.

   An active attacker that impersonates the server may use the AT-PIDREQ
   attribute to attempt to learn the peer's permanent identity.

   However, itÆs a matter of policy for the peer to accept to respond to
   such requests or not: the peer can refuse to send its permanent
   identity if it believes that the server should be able to recognize
   its temporary identity. If identity protection is a concern then the
   peer MUST NOT send its permanent identity. Any other policy allows
   identity protection compromise.

   If the peer and server cannot guarantee that the pseudonym will be
   maintained reliably and identity privacy is required then additional
   protection from an external security mechanism such as Protected
   Extensible Authentication Protocol (PEAP) [PEAP] may be used. If an
   external security mechanism is in use the identity privacy features
   of EAP-PSK may not be useful. The security considerations of using
   an external security mechanism with EAP-PSK are beyond the scope of
   this document.



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   It should also be kept in mind that the layers below EAP-PSK may
   disclose elements that can lead to identity protection compromise
   (e.g. the peerÆs [IEEE 802.3] Medium Access Control Address).


5.2 Mutual Authentication

   EAP-PSK provides mutual authentication.

   The server believes the peer is authentic because it can calculate a
   valid MAC and the peer believes that the server is authentic because
   it can calculate a correct MAC.

   The authentication protocol used in EAP-PSK, MAP1, enjoys a security
   proof in the provable security paradigm, see [EAKD].

   The MAC algorithm used in the instantiation of MAP1 within EAP-PSK,
   OMAC1, also enjoys a security proof in the provable security
   paradigm, see [OMAC].

   The underlying block cipher used, AES-128, is widely believed to be a
   secure block cipher.

   Finally, the key used for mutual authentication, KCK, is only used
   for that purpose, making thus this part cryptographically independent
   of the other parts.

5.3 Key Derivation

   EAP-PSK supports key derivation.

   The key hierarchy is specified in Section 2.

   The mechanism used for key derivation is the modified counter mode.

   The instantiation of the modified counter in EAP-PSK (i.e. the
   selected ris and cis) comply with the conditions stated in [SOBMMO]
   so that the security proof in the provable security paradigm of
   [SOBMMO] holds.

   The underlying block cipher used, AES-128, is widely believed to be a
   secure block cipher.

   The key derivation mechanism uses two different keys KDK1 and KDK2:

     1. KDK1 is used to produce the input blocks that are fed to the
        modified counter mode
     2. KDK2 is used in the modified counter mode to expand the input
        blocks


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   The input blocks are produced from the MAC sent by the peer during
   the authentication part of EAP-PSK so that neither the peer nor the
   server have full control over them, since this MAC is calculated over
   fields that include random values chosen respectively by the peer and
   the server. The input blocks are believed to be cryptographically
   separated from one another because they are produced as MAC of
   distinct messages (see section 2) and the MAC algorithm used (OMAC1)
   is proved to be a secure PRF if the underlying block cipher is a
   secure PRP, see [OMAC].

   The key derivation scheme is believed to be secure since the modified
   counter mode is proved to be a PRF if the underlying block cipher is
   a secure PRP.

5.4 Dictionary Attacks

   Because EAP-PSK is not a password protocol, it is not vulnerable to
   dictionary attacks: EAP-PSKÆs PSK MUST NOT be derived from a
   password. Derivation of EAP-PSKÆs PSK may lead to dictionary attacks.

   In case, EAP-PSKÆs PSK is however derived from a password, guidance
   is provided in Annex A how to do so. It is also believed that MAP1
   makes dictionary attacks harder, since the first value that is MACed
   should not be predictable neither for the peer nor the server or
   anybody else.

5.5 Protected channel

   EAP-PSK provides a protected channel over which the peer and the
   server can securely exchange information, in case of a successful
   authentication.

   This protected channel provides confidentiality, data origin
   authentication, replay protection and confirmation of the end of the
   conversation.

5.6 Negotiation attacks

   EAP-PSK does not protect from negotiation attacks since it currently
   does not provide version negotiation as only one version is
   specified.

5.7 Fast reconnect

   EAP-PSK shall provide fast reconnect. TBC.

5.8 Man-in-the-middle Attacks

   Due to the use of symmetric cryptography and the security proofs of
   its cryptographic components, EAP-PSK is believed not to be
   vulnerable to man-in-the-middle attacks.
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   There are man-in-the-middle attacks associated with the use of any
   EAP method within a tunneled protocol such as PEAP, or within a
   sequence of EAP methods followed by each other (see [MTAP]). This
   specification does not address these attacks. If EAP-PSK is used with
   a tunneling protocol or as part of a sequence of methods, there
   should be cryptographic binding provided between the protocols and
   EAP-PSK to prevent man-in-the-middle attacks. However the mechanism
   how the binding is provided is beyond the scope of this document.

5.9 Generating Random Numbers

   An EAP-PSK implementation SHOULD use a good source of randomness to
   generate the random numbers required in the protocol. Please see [RFC
   1750] for more information on generating random numbers for security
   applications.

6. Security claims

   This section provides the security claims required by [EAP].

   [a] Intended use. EAP-PSK is intended for use over both physically
   insecure networks and physically or otherwise secure networks.
   Applicable media include but are not limited to PPP, IEEE 802 wired
   networks and IEEE 802.11.

   [b] Mechanism. EAP-PSK is based on symmetric cryptography and uses a
   384 bit Pre-shared Key

   [c] Security claims. The security properties of the method are
   discussed in Section 5.

   [d] Key strength. EAP-PSK supports key derivation with 128-bit
   effective key strength.

   [e] Description of key hierarchy. Please see Section 2.

   [f] Indication of vulnerabilities. Vulnerabilities are discussed in
   Section 5.


7. Acknowledgements

   This EAP method has been inspired by [EAP-SIM] and [EAP-Archie]. It
   also considerably reused extracts of these documents. Many thanks to
   their respective authors.

   Many thanks to Laurent Butti, J‰rŸme Razniewski and Olivier Charles
   for their feedback on this document.


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8. References

   [AES]         Federal Information Processing Standards (FIPS)
                 Publication 197, " Specification for the Advanced
                 Encryption Standard (AES)", National Institute of
                 Standards and Technology, November 26, 2001.

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

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

   [EAP-AKA]     Arkko, J. Haverinen, H., ôEAP AKA Authenticationö,
                 Internet-Draft (work in progress), October 2003,
                 draft-arkko-pppext-eap-aka-11.txt

   [EAP-Archie]  Walker, J. and Housley, R., ôThe EAP Archie Protocolö,
                 Internet-Draft (work in progress), June 2003, draft-
                 jwalker-eap-archie-01.txt,

   [EAP-SecurID] Josefsson, S., ôThe EAP SecurID(r) Mechanismö,
                 Internet-Draft (work in progress), February 2002,
                 draft-josefsson-eap-securid-01.txt

   [EAP-SIM]     Haverinen, H. Salowey, J., "EAP SIM Authentication",
                 Internet-Draft (work in progress),October 2003, draft-
                 haverinen-pppext-eap-sim-12.txt

   [EAP-SKE]     Salgarelli, L., ôEAP SKE authentication and key
                 exchange protocolö, Internet-Draft (work in progress),
                 May 2003, draft-salgarelli-pppext-eap-ske-03.txt


   [EAP-SRP]     Carlson, J. et al., ôEAP SRP-SHA1 Authentication
                 Protocolô,Internet-Draft (work in progress), draft-
                 ietf-pppext-eap-srp-04.txt

   [EAKD]        Bellare, M, and P. Rogaway, "Entity Authentication and
                 Key Distribution", CRYPTO 93, LNCS 773, pp232-249,
                 Springer-Verlag, Berlin, 1994.

   [EKMF]        Aboba, B., et al., "EAP Key Management Framework",
                 draft-ietf-eap-keying-01.txt, October 2003

   [HAC]         Menezes, A. et al., ôHandbook of Applied Cryptographyö,
                 CRC Press, 1996.

   [IEEE 802.1X] IEEE STD 802.1X, Standards for Local and Metropolitan
                 Area Networks: Port Based Access Control, June 14, 2001

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   [IEEE 802.3]  Institute of Electrical and Electronics Engineers,
                 "Information Technology - IEEE Standard for Information
                 technology--Telecommunications and information exchange
                 between systems--Local and metropolitan area networksù
                 Specific requirements--Part 3: Carrier Sense Multiple
                 Access with Collision Detection (CSMA/CD) Access Method
                 and Physical Layer Specificationsö

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

   [IEEE 802REQ] Stanley, Dorothy et al., ôEAP Method Requirements for
                 Wireless LANsö, Internet-Draft (work in progress),
                 January 2004, draft-walker-ieee802-req-00.txt

   [LEAP]        Macnally, C., ôCisco LEAP protocol descriptionö,
                 September 2001

   [LEAPVUL]     Wright, J., ôWeaknesses in LEAP Challenge/Responseö,
                 Defcon 2003

   [MTAP]        Asokan, N. et al., ôMan-in-the-middle in Tunnelled
                 Authentication Protocolsö,
                 http://eprint.iacr.org/2002/163

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

   [OMAC]        Iwata, T., and Kurosawa., K., ôOMAC: One-Key CBC MACö
                 Fast Software Encryption, FSE 2003, LNCS, Springer-
                 Verlag.


   [PEAP]        Palekar, A. et al., ôProtected EAP Protocol (PEAP)ö,
                 Internet-Draft (work in progress), October 2003, draft-
                 josefsson-pppext-eap-tls-eap-07.txt

   [PKCS5]       RSA laboratories, ôPKCS #5 v2.0: Password-Based
                 Cryptography Standardö

   [PWD]         National Institute of Standards and Technology (NIST).
                 ôFIPS PUB 112: Password Usageö. May 30, 1985.

   [RFC1750]     Eastlake, D., Crocker, S. and J. Schiller, "Randomness
                 Recommendations for Security", RFC 1750, December
                 1994.
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   [RFC 2219]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SOBMMO]      Gilbert, H., ôThe Security of One-Block-to-Many Modes
                 of Operationö, Fast Software Encryption, FSE 2003,
                 LNCS, Springer-Verlag.

   [WPA]         Wi-Fi Alliance, ôWi-Fi Protected Accessö, version 2.0,
                 April 2003

9. Authors' Addresses

   Florent Bersani                   florent.bersani@francetelecom.com
   France Telecom R&D
   38, rue du G‰n‰ral Leclerc
   92794 Issy Les Moulineaux Cedex 9
   France

10. Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

Annex A: Work still to be done on this document

1. Editorial

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   Clarify the conventions on the cryptographic calculations.

   Make this draft more self-contained.

   Complete the cross-references in the text (page, section,   reference
   numbers)

   Harmonize the terminology (e. g. octets or bytes)

   Complete the terminology section (e.g. with temporary identity)

   Number the figures

2. Security

   Study alternative ways to produce the input blocks to the key
   derivation procedure.

   Is it possible to use the KCK to produce the input blocks to the key
   derivation without endangering the security properties of the
   protocol?

   Specify the mechanism to prevent abrupt ending of the conversation on
   the protected channel.

   Discuss use of other cryptographic algorithms

   Discuss the choice of AES-128-128 and OMAC1 and a tag length of 128
   bits (e.g. should the tag be truncated or not,à).

   Study DOS attacks resistance

3. Technical

   Discuss possibility to enhance network efficiency by removing the
   additional identity protection message requesting the permanent
   identity and replacing it with a query attribute included in the
   first message of the basic message flow.

   Specify how fast reconnect should be implemented

   Introduce version negotiation

   Is it desirable to have all attributes aligned on 32 bit boundaries?

   Harmonize with other standards or draft standards (e. g. EAP and EAP
   Key management framework)

   Specify all that remain TBC, e.g. the different notification
   messages.

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Annex B: Guidance for PSK generation from a password

   It is formally discouraged to use a password to generate a PSK, since
   this commonly lead to exhaustive search or dictionary attacks that
   would not otherwise be possible.

   However, we provide guidance on how to generate the PSK from a
   password.

   Guidance on how passwords should be selected is provided in [PWD].

   The technique presented herein is drawn from [PKCS5]. It is intended
   to mitigate the risks associated with password usage in cryptography,
   typically dictionary attacks.

   If the binary representation in ASCII of the password is strictly
   fewer than 128 bit long (which by the way means that the chosen
   password is probably weak because it is too short) then its is padded
   to 128 bits with zeroes.

   If the binary representation in ASCII of the password is strictly
   more than 128 bit long, then it is hashed down to exactly 128 bit
   using the Matyas-Meyer-Oseas hash (see [HAC]) with
   IV=0x0123456789ABCDEFFEDCBA9876543210 (this value has been
   arbitrarily selected).

   We now assume that we have a 128 bit number derived from the initial
   password (that can be the password itself if its binary
   representation in ASCII is exactly 128 bit long). We shall call this
   number P128.

   EAP-PSKÆs PSK is derived thanks to PBKDF2 instantiated with
   (following the notations in [PKCS5]):
     1. P128 as P
     2. The first 96 bits of the binary ASCII representation of the
        peerÆs NAI as Salt (therefore before using a password within
        EAP-PSK, two parties should agree as to whom is the peer and
        whom is the server for the procedure described in this annex).
     3. 5000 as c
     4. 48 as dkLen











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