<pre>Network Working Group                                          T. Clancy
Request for Comments: 4746                                           LTS
Category: Informational                                       W. Arbaugh
                                                                     UMD
                                                           November 2006


               <span class="h1">Extensible Authentication Protocol (EAP)</span>
                    <span class="h1">Password Authenticated Exchange</span>

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines an Extensible Authentication Protocol (EAP)
   method called EAP-PAX (Password Authenticated eXchange).  This method
   is a lightweight shared-key authentication protocol with optional
   support for key provisioning, key management, identity protection,
   and authenticated data exchange.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
      <a href="#section-1.1">1.1</a>. Language Requirements ......................................<a href="#page-3">3</a>
      <a href="#section-1.2">1.2</a>. Terminology ................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Overview ........................................................<a href="#page-5">5</a>
      <a href="#section-2.1">2.1</a>. PAX_STD Protocol ...........................................<a href="#page-6">6</a>
      <a href="#section-2.2">2.2</a>. PAX_SEC Protocol ...........................................<a href="#page-7">7</a>
      <a href="#section-2.3">2.3</a>. Authenticated Data Exchange ................................<a href="#page-9">9</a>
      <a href="#section-2.4">2.4</a>. Key Derivation ............................................<a href="#page-10">10</a>
      <a href="#section-2.5">2.5</a>. Verification Requirements .................................<a href="#page-11">11</a>
      <a href="#section-2.6">2.6</a>. PAX Key Derivation Function ...............................<a href="#page-12">12</a>
   <a href="#section-3">3</a>. Protocol Specification .........................................<a href="#page-13">13</a>
      <a href="#section-3.1">3.1</a>. Header Specification ......................................<a href="#page-13">13</a>
           <a href="#section-3.1.1">3.1.1</a>. Op-Code ............................................<a href="#page-13">13</a>
           <a href="#section-3.1.2">3.1.2</a>. Flags ..............................................<a href="#page-14">14</a>



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


           <a href="#section-3.1.3">3.1.3</a>. MAC ID .............................................<a href="#page-14">14</a>
           <a href="#section-3.1.4">3.1.4</a>. DH Group ID ........................................<a href="#page-14">14</a>
           <a href="#section-3.1.5">3.1.5</a>. Public Key ID ......................................<a href="#page-15">15</a>
           <a href="#section-3.1.6">3.1.6</a>. Mandatory to Implement .............................<a href="#page-15">15</a>
      <a href="#section-3.2">3.2</a>. Payload Formatting ........................................<a href="#page-16">16</a>
      <a href="#section-3.3">3.3</a>. Authenticated Data Exchange (ADE) .........................<a href="#page-18">18</a>
      <a href="#section-3.4">3.4</a>. Integrity Check Value (ICV) ...............................<a href="#page-19">19</a>
   <a href="#section-4">4</a>. Security Considerations ........................................<a href="#page-19">19</a>
      <a href="#section-4.1">4.1</a>. Server Certificates .......................................<a href="#page-20">20</a>
      <a href="#section-4.2">4.2</a>. Server Security ...........................................<a href="#page-20">20</a>
      <a href="#section-4.3">4.3</a>. EAP Security Claims .......................................<a href="#page-21">21</a>
           <a href="#section-4.3.1">4.3.1</a>. Protected Ciphersuite Negotiation ..................<a href="#page-21">21</a>
           <a href="#section-4.3.2">4.3.2</a>. Mutual Authentication ..............................<a href="#page-21">21</a>
           <a href="#section-4.3.3">4.3.3</a>. Integrity Protection ...............................<a href="#page-21">21</a>
           <a href="#section-4.3.4">4.3.4</a>. Replay Protection ..................................<a href="#page-21">21</a>
           <a href="#section-4.3.5">4.3.5</a>. Confidentiality ....................................<a href="#page-21">21</a>
           <a href="#section-4.3.6">4.3.6</a>. Key Derivation .....................................<a href="#page-21">21</a>
           <a href="#section-4.3.7">4.3.7</a>. Key Strength .......................................<a href="#page-22">22</a>
           <a href="#section-4.3.8">4.3.8</a>. Dictionary Attack Resistance .......................<a href="#page-22">22</a>
           <a href="#section-4.3.9">4.3.9</a>. Fast Reconnect .....................................<a href="#page-22">22</a>
           <a href="#section-4.3.10">4.3.10</a>. Session Independence ..............................<a href="#page-22">22</a>
           <a href="#section-4.3.11">4.3.11</a>. Fragmentation .....................................<a href="#page-23">23</a>
           <a href="#section-4.3.12">4.3.12</a>. Channel Binding ...................................<a href="#page-23">23</a>
           <a href="#section-4.3.13">4.3.13</a>. Cryptographic Binding .............................<a href="#page-23">23</a>
           <a href="#section-4.3.14">4.3.14</a>. Negotiation Attack Prevention .....................<a href="#page-23">23</a>
   <a href="#section-5">5</a>. IANA Considerations ............................................<a href="#page-23">23</a>
   <a href="#section-6">6</a>. Acknowledgments ................................................<a href="#page-24">24</a>
   <a href="#section-7">7</a>. References .....................................................<a href="#page-24">24</a>
      <a href="#section-7.1">7.1</a>. Normative References ......................................<a href="#page-24">24</a>
      <a href="#section-7.2">7.2</a>. Informative References ....................................<a href="#page-25">25</a>
   <a href="#appendix-A">Appendix A</a>. Key Generation from Passwords ........................ <a href="#page-27">27</a>
   <a href="#appendix-B">Appendix B</a>. Implementation Suggestions ........................... <a href="#page-27">27</a>
     <a href="#appendix-B.1">B.1</a>. WiFi Enterprise Network ................................... <a href="#page-27">27</a>
     <a href="#appendix-B.2">B.2</a>. Mobile Phone Network ...................................... <a href="#page-28">28</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   EAP-PAX (Password Authenticated eXchange) is an Extensible
   Authentication Protocol (EAP) method [<a href="./rfc3748" title="&quot;Extensible Authentication Protocol (EAP)&quot;">RFC3748</a>] designed for
   authentication using a shared key.  It makes use of two separate
   subprotocols, PAX_STD and PAX_SEC.  PAX_STD is a simple, lightweight
   protocol for mutual authentication using a shared key, supporting
   Authenticated Data Exchange (ADE).  PAX_SEC complements PAX_STD by
   providing support for shared-key provisioning and identity protection
   using a server-side public key.






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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   The idea motivating EAP-PAX is a desire for device authentication
   bootstrapped by a simple Personal Identification Number (PIN).  If a
   weak key is used or a expiration period has elapsed, the
   authentication server forces a key update.  Rather than using a
   symmetric key exchange, the client and server perform a Diffie-
   Hellman key exchange, which provides forward secrecy.

   Since implementing a Public Key Infrastructure (PKI) can be
   cumbersome, PAX_SEC defines multiple client security policies,
   selectable based on one's threat model.  In the weakest mode, PAX_SEC
   allows the use of raw public keys completely eliminating the need for
   a PKI.  In the strongest mode, PAX_SEC requires that EAP servers use
   certificates signed by a trusted Certification Authority (CA).  In
   the weaker modes, during provisioning PAX_SEC is vulnerable to a
   man-in-the-middle dictionary attack.  In the strongest mode, EAP-PAX
   is provably secure under the Random Oracle model.

   EAP-PAX supports the generation of strong key material; mutual
   authentication; resistance to desynchronization, dictionary, and
   man-in-the-middle attacks; ciphersuite extensibility with protected
   negotiation; identity protection; and the authenticated exchange of
   data, useful for implementing channel binding.  These features
   satisfy the EAP method requirements for wireless LANs [<a href="./rfc4017" title="&quot;Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs&quot;">RFC4017</a>],
   making EAP-PAX ideal for wireless environments such as IEEE 802.11
   [<a href="#ref-IEEE.80211" title="&quot;Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications&quot;">IEEE.80211</a>].

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Language Requirements</span>

   In this document, several words are used to signify the requirements
   of the specification.  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
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Terminology</span>

   This section describes the various variables and functions used in
   the EAP-PAX protocol.  They will be referenced frequently in later
   sections.

   Variables:

   CID
      User-supplied client ID, specified as a Network Access Identifier
      (NAI) [<a href="./rfc4282" title="&quot;The Network Access Identifier&quot;">RFC4282</a>], restricted to 65535 octets

   g
      public Diffie-Hellman generator, typically the integer 2 [<a href="./rfc2631" title="&quot;Diffie-Hellman Key Agreement Method&quot;">RFC2631</a>]



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   M
      128-bit random integer generated by the server

   N
      128-bit random integer generated by the client

   X
      256-bit random integer generated by the server

   Y
      256-bit random integer generated by the client

   Keys:

   AK
      authentication key shared between the client and EAP server

   AK'
      new authentication key generated during a key update

   CertPK
      EAP server's certificate containing public key PK

   CK
      Confirmation Key generated from the MK and used during
      authentication to prove knowledge of AK

   EMSK
      Extended Master Session Key also generated from the MK and
      containing additional keying material

   IV
      Initialization Vector used to seed ciphers; exported to the
      authenticator

   MID
      Method ID used to construct the EAP Session ID and consequently
      name all the exported keys [<a href="#ref-IETF.KEY" title="&quot;Extensible Authentication Protocol (EAP) Key Management Framework&quot;">IETF.KEY</a>]

   MK
      Master Key between the client and EAP server from which all other
      EAP method session keys are derived

   MSK
      Master Session Key generated from the MK and exported by the EAP
      method to the authenticator





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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   PK
      EAP server's public key

   Operations:

   enc_X(Y)
      encryption of message Y with public key X

   MAC_X(Y)
      keyed message authentication code computed over message Y with
      symmetric key X

   PAX-KDF-W(X, Y, Z)
      PAX Key Derivation Function computed using secret X, identifier Y,
      and seed Z, and producing W octets of output

   ||
      string or binary data concatenation

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Overview</span>

   The EAP framework [<a href="./rfc3748" title="&quot;Extensible Authentication Protocol (EAP)&quot;">RFC3748</a>] defines four basic steps that occur
   during the execution of an EAP conversation between client and
   server.  The first phase, discovery, is handled by the underlying
   link-layer protocol.  The authentication phase is defined here.  The
   key distribution and secure association phases are handled
   differently depending on the underlying protocol, and are not
   discussed in this document.

        +--------+                                     +--------+
        |        |                EAP-Request/Identity |        |
        | CLIENT |&lt;------------------------------------| SERVER |
        |        |                                     |        |
        |        | EAP-Response/Identity               |        |
        |        |------------------------------------&gt;|        |
        |        |                                     |        |
        |        |        EAP-PAX (STD or SEC)         |        |
        |        |&lt;-----------------------------------&gt;|        |
        |        | ...                             ... |        |
        |        |&lt;-----------------------------------&gt;|        |
        |        |                                     |        |
        |        |          EAP-Success or EAP-Failure |        |
        |        |&lt;------------------------------------|        |
        +--------+                                     +--------+

                    Figure 1: EAP-PAX Packet Exchanges





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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   There are two distinct subprotocols that can be executed.  The first,
   PAX_STD, is used during typical authentications.  The second,
   PAX_SEC, provides more secure features such as key provisioning and
   identity protection.

   PAX_STD and PAX_SEC have two modes of operation.  When an AK update
   is being performed, the client and server exchange Diffie-Hellman
   exponents g^X and g^Y, which are computed modulo prime P or over an
   elliptic curve multiplicative group.  When no update is being
   performed, and only session keys are being derived, X and Y are
   exchanged.  Using Diffie-Hellman during the key update provides
   forward secrecy, and secure key derivation when a weak provisioned
   key is used.

   The main deployment difference between PAX_STD and PAX_SEC is that
   PAX_SEC requires a server-side public key.  More specifically, that
   means a private key known only to the server with corresponding
   public key being transmitted to the client during each PAX_SEC
   session.  For every authentication, the client is required to compute
   computationally intensive public-key operations.  PAX_STD, on the
   other hand, uses purely symmetric operations, other than a possible
   Diffie-Hellman exchange.

   Each of the protocols is now defined.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  PAX_STD Protocol</span>

   PAX_STD is a simple nonce-based authentication using the strong
   long-term key.  The client and server each exchange 256 bits of
   random data, which is used to seed the PAX-KDF for generation of
   session keys.  The randomly exchanged data in the protocol differs
   depending on whether a key update is being performed.  If no key
   update is being performed, then let:

   o  A = X
   o  B = Y
   o  E = X || Y

   Thus, A and B are 256-bit values and E is their 512-bit
   concatenation.  To provide forward secrecy and security, let the
   following be true when a key update is being performed:

   o  A = g^X
   o  B = g^Y
   o  E = g^(XY)

   Here A and B are Diffie-Hellman exponents whose length is determined
   by the Diffie-Hellman group size.  The value A is data transmitted



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   from the server to the client, and B is data transmitted from the
   client to the server.  The value E is the entropy computed by each
   that is used in <a href="#section-2.4">Section 2.4</a> to perform key derivation.

   The full protocol is as follows:

   o  PAX_STD-1 : client &lt;- server : A
   o  PAX_STD-2 : client -&gt; server : B, CID, MAC_CK(A, B, CID),
      [optional ADE]
   o  PAX_STD-3 : client &lt;- server : MAC_CK(B, CID), [optional ADE]
   o  PAX-ACK : client -&gt; server : [optional ADE]

   See <a href="#section-2.3">Section 2.3</a> for more information on the ADE component, and
   <a href="#section-2.4">Section 2.4</a> for the key derivation process, including derivation of
   CK.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  PAX_SEC Protocol</span>

   PAX_SEC is the high-security protocol designed to provide identity
   protection and support for provisioning.  PAX_SEC requires a server-
   side public key, and public-key operations for every authentication.

   PAX_SEC can be performed with and without key update.  Let A, B, and
   E be defined as in the previous section.

   The exchanges for PAX_SEC are as follows:

   o  PAX_SEC-1 : client &lt;- server : M, PK or CertPK
   o  PAX_SEC-2 : client -&gt; server : Enc_PK(M, N, CID)
   o  PAX_SEC-3 : client &lt;- server : A, MAC_N(A, CID)
   o  PAX_SEC-4 : client -&gt; server : B, MAC_CK(A, B, CID), [optional
      ADE]
   o  PAX_SEC-5 : client &lt;- server : MAC_CK(B, CID), [optional ADE]
   o  PAX-ACK : client -&gt; server : [optional ADE]

   See <a href="#section-2.3">Section 2.3</a> for more information on the ADE component, and
   <a href="#section-2.4">Section 2.4</a> for the key derivation process, including derivation of
   CK.

   Use of CertPK is optional in PAX_SEC; however, careful consideration
   should be given before omitting the CertPK.  The following table
   describes the risks involved when using PAX_SEC without a
   certificate.








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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


        Certificate    |    Provisioning     |       Identity
            Mode       |                     |      Protection
     ==================+=====================+======================
       No Certificate  |    MiTM offline     |   ID reveal attack
                       |  dictionary attack  |
     ------------------+---------------------+---------------------
        Self-Signed    |    MiTM offline     |   ID reveal attack
        Certificate    |  dictionary attack  |
     ------------------+---------------------+---------------------
       Certificate/PK  |    MiTM offline     |   ID reveal attack
          Caching      |  dictionary attack  |  during first auth
     ------------------+---------------------+---------------------
         CA-Signed     |   secure mutual     |   secure mutual
        Certificate    |   authentication    |   authentication

                Figure 2: Table of Different Security Modes

   When using PAX_SEC to support provisioning with a weak key, use of a
   CA-signed certificate is RECOMMENDED.  When not using a CA-signed
   certificate, the initial authentication is vulnerable to an offline
   man-in-the-middle (MiTM) dictionary attack.

   When using PAX_SEC to support identity protection, use of either a
   CA-signed certificate or key caching is RECOMMENDED.  Caching
   involves a client recording the public key of the EAP server and
   verifying its consistency between sessions, similar to Secure SHell
   (SSH) Protocol [<a href="./rfc4252" title="&quot;The Secure Shell (SSH) Authentication Protocol&quot;">RFC4252</a>].  Otherwise, an attacker can spoof an EAP
   server during a session and gain knowledge of a client's identity.

   Whenever certificates are used, clients MUST validate that the
   certificate's extended key usage, KeyPurposeID, is either
   "eapOverPPP" or "eapOverLAN" [<a href="./rfc3280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC3280</a>][RFC4334].  If the underlying
   EAP transport protocol is known, then the client MUST differentiate
   between these values.  For example, an IEEE 802.11 supplicant SHOULD
   require KeyPurposeID == eapOverLAN.  By not distinguishing, a client
   could accept as valid an unauthorized server certificate.

   When using EAP-PAX with Wireless LAN, clients SHOULD validate that
   the certificate's wlanSSID extension matches the SSID of the network
   to which it is currently authenticating.

   In order to facilitate discussion of packet validations, three client
   security policies for PAX_SEC are defined.

   open
      Clients support both use of PK and CertPK.  If CertPK is used, the
      client MUST validate the KeyPurposeID.




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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   caching
      Clients save PK for each EAP server the first time it encounters
      the server, and SHOULD NOT authenticate to EAP servers whose
      public key has been changed.  If CertPK is used, the client MUST
      validate the KeyPurposeID.

   strict
      In strict mode, clients require servers to present a valid
      certificate signed by a trusted CA.  As with the other modes, the
      KeyPurposeID MUST be validated.

   Servers SHOULD support the PAX_SEC mode of operation, and SHOULD
   support both the use of PK and CertPK with PAX_SEC.  Clients MUST
   support PAX_SEC, and MUST be capable of accepting both raw public
   keys and certificates from the server.  Local security policy will
   define which forms of key or certificate authentications are
   permissible.  Default configurations SHOULD require a minimum of the
   caching security policy, and MAY require strict.

   The ability to perform key management on the AK is built in to EAP-
   PAX through the use of AK'.  However, key management of the server
   public key is beyond the scope of this document.  If self-signed
   certificates are used, the deployers should be aware that expired
   certificates may be difficult to replace when the caching security
   mode is used.

   See <a href="#section-4">Section 4</a> for further discussion on security considerations.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Authenticated Data Exchange</span>

   Messages PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and PAX_ACK
   contain optional component ADE.  This component is used to convey
   authenticated data between the client and server during the
   authentication.

   The Authenticated Data Exchanged (ADE) can be used in a variety of
   ways, including the implementation of channel bindings.  Channel
   bindings allow link-layer network properties to be securely validated
   by the EAP client and server during the authentication session.

   It is important to note that ADE is not encrypted, so any data
   included will not be confidential.  However, since these packets are
   all protected by the Integrity Check Value (ICV), authenticity is
   guaranteed.







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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   The ADE element consists of an arbitrary number of subelements, each
   with length and type specified.  If the number and size of
   subelements is too large, packet fragmentation will be necessary.
   Vendor-specific options are supported.  See <a href="#section-3.3">Section 3.3</a>.

   Note that more than 1.5 round-trips may be necessary to execute a
   particular authenticated protocol within EAP-PAX.  In this case,
   instead of sending an EAP-Success after receiving the PAX_ACK, the
   server can continue sending PAX_ACK messages with attached elements.
   The client responds to these PAX_ACK messages with PAX_ACK messages
   possibly containing more ADE elements.  Such an execution could look
   something like the following:

        +--------+                                     +--------+
        |        |                           PAX_STD-1 |        |
        |        |&lt;------------------------------------|        |
        |        | PAX_STD-2(ADE[1])                   |        |
        |        |------------------------------------&gt;|        |
        |        |                   PAX_STD-3(ADE[2]) |        |
        |        |&lt;------------------------------------|        |
        |        | PAX_ACK(ADE[3])                     |        |
        |        |------------------------------------&gt;|        |
        |        |                     PAX_ACK(ADE[4]) |        |
        |        |&lt;------------------------------------|        |
        |        |                                     |        |
        |        |                 ...                 |        |
        |        |                                     |        |
        |        | PAX_ACK(ADE[i])                     |        |
        |        |------------------------------------&gt;|        |
        |        |                   PAX_ACK(ADE[i+1]) |        |
        |        |&lt;------------------------------------|        |
        |        |                                     |        |
        |        |                 ...                 |        |
        |        |                                     |        |
        |        |          EAP-Success or EAP-Failure |        |
        |        |&lt;------------------------------------|        |
        +--------+                                     +--------+

          Figure 3: Extended Diagram of EAP-PAX Packet Exchanges

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Key Derivation</span>

   Keys are derived independently of which authentication mechanism was
   used.  The process uses the entropy value E computed as described
   above.  Session and authentication keys are computed as follows:

   o  AK' = PAX-KDF-16(AK, "Authentication Key", E)
   o  MK = PAX-KDF-16(AK, "Master Key", E)



<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   o  CK = PAX-KDF-16(MK, "Confirmation Key", E)
   o  ICK = PAX-KDF-16(MK, "Integrity Check Key", E)
   o  MID = PAX-KDF-16(MK, "Method ID", E)
   o  MSK = PAX-KDF-64(MK, "Master Session Key", E)
   o  EMSK = PAX-KDF-64(MK, "Extended Master Session Key", E)
   o  IV = PAX-KDF-64(0x00^16, "Initialization Vector", E)

   The IV is computed using a 16-octet NULL key.  The value of AK' is
   only used to replace AK if a key update is being performed.  The EAP
   Method ID is represented in ASCII as 32 hexadecimal characters
   without any octet delimiters such as colons or dashes.

   The EAP Key Management Framework [<a href="#ref-IETF.KEY" title="&quot;Extensible Authentication Protocol (EAP) Key Management Framework&quot;">IETF.KEY</a>] recommends specification
   of key names and scope.  The EAP-PAX Method-ID is the MID value
   computed as described above.  The EAP peer name is the CID value
   exchanged in PAX_STD-2 and PAX_SEC-2.  The EAP server name is an
   empty string.

<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>.  Verification Requirements</span>

   In order for EAP-PAX to be secure, MACs must be properly verified
   each step of the way.  Any packet with an ICV (see <a href="#section-3.4">Section 3.4</a>) that
   fails validation must be silently discarded.  After ICV validation,
   the following checks must be performed:

   PAX_STD-2
      The server MUST validate the included MAC, as it serves to
      authenticate the client to the server.  If this validation fails,
      the server MUST send an EAP-Failure message.

   PAX_STD-3
      The client MUST validate the included MAC, as it serves to
      authenticate the server to the client.  If this validation fails,
      the client MUST send an EAP-Failure message.

   PAX_SEC-1
      The client MUST validate PK or CertPK in a manner specified by its
      local security policy (see <a href="#section-2.2">Section 2.2</a>).  If this validation
      fails, the client MUST send an EAP-Failure message.

   PAX_SEC-2
      The server MUST verify that the decrypted value of M matches the
      value transmitted in PAX_SEC-1.  If this validation fails, the
      server MUST send an EAP-Failure message.







<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   PAX_SEC-3
      The client MUST validate the included MAC, as it serves to prevent
      replay attacks.  If this validation fails, the client MUST send an
      EAP-Failure message.

   PAX_SEC-4
      The server MUST validate the included MAC, as it serves to
      authenticate the client to the server.  If this validation fails,
      the server MUST send an EAP-Failure message.

   PAX_SEC-5
      The client MUST validate the included MAC, as it serves to
      authenticate the server to the client.  If this validation fails,
      the client MUST send an EAP-Failure message.

   PAX-ACK
      If PAX-ACK is received in response to a message fragment, the
      receiver continues the protocol execution.  If PAX-ACK is received
      in response to PAX_STD-3 or PAX_SEC-5, then the server MUST send
      an EAP-Success message.  This indicates a successful execution of
      PAX.

<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a>.  PAX Key Derivation Function</span>

   The PAX-KDF is a secure key derivation function used to generate
   various keys from the provided entropy and shared key.

   PAX-KDF-W(X, Y, Z)

   W  length, in octets, of the desired output
   X  secret key used to protect the computation
   Y  public identifier for the key being derived
   Z  exchanged entropy used to seed the KDF

   Let's define some variables and functions:

   o  M_i = MAC_X(Y || Z || i), where i is an 8-bit unsigned integer
   o  L = ceiling(W/16)
   o  F(A, B) = first A octets of binary data B

   We define PAX-KDF-W(X, Y, Z) = F(W, M_1 || M_2 || ... || M_L).

   Consequently for the two values of W used in this document, we have:

   o  PAX-KDF-16(X, Y, Z) = MAC_X(Y || Z || 0x01)
   o  PAX-KDF-64(X, Y, Z) = MAC_X(Y || Z || 0x01) || MAC_X(Y || Z ||
      0x02) || MAC_X(Y || Z || 0x03) || MAC_X(Y || Z || 0x04)




<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   The MAC used in the PRF is extensible and is the same MAC used in the
   rest of the protocol.  It is specified in the EAP-PAX header.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Protocol Specification</span>

   In this section, the packet format and content for the EAP-PAX
   messages are defined.

   EAP-PAX packets have the following structure:

    --- bit offset ---&gt;
     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      |    OP-Code    |     Flags     |    MAC ID     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  DH Group ID  | Public Key ID |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    ...                         Payload                           ...
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ...                           ICV                             ...
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: EAP-PAX Packet Structure

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Header Specification</span>

   The Code, Identifier, Length, and Type fields are all part of the EAP
   header, and defined in [<a href="./rfc3748" title="&quot;Extensible Authentication Protocol (EAP)&quot;">RFC3748</a>].  IANA has allocated EAP Method Type
   46 for EAP-PAX; thus, the Type field in the EAP header MUST be 46.

<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>.  Op-Code</span>

   The OP-Code field is one of the following values:

   o  0x01 : PAX_STD-1
   o  0x02 : PAX_STD-2
   o  0x03 : PAX_STD-3
   o  0x11 : PAX_SEC-1
   o  0x12 : PAX_SEC-2
   o  0x13 : PAX_SEC-3
   o  0x14 : PAX_SEC-4



<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   o  0x15 : PAX_SEC-5
   o  0x21 : PAX-ACK

<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>.  Flags</span>

   The flags field is broken up into 8 bits each representing a binary
   flag.  The field is defined as the Logical OR of the following
   values:

   o  0x01 : more fragments (MF)
   o  0x02 : certificate enabled (CE)
   o  0x04 : ADE Included (AI)
   o  0x08 - 0x80 : reserved

   The MF flag is set if the current packet required fragmentation, and
   further fragments need to be transmitted.  If a packet does not
   require fragmentation, the MF flag is not set.

   When a payload requires fragmentation, each fragment is transmitted,
   and the receiving party responds with a PAX-ACK packet for each
   received fragment.

   When using PAX_STD, the CE flag MUST be zero.  When using PAX_SEC,
   the CE flag MUST be set if PAX_SEC-1 includes CertPK.  It MUST NOT be
   set if PAX_SEC-1 includes PK.  If CE is set in PAX_SEC-1, it MUST be
   set in PAX_SEC-2, PAX_SEC-3, PAX_SEC-4, and PAX_SEC-5.  If either
   party detects an inconsistent value of the CE flag, he MUST send an
   EAP-Failure message and discontinue the session.

   The AI flag indicates the presence of an ADE element.  AI MUST only
   be set on packets PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and
   PAX_ACK if an ADE element is included.  On packets of other types,
   ADE elements MUST be silently discarded as they cannot be
   authenticated.

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>.  MAC ID</span>

   The MAC field specifies the cryptographic hash used to generate the
   keyed hash value.  The following are currently supported:

   o  0x01 : HMAC_SHA1_128 [<a href="#ref-FIPS198" title="&quot;The Keyed-Hash Message Authentication Code (HMAC)&quot;">FIPS198</a>] [<a href="#ref-FIPS180" title="&quot;Secure Hash Standard&quot;">FIPS180</a>]
   o  0x02 : HMAC_SHA256_128 [<a href="#ref-FIPS180" title="&quot;Secure Hash Standard&quot;">FIPS180</a>]

<span class="h4"><a class="selflink" id="section-3.1.4" href="#section-3.1.4">3.1.4</a>.  DH Group ID</span>

   The Diffie-Hellman group field specifies the group used in the
   Diffie-Hellman computations.  The following are currently supported:




<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   o  0x00 : NONE (iff not performing a key update)
   o  0x01 : 2048-bit MODP Group (IANA DH Group 14) [<a href="./rfc3526" title="&quot;More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)&quot;">RFC3526</a>]
   o  0x02 : 3072-bit MODP Group (IANA DH Group 15) [<a href="./rfc3526" title="&quot;More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)&quot;">RFC3526</a>]
   o  0x03 : NIST ECC Group P-256 [<a href="#ref-FIPS186" title="&quot;Digital Signature Standard (DSS)&quot;">FIPS186</a>]

   If no key update is being performed, the DH Group ID field MUST be
   zero.  Otherwise, the DH Group ID field MUST NOT be zero.

<span class="h4"><a class="selflink" id="section-3.1.5" href="#section-3.1.5">3.1.5</a>.  Public Key ID</span>

   The Public Key ID field specifies the cipher used to encrypt the
   client's EAP-Response in PAX_SEC-2.

   The following are currently supported:

   o  0x00 : NONE (if using PAX_STD)
   o  0x01 : RSAES-OAEP [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>]
   o  0x02 : RSA-PKCS1-V1_5 [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>]
   o  0x03 : El-Gamal Over NIST ECC Group P-256 [<a href="#ref-FIPS186" title="&quot;Digital Signature Standard (DSS)&quot;">FIPS186</a>]

   If PAX_STD is being executed, the Public Key ID field MUST be zero.
   If PAX_SEC is being executed, the Public Key ID field MUST NOT be
   zero.

   When using RSAES-OAEP, the hash algorithm and mask generation
   algorithm used SHALL be the MAC specified by the MAC ID, keyed using
   an all-zero key.  The label SHALL be null.

   The RSA-based schemes specified here do not dictate the length of the
   public keys.  DER encoding rules will specify the key size in the key
   or certificate [<a href="#ref-X.690" title="&quot;Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)&quot;">X.690</a>].  Key sizes SHOULD be used that reflect the
   desired level of security.

<span class="h4"><a class="selflink" id="section-3.1.6" href="#section-3.1.6">3.1.6</a>.  Mandatory to Implement</span>

   The following ciphersuite is mandatory to implement and achieves
   roughly 112 bits of security:

   o  HMAC_SHA1_128
   o  IANA DH Group 14 (2048 bits)
   o  RSA-PKCS1-V1_5 (RECOMMEND 2048-bit public key)

   The following ciphersuite is RECOMMENDED and achieves 128 bits of
   security:

   o  HMAC_SHA256_128
   o  IANA DH Group 15 (3072 bits)
   o  RSAES-OAEP (RECOMMEND 3072-bit public key)



<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Payload Formatting</span>

   This section describes how to format the payload field.  Depending on
   the packet type, different values are transmitted.  Sections <a href="#section-2.1">2.1</a> and
   2.2 define the fields, and in what order they are to be concatenated.
   For simplicity and since many field lengths can vary with the
   ciphersuite, each value is prepended with a 2-octet length value
   encoded as an integer as described below.  This length field MUST
   equal the length in octets of the subsequent value field.

              --- octet offset ---&gt;
               0                   1
               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
              +---+---------------------
              |len|  value ....
              +---+--------

                Figure 5: Length Encoding for Data Elements

   All integer values are stored as octet arrays in network-byte order,
   with the most significant octet first.  Integers are padded on the
   most significant end to reach octet boundaries.

   Public keys and certificates SHALL be in X.509 format [<a href="./rfc3280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC3280</a>]
   encoded using the Distinguished Encoding Rules (DER) format [<a href="#ref-X.690" title="&quot;Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)&quot;">X.690</a>].

   Strings are not null-terminated and are encoded using UTF-8.  Binary
   data, such as message authentication codes, are transmitted as-is.

   MACs are computed by concatenating the specified values in the
   specified order.  Note that for MACs, length fields are not included,
   though the resulting MAC will itself have a length field.  Values are
   encoded as described above, except that no length field is specified.

   To illustrate this process, an example is presented.  What follows is
   the encoding of the payload for PAX_STD-2.  The three basic steps
   will be computing the MAC, forming the payload, and encrypting the
   payload.

   To create the MAC, we first need to form the buffer that will be
   MACed.  For this example, assume that no key update is being done and
   HMAC_SHA1_128 is used such that the result will be a 16-octet value.









<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   --- octet offset ---&gt;
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       32-octet integer A                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       32-octet integer B                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ...                    variable length CID                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  ||
                  ||
           CK --&gt; MAC
                  ||
                  \/

   --- octet offset ---&gt;
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      16-octet MAC output      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 6: Example Encoding of PAX_STD-2 MAC Data

   With this, we can now create the encoded payload:

   --- octet offset ---&gt;
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |32 |                     32-octet integer B
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | L |                                                       |
   +-+-+-+-+                                                       +
   |                                                               |
   ...                        L-octet CID                        ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |16 |       MAC computed above      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 7: Example Encoding of PAX_STD-2 Packet





<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   These 52+L octets are then attached to the packet as the payload.
   The ICV is then computed by MACing the packet headers and payload,
   and appended after the payload (see <a href="#section-3.4">Section 3.4</a>).

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Authenticated Data Exchange (ADE)</span>

   This section describes the formatting of the ADE elements.  ADE
   elements can only occur on packets of type PAX_STD-2, PAX_STD-3,
   PAX_SEC-4, PAX_SEC-5, and PAX_ACK.  Values included in other packets
   MUST be silently ignored.

   The ADE element is preceded by its 2-octet length L.  Each subelement
   has first a 2-octet length Li followed by a 2-octet type Ti.  The
   entire ADE element looks as follows:

   --- octet offset ---&gt;
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | L |L1 |T1 |                                                   |
   +-+-+-+-+-+-+                                                   +
   |                                                               |
   ...                 subADE-1, type T1, length L1              ...
   |                                                               |
   +                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   |L2 |T2 |                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   +
   |                                                               |
   ...                 subADE-2, type T2, length L2              ...
   |                                                               |
   +         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         | more subADE elements...                           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 8: Encoding of ADE Components

   The following type values have been allocated:

   o  0x01 : Vendor Specific
   o  0x02 : Client Channel Binding Data
   o  0x03 : Server Channel Binding Data










<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   The first three octets of a subADE utilizing type code 0x01 must be
   the vendor's Enterprise Number [<a href="./rfc3232" title="&quot;Assigned Numbers: RFC 1700 is Replaced by an On-line Database&quot;">RFC3232</a>] as registered with IANA.
   The format for such a subADE is as follows:

   --- octet offset ---&gt;
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Li | 1 | ENi |                                                 |
   +-+-+-+-+-+-+-+                                                 +
   |                                                               |
   ...   subADE-i, type Vendor Specific, length Li, vendor ENi  ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 9: Encoding of Vendor-specific ADE

   Channel binding subADEs have yet to be defined.  Future IETF
   documents will specify the format for these subADE fields.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Integrity Check Value (ICV)</span>

   The ICV is computed as the MAC over the entire EAP packet, including
   the EAP header, the EAP-PAX header, and the EAP-PAX payload.  The MAC
   is keyed using the 16-octet ICK, using the MAC type specified by the
   MAC ID in the EAP-PAX header.  For packets of type PAX_STD-1,
   PAX_SEC-1, PAX_SEC-2, and PAX_SEC-3, where the MK has not yet been
   derived, the MAC is keyed using a zero-octet NULL key.

   If the ICV field is incorrect, the receiver MUST silently discard the
   packet.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Security Considerations</span>

   Any authentication protocol, especially one geared for wireless
   environments, must assume that adversaries have many capabilities.
   In general, one must assume that all messages between the client and
   server are delivered via the adversary.  This allows passive
   attackers to eavesdrop on all traffic, while active attackers can
   modify data in any way before delivery.

   In this section, we discuss the security properties and requirements
   of EAP-PAX with respect to this threat model.  Also note that the
   security of PAX can be proved using under the Random Oracle model.







<span class="grey">Clancy &amp; Arbaugh             Informational                     [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Server Certificates</span>

   PAX_SEC can be used in several configurations.  It can be used with
   or without a server-side certificate.  <a href="#section-2.2">Section 2.2</a> details the
   possible modes and the resulting security risk.

   When using PAX_SEC for identity protection and not using a CA-signed
   certificate, an attacker can convince a client to reveal his
   username.  To achieve this, an attacker can simply forge a PAX_SEC-1
   message and send it to the client.  The client would respond with a
   PAX_SEC-2 message containing his encrypted username.  The attacker
   can then use his associated private key to decrypt the client's
   username.  Use of key caching can reduce the risk of identity
   revelation by allowing clients to detect when the EAP server to which
   they are accustom has a different public key.

   When provisioning with PAX_SEC and not using a CA-signed certificate,
   an attacker could first forge a PAX_SEC-1 message and send it to the
   client.  The client would respond with a PAX_SEC-2 message.  Using
   the decrypted value of N, an attacker could forge a PAX_SEC-3
   message.  Once the client responds with a PAX_SEC-4 message, an
   attacker can guess values of the weak AK and compute CK = PAX-KDF(AK,
   "Confirmation Key", g^XY).  Given enough time, the attacker can
   obtain both the old AK and new AK' and forge a responding PAX_SEC-5.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Server Security</span>

   In order to maintain a reasonable security policy, the server should
   manage five pieces of information concerning each user, most
   obviously, the username and current key.  In addition, the server
   must keep a bit that indicates whether the current key is weak.  Weak
   keys must be updated prior to key derivation.  Also, the server
   should track the date of last key update.  To implement the coarse-
   grained forward secrecy, the authentication key must be updated on a
   regular basis, and this field can be used to expire keys.  Last, the
   server should track the previous key, to prevent attacks where an
   adversary desynchronizes the key state by interfering with PAX-ACK
   packets.  See <a href="#appendix-B">Appendix B</a> for more suggested implementation strategies
   that prevent key desynchronization attacks.

   Since the client keys are stored in plaintext on the server, special
   care should be given to the overall security of the authentication
   server.  An operating system-level attack yielding root access to an
   intruder would result in the compromise of all client credentials.







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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  EAP Security Claims</span>

   This section describes EAP-PAX in terms of specific security
   terminology as required by [<a href="./rfc3748" title="&quot;Extensible Authentication Protocol (EAP)&quot;">RFC3748</a>].

<span class="h4"><a class="selflink" id="section-4.3.1" href="#section-4.3.1">4.3.1</a>.  Protected Ciphersuite Negotiation</span>

   In the initial packet from the server, the server specifies the
   ciphersuite in the packet header.  The server is in total control of
   the ciphersuite; thus, a client not supporting the specified
   ciphersuite will not be able to authenticate.  In addition, each
   client's local security policy should specify secure ciphersuites the
   client will accept.  The ciphersuite specified in PAX_STD-1 and
   PAX_SEC-1 MUST remain the same in successive packets within the same
   authentication session.  Since later packets are covered by an ICV
   keyed with the ICK, the server can verify that the originally
   transmitted ciphersuite was not altered by an adversary.

<span class="h4"><a class="selflink" id="section-4.3.2" href="#section-4.3.2">4.3.2</a>.  Mutual Authentication</span>

   Both PAX_STD and PAX_SEC authenticate the client and the server, and
   consequently achieve explicit mutual authentication.

<span class="h4"><a class="selflink" id="section-4.3.3" href="#section-4.3.3">4.3.3</a>.  Integrity Protection</span>

   The ICV described in <a href="#section-3.4">Section 3.4</a> provides integrity protection once
   the integrity check key has been derived.  The header values in the
   unprotected packets can be verified when an ICV is received later in
   the session.

<span class="h4"><a class="selflink" id="section-4.3.4" href="#section-4.3.4">4.3.4</a>.  Replay Protection</span>

   EAP-PAX is inherently designed to avoid replay attacks by
   cryptographically binding each packet to the previous one.  Also the
   EAP sequence number is covered by the ICV to further strengthen
   resistance to replay attacks.

<span class="h4"><a class="selflink" id="section-4.3.5" href="#section-4.3.5">4.3.5</a>.  Confidentiality</span>

   With identity protection enabled, PAX_SEC provides full
   confidentiality.

<span class="h4"><a class="selflink" id="section-4.3.6" href="#section-4.3.6">4.3.6</a>.  Key Derivation</span>

   Session keys are derived using the PAX-KDF and fresh entropy supplied
   by both the client and the server.  Since the key hierarchy is
   derived from the shared password, only someone with knowledge of that
   password or the capability of guessing it is capable of deriving the



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   session keys.  One of the main benefits of PAX_SEC is that it allows
   you to bootstrap a strong shared secret using a weak password while
   preventing offline dictionary attacks.

<span class="h4"><a class="selflink" id="section-4.3.7" href="#section-4.3.7">4.3.7</a>.  Key Strength</span>

   Authentication keys are 128 bits.  The key generation is protected by
   a Diffie-Hellman key exchange.  It is believed that a 3000-bit MODP
   public-key scheme is roughly equivalent [<a href="./rfc3766" title="&quot;Determining Strengths For Public Keys Used For Exchanging Symmetric Keys&quot;">RFC3766</a>] to a 128-bit
   symmetric-key scheme.  Consequently, EAP-PAX requires the use of a
   Diffie-Hellman group with modulus larger than 3000.  Also, the
   exponent used as the private DH parameter must be at least twice as
   large as the key eventually generated.  Consequently, EAP-PAX uses
   256-bit DH exponents.  Thus, the authentication keys contain the full
   128 bits of security.

   Future ciphersuites defined for EAP-PAX MUST contain a minimum of 128
   bits of security.

<span class="h4"><a class="selflink" id="section-4.3.8" href="#section-4.3.8">4.3.8</a>.  Dictionary Attack Resistance</span>

   EAP-PAX is resistant to dictionary attacks, except for the case where
   a weak password is initially used and the server is not using a
   certificate for authentication.  See <a href="#section-4.1">Section 4.1</a> for more information
   on resistance to dictionary attacks.

<span class="h4"><a class="selflink" id="section-4.3.9" href="#section-4.3.9">4.3.9</a>.  Fast Reconnect</span>

   Although a specific fast reconnection option is not included,
   execution of PAX_STD requires very little computation time and is
   therefore bound primarily by the latency of the Authentication,
   Authorization, and Accounting (AAA) server.

<span class="h4"><a class="selflink" id="section-4.3.10" href="#section-4.3.10">4.3.10</a>.  Session Independence</span>

   This protocol easily achieves backward secrecy through, among other
   things, use of the PAX-KDF.  Given a current session key, attackers
   can discover neither the entropy used to generate it nor the key used
   to encrypt that entropy as it was transmitted across the network.

   This protocol has coarse-grained forward secrecy.  Compromised
   session keys are only useful on data for that session, and one cannot
   derive AK from them.  If an attacker can discover AK, that value can
   only be used to compromise session keys derived using that AK.
   Reasonably frequent password updates will help mitigate such attacks.

   Session keys are independently generated using fresh nonces for each
   session, and therefore the sessions are independent.



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


<span class="h4"><a class="selflink" id="section-4.3.11" href="#section-4.3.11">4.3.11</a>.  Fragmentation</span>

   Fragmentation and reassembly is supported through the fragmentation
   flag in the header.

<span class="h4"><a class="selflink" id="section-4.3.12" href="#section-4.3.12">4.3.12</a>.  Channel Binding</span>

   EAP-PAX can be extended to support channel bindings through the use
   of its subADE fields.

<span class="h4"><a class="selflink" id="section-4.3.13" href="#section-4.3.13">4.3.13</a>.  Cryptographic Binding</span>

   EAP-PAX does not include any cryptographic binding.  This is relevant
   only for tunneled methods.

<span class="h4"><a class="selflink" id="section-4.3.14" href="#section-4.3.14">4.3.14</a>.  Negotiation Attack Prevention</span>

   EAP is susceptible to an attack where an attacker uses NAKs to
   convince an EAP client and server to use a less secure method, and
   can be prevented using method-specific integrity protection on NAK
   messages.  Since EAP-PAX does not have suitable keys derived for this
   integrity protection at the beginning of a PAX conversation, this is
   not included.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  IANA Considerations</span>

   This document requires IANA to maintain the namespace for the
   following header fields: MAC ID, DH Group ID, Public Key ID, and ADE
   type.  The initial namespace populations are as follows.

   MAC ID Namespace:

   o  0x01 : HMAC_SHA1_128
   o  0x02 : HMAC_SHA256_128

   DH Group ID Namespace:

   o  0x00 : NONE
   o  0x01 : IANA DH Group 14
   o  0x02 : IANA DH Group 15
   o  0x03 : NIST ECC Group P-256










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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   Public Key ID Namespace:

   o  0x00 : NONE
   o  0x01 : RSAES-OAEP
   o  0x02 : RSA-PKCS1-V1_5
   o  0x03 : El-Gamal Over NIST ECC Group P-256

   ADE Type Namespace:

   o  0x01 : Vendor Specific
   o  0x02 : Client Channel Binding Data
   o  0x03 : Server Channel Binding Data

   Allocation of values for these namespaces shall be reviewed by a
   Designated Expert appointed by the IESG.  The Designated Expert will
   post a request to the EAP WG mailing list (or a successor designated
   by the Designated Expert) for comment and review, including an
   Internet-Draft.  Before a period of 30 days has passed, the
   Designated Expert will either approve or deny the registration
   request and publish a notice of the decision to the EAP WG mailing
   list or its successor, as well as informing IANA.  A denial notice
   must be justified by an explanation and, in the cases where it is
   possible, concrete suggestions on how the request can be modified so
   as to become acceptable.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Acknowledgments</span>

   The authors would like to thank Jonathan Katz for discussion with
   respect to provable security, Bernard Aboba for technical guidance,
   Jari Arkko for his expert review, and Florent Bersani for feedback
   and suggestions.  Finally, the authors would like to thank the
   Defense Information Systems Agency for initially funding this work.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  References</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Normative References</span>

   [<a id="ref-FIPS180">FIPS180</a>]    National Institute for Standards and Technology, "Secure
                Hash Standard", Federal Information Processing Standard
                180-2, August 2002.

   [<a id="ref-FIPS186">FIPS186</a>]    National Institute for Standards and Technology,
                "Digital Signature Standard (DSS)", Federal Information
                Processing Standard 186, May 1994.

   [<a id="ref-FIPS198">FIPS198</a>]    National Institute for Standards and Technology, "The
                Keyed-Hash Message Authentication Code (HMAC)", Federal
                Information Processing Standard 198, March 2002.



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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   [<a id="ref-RFC2119">RFC2119</a>]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC3232">RFC3232</a>]    Reynolds, J., "Assigned Numbers: <a href="./rfc1700">RFC 1700</a> is Replaced by
                an On-line Database", <a href="./rfc3232">RFC 3232</a>, January 2002.

   [<a id="ref-RFC3280">RFC3280</a>]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
                X.509 Public Key Infrastructure Certificate and
                Certificate Revocation List (CRL) Profile", <a href="./rfc3280">RFC 3280</a>,
                April 2002.

   [<a id="ref-RFC3447">RFC3447</a>]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                Standards (PKCS) #1: RSA Cryptography Specifications
                Version 2.1", <a href="./rfc3447">RFC 3447</a>, February 2003.

   [<a id="ref-RFC3526">RFC3526</a>]    Kivinen, T. and M. Kojo, "More Modular Exponential
                (MODP) Diffie-Hellman groups for Internet Key Exchange
                (IKE)", <a href="./rfc3526">RFC 3526</a>, May 2003.

   [<a id="ref-RFC3748">RFC3748</a>]    Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                H. Levkowetz, "Extensible Authentication Protocol
                (EAP)", <a href="./rfc3748">RFC 3748</a>, June 2004.

   [<a id="ref-RFC4282">RFC4282</a>]    Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                Network Access Identifier", <a href="./rfc4282">RFC 4282</a>, December 2005.

   [<a id="ref-RFC4334">RFC4334</a>]    Housley, R. and T. Moore, "Certificate Extensions and
                Attributes Supporting Authentication in Point-to-Point
                Protocol (PPP) and Wireless Local Area Networks (WLAN)",
                <a href="./rfc4334">RFC 4334</a>, February 2006.

   [<a id="ref-X.690">X.690</a>]      International Telecommunications Union, "Information
                technology - ASN.1 encoding rules: Specification of
                Basic Encoding Rules (BER), Canonical Encoding Rules
                (CER) and Distinguished Encoding Rules (DER)", Data
                Networks and Open System Communication Recommendation
                X.690, July 2002.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Informative References</span>

   [<a id="ref-IETF.KEY">IETF.KEY</a>]   Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
                Levkowetz, "Extensible Authentication Protocol (EAP) Key
                Management Framework", Work in Progress.








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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   [<a id="ref-IEEE.80211">IEEE.80211</a>] Institute of Electrical and Electronics Engineers,
                "Information technology - Telecommunications and
                information exchange between systems - Local and
                metropolitan area networks - Specific Requirements Part
                11:  Wireless LAN Medium Access Control (MAC) and
                Physical Layer (PHY) Specifications", IEEE Standard
                802.11-1997, 1997.

   [<a id="ref-RFC2631">RFC2631</a>]    Rescorla, E., "Diffie-Hellman Key Agreement Method", <a href="./rfc2631">RFC</a>
                <a href="./rfc2631">2631</a>, June 1999.

   [<a id="ref-RFC3766">RFC3766</a>]    Orman, H. and P. Hoffman, "Determining Strengths For
                Public Keys Used For Exchanging Symmetric Keys", <a href="https://www.rfc-editor.org/bcp/bcp86">BCP 86</a>,
                <a href="./rfc3766">RFC 3766</a>, April 2004.

   [<a id="ref-RFC4017">RFC4017</a>]    Stanley, D., Walker, J., and B. Aboba, "Extensible
                Authentication Protocol (EAP) Method Requirements for
                Wireless LANs", <a href="./rfc4017">RFC 4017</a>, March 2005.

   [<a id="ref-RFC4252">RFC4252</a>]    Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
                Authentication Protocol", <a href="./rfc4252">RFC 4252</a>, January 2006.






























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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Key Generation from Passwords</span>

   If a 128-bit key is not available to bootstrap the authentication
   process, then one must be generated from some sort of weak preshared
   key.  Note that the security of the hashing process is unimportant,
   as long as it does not significantly decrease the password's entropy.
   Resistance to dictionary attacks is provided by PAX_SEC.
   Consequently, computing the SHA-1 of the password and truncating the
   output to 128 bits is RECOMMENDED as a means of converting a weak
   password to a key for provisioning.

   When using other preshared credentials, such as a Kerberos Data
   Encryption Standard (DES) key, or an MD4-hashed Microsoft Challenge
   Handshake Authentication Protocol (MSCHAP) password, to provision
   clients, these keys SHOULD still be put through SHA-1 before being
   used.  This serves to protect the credentials from possible
   compromise, and also keeps things uniform.  As an example, consider
   provisioning using an existing Kerberos credential.  The initial key
   computation could be SHA1_128(string2key(password)).  The KDC,
   storing string2key(password), would also be able to compute this
   initial key value.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Implementation Suggestions</span>

   In this section, two implementation strategies are discussed.  The
   first describes how best to implement and deploy EAP-PAX in an
   enterprise network for IEEE 802.11i authentication.  The second
   describes how to use EAP-PAX for device authentication in a 3G-style
   mobile phone network.

<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  WiFi Enterprise Network</span>

   For the purposes of this section, a wireless enterprise network is
   defined to have the following characteristics:

   o  Users wish to obtain network access through IEEE 802.11 access
      points.

   o  Users can possibly have multiple devices (laptops, PDAs, etc.)
      they wish to authenticate.

   o  A preexisting authentication framework already exists, for
      example, a Microsoft Active Directory domain or a Kerberos realm.

   Two of the biggest challenges in an enterprise WiFi network is key
   provisioning and support for multiple devices.  Consequently, it is
   recommended that the client's Network Access Identifier (NAI) have




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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


   the format username/KID@realm, where KID is a key ID that can be used
   to distinguish between different devices.

   The client's supplicant can use a variety of sources to automatically
   generate the KID.  Two of the better choices would likely be the
   computer's NETBIOS name, or local Ethernet adapter's MAC address.
   The wireless adapter's address may be a suboptimal choice, as the
   user may only have one PCCARD adapter for multiple systems.

   With an authentication system already in place, there is a natural
   choice for the provisioned key.  Clients can authenticate using their
   preexisting password.  When the server is presented with a new KID,
   it can create a new key record on the server and use the user's
   current password as the provisioned key.  For example, for Active
   Directory, the supplicant could use Microsoft's NtPasswordHash
   function to generate a key verifiable by the server.  It is suggested
   that this key then be fed through SHA1_128 before being used in a
   non-Microsoft authentication protocol.

   After a key update, the server should keep track of both the old and
   new authentication keys.  When two keys exist, the server should
   attempt to use both to validate the MACs on transmitted packets.
   Once a client successfully authenticates using the new key, the
   server should discard the old key.  This prevents desynchronization
   attacks.

<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  Mobile Phone Network</span>

   In a mobile phone system, we no longer need to worry about supporting
   multiple keys per identity.  Presumably, each mobile device has a
   unique identity.  However, if multiple devices per identity are
   desired, a method similar to that presented in Section B.1 could be
   used.

   Provisioning could easily be accomplished by issuing customers a 6-
   digit PIN they could type into their phone's keypad.















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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


Authors' Addresses

   T. Charles Clancy
   DoD Laboratory for Telecommunications Sciences
   8080 Greenmeade Drive
   College Park, MD  20740
   USA

   EMail: clancy@ltsnet.net


   William A. Arbaugh
   University of Maryland
   Department of Computer Science
   College Park, MD  20742
   USA

   EMail: waa@cs.umd.edu

































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<span class="grey"><a href="./rfc4746">RFC 4746</a>                        EAP-PAX                    November 2006</span>


Full Copyright Statement

   Copyright (C) The IETF Trust (2006).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






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</pre>