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TLS Working Group                                                 Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                              Y. Sheffer
Expires: June 21, 2012                                       Independent
                                                           H. Tschofenig
                                                              P. Gutmann
                                                  University of Auckland
                                                       December 19, 2011

 A Flexible Authentication Framework for the  Transport Layer Security
   (TLS) Protocol using the Extensible Authentication Protocol (EAP)


   Many of today's Web security problems have their root in the
   widespread usage of weak authentication mechanisms bundled with the
   usage of password based credentials.  Dealing with both of these
   problems is the basis of this publication.

   This document extends the Transport Layer Security (TLS) protocol
   with a flexible and widely deployed authentication framework, namely
   the Extensible Authentication Protocol (EAP), to improve security of
   Web- as well as non-Web-based applications.  The EAP framework allows
   so-called EAP methods, i.e. authentication and key exchange
   protocols, to be plugged into EAP without having to re-design the
   underlying protocol.  The benefit of such an easy integration is the
   ability to run authentication protocols that fit a specific
   deployment environment, both from a credential choice as well as from
   the security and performance characteristics of the actual protocol.

   This work follows the example of IKEv2, where EAP has been added to
   allow clients to seamlessly use different forms of authentication
   credentials, such as passwords, token cards, and shared secrets.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months

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   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."

   This Internet-Draft will expire on June 21, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  EAP Applicability  . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Comparison with Design Alternatives  . . . . . . . . . . .  5
     1.3.  Conventions Used in This Document  . . . . . . . . . . . .  5
   2.  Operating Environment  . . . . . . . . . . . . . . . . . . . .  6
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  The tee_supported Extension  . . . . . . . . . . . . . . .  8
     3.2.  The EapFinished Handshake Message  . . . . . . . . . . . .  8
     3.3.  The EapMsg Handshake Message . . . . . . . . . . . . . . .  9
     3.4.  Calculating the EapFinished message  . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     4.1.  EapFinished vs. Finished . . . . . . . . . . . . . . . . . 11
     4.2.  Identity Protection  . . . . . . . . . . . . . . . . . . . 11
     4.3.  Mutual Authentication  . . . . . . . . . . . . . . . . . . 12
   5.  Performance Considerations . . . . . . . . . . . . . . . . . . 13
   6.  Operational Considerations . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  Changes from Previous Versions . . . . . . . . . . . . . . . . 17
     9.1.  Changes in version -02 . . . . . . . . . . . . . . . . . . 17
     9.2.  Changes in version -01 . . . . . . . . . . . . . . . . . . 17
     9.3.  Changes from the protocol model draft  . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   This document describes a new extension to [TLS].  This extension
   allows a TLS client to authenticate using [EAP] instead of performing
   the authentication at the application level.  The extension follows
   [TLS-EXT].  For the remainder of this document we will refer to this
   extension as TEE (TLS with EAP Extension).

   TEE extends the TLS handshake beyond the regular setup, to allow the
   EAP protocol to run between the TLS server (called an "authenticator"
   in EAP) and the TLS client (called either a "supplicant" or a
   "peer").  This allows the TLS architecture to handle client
   authentication before exposing the server application software to an
   unauthenticated client.  In doing this, we follow the approach taken
   for IKEv2 in [RFC5996].  However, similar to regular TLS, we protect
   the user identity by only sending the client identity after the
   server has authenticated.  In this our solution differs from that of

   Currently used applications that rely on non-certificate user
   credentials use TLS to authenticate the server only.  After that, the
   application takes over, and presents a login screen where the user is
   expected to present their credentials.

   This creates several problems.  It allows a client to access the
   application before authentication, thus creating a potential for
   anonymous attacks on non-hardened applications.  Additionally, web
   pages are not particularly well suited for long shared secrets and
   for interfacing with certain devices such as USB tokens.

   TEE allows full mutual authentication to occur for all these
   applications within the TLS exchange.  The application receives
   control only when the user is identified and authenticated.  The
   authentication can be built into the server infrastructure by
   connecting to an AAA server.  The client side can be integrated into
   client software such as web browsers and mail clients.  An EAP
   infrastructure is already built into some operating systems providing
   a user interface for each authentication method within EAP.

   We intend TEE to be used for various protocols that use TLS such as
   HTTPS, in cases where certificate based client authentication is not
   practical.  This includes web-based mail services, online banking,
   premium content websites and mail clients.

   Another class of applications that may see benefit from TEE are TLS
   based VPN clients used as part of so-called "SSL VPN" products.  No
   such client protocols have so far been standardized.

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1.1.  EAP Applicability

   Section 1.3 of [EAP] states that EAP is only applicable for network
   access authentication, rather than for "bulk data transfer".  It then
   goes on to explain why the transport properties of EAP indeed make it
   unsuitable for bulk data transfer, e.g. for large file transport.
   Our proposed use of EAP falls squarely within the applicability as
   defined, since we make no further use of EAP beyond access

1.2.  Comparison with Design Alternatives

   It has been suggested to implement EAP authentication as part of the
   protected application, rather than as part of the TLS handshake.  A
   BCP document could be used to describe a secure way of doing this.
   The drawbacks we see in such an approach are listed below:
   o  EAP does not have a pre-defined transport method.  Application
      designers would need to specify an EAP transport for each
      application.  Making this a part of TLS has the benefit of a
      single specification for all protected applications.
   o  The integration of EAP and TLS is security-sensitive and should be
      standardized and interoperable.  We do not believe that it should
      be left to application designers to do this in a secure manner.
      Specifically on the server-side, integration with AAA servers adds
      complexity and is more naturally part of the underlying
   o  Our current proposal provides channel binding between TLS and EAP,
      to counter the MITM attacks described in [MITM].  TLS does not
      provide any standard way of extracting cryptographic material from
      the TLS state, and in most implementations, the TLS state is not
      exposed to the protected application.  Because of this, it is
      difficult for application designers to bind the user
      authentication to the protected channel provided by TLS.

1.3.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Operating Environment

   TEE will work between a client application and a server application,
   performing either client authentication or mutual authentication
   within the TLS exchange.

        Client                         Server
    +-------------------------+     +------------------------+
    |  |GUI| | Client | |TLS+-+-----+-+TLS|   |Server      | |
    |  +-^-+ |Software| +-^-+ |     +-+-^-+   |Application | |
    |    |   +--------+   |   |     |   |     |Software    | |
    |    |                |   |     |   |     +------------+ |
    |  +-v----------------v-+ |     |   |                    |
    |  |   EAP              | |     +---|--------------------+
    |  |  Infrastructure    | |         |
    |  +--------------------+ |         |    +--------+
    +-------------------------+         |    | AAA    |
                                        |    | Server |
                                        +-----        |

   The above diagram shows the typical deployment.  The client has
   software that either includes a UI for some EAP methods, or else is
   able to invoke some operating system EAP infrastructure that takes
   care of the user interaction.  Although the technical mechanisms have
   been standardized for a AAA client to dynamically discover a AAA
   server often the address of the AAA server is statically configured.
   Typically the AAA server communicates using the RADIUS protocol with
   EAP ([RADIUS] and [RAD-EAP]), or the Diameter protocol ([Diameter]
   and [Dia-EAP]).

   As stated in the introduction, we expect TEE to be used in both
   browsers and applications.  Further uses may be authentication and
   key generation for other protocols, and tunneling clients, which so
   far have not been standardized.

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3.  Protocol Overview

   When TLS is used with EAP, additional records are sent after the
   ChangeCipherSpec protocol message and before the Finished message,
   effectively creating an extended handshake before the application
   layer data can be sent.  Each EapMsg handshake record contains
   exactly one EAP message.  Using EAP for client authentication allows
   TLS to be used with various AAA back-end servers such as RADIUS or

   TLS with EAP may be used for securing a data connection such as HTTP
   or POP3.  We believe it has three main benefits:
   o  The ability of EAP to work with backend servers can remove that
      burden from the application layer.
   o  Moving the user authentication into the TLS handshake protects the
      presumably less secure application layer from attacks by
      unauthenticated parties.
   o  Using mutual authentication methods within EAP can help thwart
      certain classes of phishing attacks.

   The TEE extension defines the following:
   o  A new extension type called tee_supported, used to indicate that
      the communicating application (either client or server) supports
      this extension.
   o  A new message type for the handshake protocol, called EapMsg,
      which is used to carry a single EAP message.
   o  A new message type for the handshake protocol, called EapFinished,
      which is used to sign previous messages.

   The diagram below outlines the protocol structure.  For illustration
   purposes only, we use EAP Generalized Pre-Shared Key (EAP-GPSK)
   method [RFC5433].  This method is a lightweight shared-key
   authentication protocol supporting mutual authentication and key

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         Client                                               Server
         ------                                               ------

         ClientHello(*)             -------->
                                     <--------       ServerHelloDone
         EapMsg(Identity-Reply)     -------->
         EapMsg(GPSK-Reply)         -------->
         EapMsg(GPSK-Reply)         -------->
                                    <--------            EaoFinished
         EapFinished                -------->

       (*) The ClientHello and ServerHello include the tee_supported
           extension to indicate support for TEE

   The client indicates in the first message its support for TEE.  The
   server sends an EAP identity request in the reply.  The client sends
   the identity reply after the handshake completion.  The EAP request-
   response sequence continues until the client is either authenticated
   or rejected.

3.1.  The tee_supported Extension

   The tee_supported extension is a ClientHello and ServerHello
   extension as defined in section 2.3 of [TLS-EXT].  The extension_type
   field is TBA by IANA.  The extension_data is zero-length.

3.2.  The EapFinished Handshake Message

   The EapFinished message is identical in syntax to the Finished
   message described in section 7.4.9 of [TLS].  It is calculated in
   exactly the same way.

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   When TEE is used, application data cannot follow the "Finished"
   message.  Instead, it may only begin after the EapFinished message

   The HandshakeType value for the EapFinished handshake message is TBA
   by IANA.

3.3.  The EapMsg Handshake Message

   The EapMsg handshake message carries exactly one EAP message as
   defined in [EAP].

   The HandshakeType value for the EapMsg handshake message is TBA by

   The EapMsg message is used to tunnel EAP messages between the
   authentication server, which may be co-located with the TLS server,
   or else may be a separate AAA server, and the supplicant, which is
   co-located with the TLS client.  TLS on either side receives the EAP
   data from the EAP infrastructure, and treats it as opaque.  TLS does
   not make any changes to the EAP payload or make any decisions based
   on the contents of an EapMsg handshake message.

   Note that it is expected that the authentication server notifies the
   TLS server about authentication success or failure, and so TLS need
   not inspect the eap_payload within the EapMsg to detect success or

         struct {
             opaque eap_payload[4..65535];
         } EapMsg;

   eap_payload is defined in section 4 of RFC 3748.  It includes the
   Code, Identifier, Length and Data fields of the EAP packet.

3.4.  Calculating the EapFinished message

   If the EAP method is key-generating (see [RFC5247]), the Finished
   message is calculated as follows:

         struct {
             opaque verify_data[12];
         } Finished;

             PRF(MSK, finished_label, MD5(handshake_messages) +
             SHA-1(handshake_messages)) [0..11];

   The finished_label and the PRF are as defined in section 7.4.9 of

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   The handshake_messages field does not include all the data in the
   handshake.  Instead it includes only the data that has not been
   signed by the previous Finished message.  The handshake_messages
   field includes all of the octets beginning with and including the
   Finished message, up to but not including this EapFinished message.
   This is the concatenation of all the Handshake structures exchanged
   thus far, and not yet signed, as defined in section 7.4 of [TLS]and
   in this document.

   The Master Session Key (MSK) is derived by the AAA server and by the
   client if the EAP method is key-generating.  On the server-side, it
   is typically received from the AAA server over the RADIUS or Diameter
   protocol.  On the client-side, it is passed to TLS by some other

   If the EAP method is not key-generating, then the master_secret is
   used to sign the messages instead of the MSK.  For a discussion on
   the use of such methods, see Section 4.1.

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4.  Security Considerations

4.1.  EapFinished vs. Finished

   In regular TLS, the Finished message provides two functions: it signs
   all preceding messages, and it signals that application data can now
   be sent.  In TEE, it only signs, and the signal is given by

   Many EAP methods, such as EAP-TLS, EAP-IKEv2 and EAP-SIM, generate
   keys in addition to authenticating clients.  Such methods are
   resistant to man-in-the-middle (MITM) attacks, as discussed in [MITM]
   and are called key-generating methods.

   To realize the benefit of such methods, we need to verify the key
   that was generated within the EAP method.  This is referred to as the
   MSK in EAP.  In TEE, the EapFinished message signs the messages that
   have not yet been signed by the Finished message using the MSK if
   such exists.  If not, then the messages are signed with the
   master_secret as in regular TLS.

   The need for signing twice arises from the fact that we need to use
   both the master_secret and the MSK.  It was possible to use just one
   Finished record and blend the MSK into the master_secret.  However,
   this would needlessly complicate the protocol and make security
   analysis more difficult.  Instead, we have decided to follow the
   example of IKEv2, where two AUTH payloads are exchanged.

   It should be noted that using non-key-generating methods may expose
   the client to a MITM attack if the same method and credentials are
   used in some other situation, in which the EAP is done outside of a
   protected tunnel with an authenticated server.  Unless it can be
   determined that the EAP method is never used in such a situation,
   non-key-generating methods SHOULD NOT be used.  This issue is
   discussed extensively in [Compound-Authentication].

4.2.  Identity Protection

   Unlike [TLS-PSK], TEE provides identity protection for the client.
   The client's identity is hidden from a passive eavesdropper using TLS
   encryption.  Active attacks are discussed in Section 4.3.

   We could save one round-trip by having the client send its identity
   within the Client Hello message.  This is similar to TLS-PSK.
   However, we believe that identity protection is a worthy enough goal,
   so as to justify the extra round-trip.

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4.3.  Mutual Authentication

   In order to achieve our security goals, we need to have both the
   server and the client authenticate.  Client authentication is
   obviously done using the EAP method.  The server authentication can
   be done in either of two ways:
   1.  The client can verify the server certificate.  This may work well
       depending on the scenario, but implies that the client or its
       user can recognize the right DN or alternate name, and
       distinguish it from plausible alternatives.  The introduction to
       [I.D.Webauth-phishing] shows that at least in HTTPS, this is not
       always the case.
   2.  The client can use a mutually authenticated (MA) EAP method, such
       as EAP-GPSK.  The client would be authenticated to the AAA server
       and vice versa.  Additionally authenticating the application
       server to the client is not necessary and the TLS handshake may
       as well be anonymous.  Note that the authenticated client
       identity may be sent from the AAA server to the application
       server (acting as a AAA client) if the authenticated identity
       indeed matters for the purpose of the service fulfillment and
       with the user's permission.

   To summarize:
   o  Clients MUST NOT propose anonymous ciphersuites, unless they
      support MA EAP methods.
   o  Clients MUST NOT accept non-MA methods if the ciphersuite is
   o  Clients MUST NOT accept non-MA methods if they are not able to
      verify the server credentials.  Note that this document does not
      define what verification involves.  If the server DN is known and
      stored on the client, verifying certificate signature and checking
      revocation may be enough.  For web browsers, the case is not as
      clear cut, and MA methods SHOULD be used.

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5.  Performance Considerations

   Regular TLS adds two round-trips to a TCP connection.  However,
   because of the stream nature of TCP, the client does not really need
   to wait for the server's Finished message, and can begin sending
   application data immediately after its own Finished message.  In
   practice, many clients do so, and TLS only adds one round-trip of

   TEE adds as many round-trips as the EAP method requires.  For
   example, EAP-MD5 requires 1 round-trip, while EAP-GPSK requires 2
   round-trips.  Additionally, the client MUST wait for the EAP-Success
   message before sending its own Finished message, so we need at least
   3 round-trips for the entire handshake.  The best a client can do is
   two round-trips plus however many round-trips the EAP method

   It should be noted, though, that these extra round-trips save
   processing time at the application level.  Two extra round-trips take
   a lot less time than presenting a log-in web page and processing the
   user's input.

   It should also be noted, that TEE reverses the order of the Finished
   messages.  In regular TLS the client sends the Finished message
   first.  In TEE it is the server that sends the Finished message
   first.  This should not affect performance, and it is clear that the
   client may send application data immediately after the Finished

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6.  Operational Considerations

   Section 4.3 defines a dependency between the TLS state and the EAP
   state in that it mandates that certain EAP methods should not be used
   with certain TLS ciphersuites.  To avoid such dependencies, there are
   two approaches that implementations can take.  They can either not
   use any anonymous ciphersuites, or else they can use only MA EAP

   Where certificate validation is problematic, such as in browser-based
   HTTPS, we recommend the latter approach.

   In cases where the use of EAP within TLS is not known before opening
   the connection, it is necessary to consider the implications of
   requiring the user to type in credentials after the connection has
   already started.  TCP sessions may time out, because of security
   considerations, and this may lead to session setup failure.

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7.  IANA Considerations

   IANA is asked to assign an extension type value from the
   "ExtensionType Values" registry for the tee_supported extension.

   IANA is asked to assign two handshake message types from the "TLS
   HandshakeType Registry", one for "EapMsg" and one for "EapFinished".

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8.  Acknowledgments

   The authors would like to thank Josh Howlett for his comments.

   The TLS Inner Application Extension work ([TLSIA]) has inspired the
   authors to create this simplified work.  TLS/IA provides a somewhat
   different approach to integrating non-certificate credentials into
   the TLS protocol, in addition to several other features available
   from the RADIUS namespace.

   The authors would also like to thank the various contributors to
   [RFC5996] whose work inspired this one.

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9.  Changes from Previous Versions

9.1.  Changes in version -02

   o  Added discussion of alternative designs.

9.2.  Changes in version -01

   o  Changed the construction of the Finished message
   o  Replaced MS-CHAPv2 with GPSK in examples.
   o  Added open issues section.
   o  Added reference to [Compound-Authentication]
   o  Fixed reference to MITM attack

9.3.  Changes from the protocol model draft

   o  Added diagram for EapMsg
   o  Added discussion of EAP applicability
   o  Added discussion of mutually-authenticated EAP methods vs other
      methods in the security considerations.
   o  Added operational considerations.
   o  Other minor nits.

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10.  References

10.1.  Normative References

   [EAP]      Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

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

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [TLS-EXT]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

10.2.  Informative References

              Puthenkulam, J., Lortz, V., Palekar, A., and D. Simon,
              "The Compound Authentication Binding Problem",
              draft-puthenkulam-eap-binding-04 (work in progress),
              October 2003.

   [Dia-EAP]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.

              Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

              Hartman, S., "Requirements for Web Authentication
              Resistant to Phishing", draft-hartman-webauth-phishing-09
              (work in progress), August 2008.

   [MITM]     Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunneled Authentication Protocols", IACR ePrint
              Archive , October 2002.

   [RAD-EAP]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RADIUS]   Rigney, C., Willens, S., Rubens, A., and W. Simpson,

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              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

   [RFC5433]  Clancy, T. and H. Tschofenig, "EAP Generalized Pre-Shared
              Key (EAP-GPSK)", RFC 5433, February 2009.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol: IKEv2", RFC 5996,
              September 2010.

   [TLS-PSK]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [TLSIA]    Funk, P., Blake-Wilson, S., Smith, H., Tschofenig, N., and
              T. Hardjono, "TLS Inner Application Extension (TLS/IA)",
              draft-funk-tls-inner-application-extension-03 (work in
              progress), June 2006.

Nir, et al.               Expires June 21, 2012                [Page 19]

Internet-Draft        TLS with EAP Extension (TEE)         December 2011

Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  67897

   Email: ynir@checkpoint.com

   Yaron Sheffer

   Email: yaronf.ietf@gmail.com

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   New Zealand

   Email: pgut001@cs.auckland.ac.nz

Nir, et al.               Expires June 21, 2012                [Page 20]

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