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Versions: 00                                                            
TLS Working Group                                                 Y. Nir
Internet-Draft                                                Y. Sheffer
Intended status: Standards Track                             Check Point
Expires: August 25, 2007                                   H. Tschofenig
                                                       February 21, 2007

             Protocol Model for TLS with EAP Authentication

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   This document describes an extension to the TLS protocol to allow TLS
   clients to authenticate with legacy credentials using the Extensible
   Authentication Protocol (EAP).

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

   When TLS is used with EAP, additional records are sent after the
   ChangeCipherSpec protocol 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 Diameter.

   TLS with EAP may be used for securing a data connection such as HTTP
   or POP3, where the ability of EAP to work with backend servers can
   remove that burden from the application layer.

   This document is a protocol model, rather than a full protocol

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  5
   2.  Operating Environment  . . . . . . . . . . . . . . . . . . . .  6
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  The tee_supported Extension  . . . . . . . . . . . . . . .  8
     3.2.  The InterimAuth Handshake Message  . . . . . . . . . . . .  8
     3.3.  The EapMsg Handshake Message . . . . . . . . . . . . . . .  8
     3.4.  Calculating the Finished message . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
     4.1.  InterimAuth vs. Finished . . . . . . . . . . . . . . . . . 10
     4.2.  Identity Protection  . . . . . . . . . . . . . . . . . . . 10
   5.  Performance Considerations . . . . . . . . . . . . . . . . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18

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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 using a
   certificate, or alternatively 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).  The document is a protocol model as
   described in [RFC4101].

   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 a "supplicant").  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 [IKEv2].  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 defers from that of IKEv2.

   Currently used applications 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 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-in to 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 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.  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,
   taking care of all encryption and authentication.

        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.  The server is configured with the
   address and protocol of the AAA server.  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

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

   The diagram below outlines the protocol structure.  For illustration
   purposes only, we use the [I-D.dpotter-pppext-eap-mschap] EAP method

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

         ClientHello(*)             -------->
                                     <--------       ServerHelloDone
         EapMsg(Identity-Reply)     -------->
         EapMsg(MS-CHAP-v2-Reply)   -------->
                                    <--------               Finished
         Finished                   -------->

       (*) 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.

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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 InterimAuth Handshake Message

   The InterimAuth 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.

   The semantics, however, are somewhat different.  The "Finished"
   message indicates that application data may now be sent.  The
   "InterimAuth" message does not indicate this.  Instead, further
   handshake messages are needed.

   Depending on the EAP method used, the Finished message may be
   calculated differently.  See Section 3.4 for details.

   The HandshakeType value for the InterimAuth 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 the co-located with the TLS
   server, or 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.

3.4.  Calculating the Finished message

   If the EAP method is key-generating, the Finished message is
   calculated as follows:

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         struct {
             opaque verify_data[12];
         } Finished;

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

   The finished_label is defined exactly as in section 7.4.9 of [TLS].

   The handshake_messages, similar to regular TLS is all of the data
   from all messages in this handshake, including any EapMsg and
   InterimAuth messages, up to but not including this Finished message.
   This is the concatenation of all the Handshake structures, as defined
   in section 7.4 of [TLS] and here, exchanged thus far.

   The MSK is typically received from the AAA server over the RADIUS or
   Diameter protocol.

   If the EAP method is not key-generating, then the Finished message is
   calculated exactly as described in [TLS].  Such methods however, are
   NOT RECOMMENDED.  See Section 4.1 for details.

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4.  Security Considerations

4.1.  InterimAuth vs. Finished

   In regular TLS, the Finished message provides two functions: it signs
   all previous messages, and it signals that application data can now
   be used.  In TEE, we sign the previous messages twice.

   Some EAP methods, such as EAP-TLS, EAP-IKEv2 and EAP-SIM generate
   keys in addition to authenticating clients.  Such methods are said to
   be resistant to MITM attacks as discussed in [MITM].  Such methods
   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 InterimAuth message signs all previous
   messages with the master_secret, just like the Finished message in
   regular TLS.  The Finished message signs all previous messages 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 MITM method is 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.

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, and it is not sent to the server until after the server's
   identity has been authenticated by verifying the certificate.

   Active attacks are thwarted by the server authentication using a
   certificate or by using a suitable EAP method.

   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,

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   so as to justify the extra round-trip.

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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-SIM 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.  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 "InterimAuth".

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7.  Acknowledgments

   The TLS Innel Application Extension work ([TLS/IA]) 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 thanks the various contributors to
   [IKEv2] whose work inspired this one.

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8.  References

8.1.  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 1.1", RFC 4346, April 2006.

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

8.2.  Informative References

   [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.

              Potter, D. and J. Zamick, "PPP EAP MS-CHAP-V2
              Authentication Protocol",
              draft-dpotter-pppext-eap-mschap-01 (work in progress),
              January 2002.

   [IKEv2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [MITM]     Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunneled Authentication Protocols", 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,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

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   [RFC4101]  Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
              June 2005.

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

   [TLS/IA]   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.

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Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   3A Jabotinsky St.
   Ramat Gan  52520

   Email: ynir@checkpoint.com

   Yaron Sheffer
   Check Point Software Technologies Ltd.
   3A Jabotinsky St.
   Ramat Gan  52520

   Email: yaronf at checkpoint dot com

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739

   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com

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