document fixes.

[gnutls.git] / doc / protocol / draft-nir-tls-eap-01.txt

TLS Working Group                                                 Y. Nir
Internet-Draft                                                Y. Sheffer
Intended status: Standards Track                             Check Point
Expires: January 5, 2008                                   H. Tschofenig
                                                                     NSN
                                                              P. Gutmann
                                                  University of Auckland
                                                            July 4, 2007


                      TLS using EAP Authentication
                        draft-nir-tls-eap-01.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).










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Abstract

   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 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
   Diameter.

   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.
























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  EAP Applicability  . . . . . . . . . . . . . . . . . . . .  5
     1.2.  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 . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
     4.1.  InterimAuth vs. Finished . . . . . . . . . . . . . . . . . 10
     4.2.  Identity Protection  . . . . . . . . . . . . . . . . . . . 10
     4.3.  Mutual Authentication  . . . . . . . . . . . . . . . . . . 11
   5.  Performance Considerations . . . . . . . . . . . . . . . . . . 12
   6.  Operational Considerations . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Changes from Previous Versions . . . . . . . . . . . . . . . . 16
     9.1.  Changes in version -01 . . . . . . . . . . . . . . . . . . 16
     9.2.  Changes from the protocol model draft  . . . . . . . . . . 16
   10. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21























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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 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 [RFC4306].  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 IKEv2.

   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
   authentication.

1.2.  Conventions Used in This Document

   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 [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.  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 communicating application (either client or server) 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 GPSK EAP method [EAP-GPSK].

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

         ClientHello(*)             -------->
                                                      ServerHello(*)
                                                       (Certificate)
                                                   ServerKeyExchange
                                            EapMsg(Identity-Request)
                                     <--------       ServerHelloDone
         ClientKeyExchange
         (CertificateVerify)
         ChangeCipherSpec
         InterimAuth
         EapMsg(Identity-Reply)     -------->
                                                    ChangeCipherSpec
                                                         InterimAuth
                                                EapMsg(GPSK-Request)
                                    <--------
         EapMsg(GPSK-Reply)         -------->
                                                EapMsg(GPSK-Request)
                                    <--------
         EapMsg(GPSK-Reply)         -------->
                                                     EapMsg(Success)
                                    <--------               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



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   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 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.

   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
   IANA.

   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
   failure.









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         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 Finished message

   If the EAP method is key-generating (see [I-D.ietf-eap-keying]), the
   Finished message is calculated as follows:

         struct {
             opaque verify_data[12];
         } Finished;

         verify_data
             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
   [TLS].

   The handshake_messages field, unlike regular TLS, does not sign all
   the data in the handshake.  Instead it signs all the data that has
   not been signed by the previous InterimAuth message.  The
   handshake_messages field includes all of the octets beginning with
   and including the InterimAuth message, up to but not including this
   Finished 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
   method.

   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.  InterimAuth 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 those messages that have not yet been
   signed.

   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 man-in-the-middle (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 the rest of the 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 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 GPSK.  In this case, server certificate verification does not
       matter, and the TLS handshake may as well be anonymous.  Note
       that in this case, the client identity is sent to the server
       before server authentication.

   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
      anonymous.
   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
   delay.

   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
   requires.

   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
   message.























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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
   methods.

   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 "InterimAuth".












































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8.  Acknowledgments

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

   The TLS Inner 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 thank the various contributors to
   [RFC4306] whose work inspired this one.







































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9.  Changes from Previous Versions

9.1.  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

9.2.  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.  Open Issues

   Some have suggested that since the protocol is identical to regular
   TLS up to the InterimAuth message, we should call that the Finished
   message, and call the last message in the extended handshake
   something like "EapFinished".  This has the advantage that the
   construction of Finished is already well defined and will not change.
   However, the Finished message has a specific meaning as indicated by
   its name.  It means that the handshake is over and that application
   data can now be sent.  This is not true of what is in this draft
   called InterimAuth.  We'd like the opinions of reviewrs about this
   issue.

   The MSK from the EAP exchange is only used to sign the Finished
   message.  It is not used again in the data encryption.  In this we
   followed the example of IKEv2.  The reason is that TLS already has
   perfectly good ways of exchanging keys, and we do not need this
   capability from EAP methods.  Also, using the MSK in keys would
   require an additional ChangeCipherSpec and would complicate the
   protocol.  We'd like the opinions of reviewrs about this issue.

   Another response we got was that we should have a MUST requirement
   that only mutually authenticated and key-generating methods be used
   in TEE.  This would simplify the security considerations section.
   While we agree that this is a good idea, most EAP methods in common
   use are not compliant.  Additionally, such requirements assume that
   EAP packets are visible to a passive attacker.  As EAP is used in
   protected tunnels such as in L2TP, in IKEv2 and here, this assumption
   may not be required.  If we consider the server authenticated by its
   certificate, it may be acceptable to use a non-MA method.

   It has been suggested that identity protection is not important
   enough to add a roundtrip, and so we should have the client send the
   username in the ClientHello.  We are not sure about how others feel
   about this, and would like to solicit the reviewers opinion.  Note
   that if this is done, the client sends the user name before ever
   receiving any indication that the server actually supports TEE.  This
   might be acceptable in an email client, where the server is
   preconfigured, but it may be unacceptable in other uses, such as web
   browsers.











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11.  References

11.1.  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.

11.2.  Informative References

   [Compound-Authentication]
              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.

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

   [EAP-GPSK]
              Clancy, T. and H. Tschofenig, "EAP Generalized Pre-Shared
              Key (EAP-GPSK)", draft-ietf-emu-eap-gpsk-05 (work in
              progress), April 2007.

   [I-D.ietf-eap-keying]
              Aboba, B., "Extensible Authentication Protocol (EAP) Key
              Management Framework", draft-ietf-eap-keying-18 (work in
              progress), February 2007.

   [I.D.Webauth-phishing]
              Hartman, S., "Requirements for Web Authentication
              Resistant to Phishing", draft-hartman-webauth-phishing-03
              (work in progress), March 2007.



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

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 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.
   5 Hasolelim st.
   Tel Aviv  67897
   Israel

   Email: ynir@checkpoint.com


   Yaron Sheffer
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  67897
   Israel

   Email: yaronf at checkpoint dot com


   Hannes Tschofenig
   Nokia Siemens Networks
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

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


   Peter Gutmann
   University of Auckland
   Department of Computer Science
   New Zealand

   Email: pgut001@cs.auckland.ac.nz















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Full Copyright Statement

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