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PROPOSED STANDARD

Network Working Group                                       S. Santesson
Request for Comments: 4680                                     Microsoft
Updates: 4346                                             September 2006
Category: Standards Track


              TLS Handshake Message for Supplemental Data

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines a TLS handshake message for exchange of
   supplemental application data.  TLS hello message extensions are used
   to determine which supplemental data types are supported by both the
   TLS client and the TLS server.  Then, the supplemental data handshake
   message is used to exchange the data.  Other documents will define
   the syntax of these extensions and the syntax of the associated
   supplemental data types.






















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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


1.  Introduction

   Recent standards activities have proposed different mechanisms for
   transmitting supplemental application data in the TLS handshake
   message.  For example, recent proposals transfer data that is not
   processed by the TLS protocol itself, but assist the TLS-protected
   application in the authentication and authorization decisions.  One
   proposal transfers user name hints for locating credentials, and
   another proposal transfers attribute certificates and Security
   Assertions Markup Language (SAML) assertions for authorization
   checks.

   In order to avoid definition of multiple handshake messages, one for
   each new type of application-specific supplemental data, this
   specification defines a new handshake message type that bundles
   together all data objects that are to be delivered to the TLS-
   protected application and sends them in a single handshake message.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [N1].

   The syntax for the supplemental_data handshake message is defined
   using the TLS Presentation Language, which is specified in Section 4
   of [N2].

2.  Supplemental Data Handshake Message

   The new supplemental_data handshake message type is defined to
   accommodate communication of supplemental data objects as agreed
   during the exchange of extensions in the client and server hello
   messages.  See RFC 2246 (TLS 1.0) [N2] and RFC 4346 (TLS 1.1) [N3]
   for other handshake message types.

   Information provided in a supplemental data object MUST be intended
   to be used exclusively by applications and protocols above the TLS
   protocol layer.  Any such data MUST NOT need to be processed by the
   TLS protocol.











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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


      enum {
             supplemental_data(23), (255)
         } HandshakeType;

      struct {
             HandshakeType msg_type;    /* handshake type */
             uint24 length;             /* octets in message */
             select (HandshakeType) {
                    case supplemental_data:   SupplementalData;
               } body;
          } Handshake;

      struct {
            SupplementalDataEntry supp_data<1..2^24-1>;
         } SupplementalData;

      struct {
            SupplementalDataType supp_data_type;
            uint16 supp_data_length;
            select(SupplementalDataType) { }
         } SupplementalDataEntry;

      enum {
            (65535)
        } SupplementalDataType;

   supp_data_length
      This field is the length (in bytes) of the data selected by
      SupplementalDataType.

   The client MUST NOT send more than one SupplementalData handshake
   message, and the server MUST NOT send more than one SupplementalData
   handshake message.  Receiving more than one SupplementalData
   handshake message results in a fatal error, and the receiver MUST
   close the connection with a fatal unexpected_message alert.

   If present, the SupplementalData handshake message MUST contain a
   non-empty SupplementalDataEntry structure carrying data associated
   with at least one defined SupplementalDataType.  An explicit
   agreement that governs presence of any supplemental data MUST be
   concluded between client and server for each SupplementalDataType
   using the TLS extensions [N4] in the client and server hello
   messages.  Receiving an unexpected SupplementalData handshake message
   results in a fatal error, and the receiver MUST close the connection
   with a fatal unexpected_message alert.






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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


   Other documents will define specific SupplementalDataTypes and their
   associated data syntax and processing.  These same specifications
   must also specify the client and server hello message extensions that
   are used to negotiate the support for the specified supplemental data
   type.  This document simply specifies the TLS Handshake Protocol
   message that will carry the supplemental data objects.

   Different situations require the transfer of supplemental data from
   the client to the server, require the transfer of supplemental data
   from the server to the client, or both ways.  All three situations
   are fully supported.

3.  Message Flow

   The SupplementalData handshake message, if exchanged, MUST be sent as
   the first handshake message as illustrated in Figure 1 below.

     Client                                               Server

     ClientHello (with extensions) -------->

                                    ServerHello(with extensions)
                                               SupplementalData*
                                                    Certificate*
                                              ServerKeyExchange*
                                             CertificateRequest*
                                  <--------      ServerHelloDone

     SupplementalData*
     Certificate*
     ClientKeyExchange
     CertificateVerify*
     [ChangeCipherSpec]
     Finished                     -------->
                                              [ChangeCipherSpec]
                                  <--------             Finished
     Application Data             <------->     Application Data

       *  Indicates optional or situation-dependent messages.

               Figure 1.  Message Flow with SupplementalData










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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


4.  Security Considerations

   Each SupplementalDataType included in the handshake message defined
   in this specification introduces its own unique set of security
   properties and related considerations.  Security considerations must
   therefore be defined in each document that defines a supplemental
   data type.

   In some cases, the SupplementalData information may be sensitive.
   The double handshake technique can be used to provide protection for
   the SupplementalData information.  Figure 2 illustrates the double
   handshake, where the initial handshake does not include any
   extensions, but it does result in protected communications.  Then, a
   second handshake that includes the SupplementalData information is
   performed using the protected communications.  In Figure 2, the
   number on the right side indicates the amount of protection for the
   TLS message on that line.  A zero (0) indicates that there is no
   communication protection; a one (1) indicates that protection is
   provided by the first TLS session; and a two (2) indicates that
   protection is provided by both TLS sessions.

   The placement of the SupplementalData message in the TLS Handshake
   results in the server providing its SupplementalData information
   before the client is authenticated.  In many situations, servers will
   not want to provide authorization information until the client is
   authenticated.  The double handshake illustrated in Figure 2 provides
   a technique to ensure that the parties are mutually authenticated
   before either party provides SupplementalData information.























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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


   Client                                                   Server

   ClientHello (no extensions) -------->                            |0
                                       ServerHello (no extensions)  |0
                                                      Certificate*  |0
                                                ServerKeyExchange*  |0
                                               CertificateRequest*  |0
                               <--------           ServerHelloDone  |0
   Certificate*                                                     |0
   ClientKeyExchange                                                |0
   CertificateVerify*                                               |0
   [ChangeCipherSpec]                                               |0
   Finished                    -------->                            |1
                                                [ChangeCipherSpec]  |0
                               <--------                  Finished  |1
   ClientHello (w/ extensions) -------->                            |1
                                       ServerHello (w/ extensions)  |1
                                                 SupplementalData*  |1
                                                      Certificate*  |1
                                                ServerKeyExchange*  |1
                                               CertificateRequest*  |1
                               <--------           ServerHelloDone  |1
   SupplementalData*                                                |1
   Certificate*                                                     |1
   ClientKeyExchange                                                |1
   CertificateVerify*                                               |1
   [ChangeCipherSpec]                                               |1
   Finished                    -------->                            |2
                                                [ChangeCipherSpec]  |1
                               <--------                  Finished  |2
   Application Data            <------->          Application Data  |2

   *  Indicates optional or situation-dependent messages.

         Figure 2.  Double Handshake to Protect Supplemental Data
















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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


5.  IANA Considerations

   IANA has taken the following actions:

   1) Created an entry, supplemental_data(23), in the existing registry
      for HandshakeType (defined in RFC 2246 [N2]).

   2) Established a registry for TLS Supplemental Data Formats
      (SupplementalDataType).  Values in the inclusive range 0-16385
      (decimal) are assigned via RFC 2434 [N5] Standards Action.  Values
      from the inclusive range 16386-65279 (decimal) are assigned via
      RFC 2434 IETF Consensus.  Values from the inclusive range
      65280-65535 (decimal) are reserved for RFC 2434 Private Use.

6.  Normative References

   [N1]   Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

   [N2]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
          2246, January 1999.

   [N3]   Dierks, T. and E. Rescorla, "The Transport Layer Security
          (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [N4]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
          T. Wright, "Transport Layer Security (TLS) Extensions", RFC
          4366, April 2006.

   [N5]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October
          1998.

7.  Acknowledgements

   The fundamental architectural idea for the supplemental data
   handshake message was provided by Russ Housley and Eric Rescorla.














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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


Author's Address

   Stefan Santesson
   Microsoft
   Finlandsgatan 30
   164 93 KISTA
   Sweden

   EMail: stefans@microsoft.com










































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RFC 4680      TLS Handshake Message for Supplemental Data September 2006


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

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