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TLS Working Group                                             I. Hajjeh
Internet Draft                                               INEOVATION
                                                               M. Badra
                                                       LIMOS Laboratory
Intended status: Experimental                         December 15, 2007
Expires: June 2008

                                 TLS Sign

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   TLS protocol provides authentication and data protection for
   communication between two entities. However, missing from the
   protocol is a way to perform non-repudiation service.

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   This document defines extensions to the TLS protocol to allow it to
   perform non-repudiation service. It is based on [TLSSIGN] and it
   provides the client and the server the ability to sign by TLS,
   handshake and applications data using certificates such as X.509.

Table of Contents

   1. Introduction...................................................2
      1.1. Conventions used in this document.........................3
   2. TLS Sign overview..............................................3
      2.1. tls sign on off protocol..................................6
         2.1.1. bad_sign alert.......................................7
      2.2. Storing signed data.......................................7
   3. Security Considerations........................................9
   4. IANA Considerations............................................9
   5. References.....................................................9
      5.1. Normative References......................................9
      5.2. Informative References...................................10
   Author's Addresses...............................................10
   Appendix Changelog...............................................10
   Intellectual Property Statement..................................11
   Disclaimer of Validity...........................................11

1. Introduction

   Actually, TLS is the most deployed security protocol for securing
   exchanges. It provides end-to-end secure communications between two
   entities with authentication and data protection. However, what is
   missing from the protocol is a way to provide the non-repudiation

   This document describes how the non-repudiation service may be
   integrated as an optional module in TLS. This is in order to provide
   both parties with evidence that the transaction has taken place and
   to offer a clear separation with application design and development.

   TLS-Sign's design motivations included:

   O  TLS is application protocol-independent. Higher-level protocol can
      operate on top of the TLS protocol transparently.

   O  TLS is a modular nature protocol. Since TLS is developed in four
      independent protocols, the approach defined in this document can
      be added by extending the TLS protocol and with a total reuse of
      pre-existing TLS infrastructures and implementations.

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   O  Several applications like E-Business require non-repudiation proof
      of transactions. It is critical in these applications to have the
      non-repudiation service that generates, distributes, validates and
      maintains the evidence of an electronic transaction. Since TLS is
      widely used to secure these applications exchanges, the non-
      repudiation should be offered by TLS.

   O  Generic non-repudiation with TLS. TLS Sign provides a generic non-
      repudiation service that can be easily used with protocols. TLS
      Sign minimizes both design and implementation of the signature
      service and that of the designers and implementators who wish to
      use this module.

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 RFC-2119 [RFC2119].

2. TLS Sign overview

   TLS Sign is integrated as a higher-level module of the TLS Record
   protocol. It is optionally used if the two entities agree. This is
   negotiated by extending Client and Server Hello messages in the same
   way defined in [TLSEXT].

   In order to allow a TLS client to negotiate the TLS Sign, a new
   extension type should be added to the Extended Client and Server
   Hellos messages. TLS clients and servers MAY include an extension of
   type 'signature' in the Extended Client and Server Hellos messages.
   The 'extension_data' field of this extension contains a
   'signature_request' where:

      enum {
            pkcs7(0), smime(1), xmldsig(2), (255);
         } ContentFormat;

      struct {
              ContentFormat content_format;
              SignMethod sign_meth;
              SignType sign_type<2..2^16-1>;
           } SignatureRequest;

      enum {
            ssl_client_auth_cert(0), ssl_client_auth_cert_url(1), (255);
         } SignMethod;

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      uint8 SignType[2];

   The client initiates the TLS Sign module by sending the
   ExtendedClientHello including the 'signature' extension. This
   extension contains:

   - the SignType carrying the type of the non repudiation proof. It can
   have one of these two values:

   SignType non_repudiation_with_proof_of_origin      = { 0x00, 0x01 };
   SignType non_repudiation_without_proof_of_origin   = { 0x00, 0x02 };

   - the ContentFormat carrying the format of signed data. It can be

            ContentFormat PKCS7   = { 0x00, 0xA1 };
            ContentFormat SMIME   = { 0x00, 0xA2 };
            ContentFormat XMLDSIG = { 0x00, 0xA3 };

         o if the value of the ContentFormat is PKCS7, then the PKCS7
         Content_type is of type signed-data.

         o if the value of the ContentFormat is S/MIME, then S/MIME
         Content_type is of type SignedData

         o if the value of the ContentFormat is XMLDSIG, then XMLDSIG
         signatureMethod algorithms.

   - the SignMethod carrying the signature method that is used to sign
   the application data (e.g. X509  authentication certificate).

            SignMethod X509 = { 0x00, 0xB1 };

   Actually, this document uses the same certificate used in client
   authentication. Any new signature method MAY be added in future
   versions (e.g. delegated attributes certificates).

   The server MAY reject the connection by sending the error alert
   "unsupported_extension" [TLSEXT] and closing the connection.

   The client and the server MAY use the same certificates used by the
   Handshake protocol. Several cases are possible:

   - If the server has an interest in getting non-repudiation data from
   the client and that the cipher_suites list sent by the client does
   not include any cipher_suite with signature ability, the server MUST
   (upon reception of tls_sign_on_off protocol message not followed by a

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   certificate with a type equals to ExtendedServerHello.sign_method)
   close the connection by sending a fatal error.

   - If the server has an interest in getting non-repudiation data from
   the client and that the cipher_suites list sent by the client
   includes at least a cipher_suite with signature ability, the server
   SHOULD select a cipher_suite with signature ability and MUST provide
   a certificate (e.g., RSA) that MAY be used for key exchange. Further,
   the server MUST request a certificate from the client using the TLS
   certificate request message (e.g., an RSA or a DSS signature-capable
   certificate). If the client does not send a certificate during the
   TLS Handshake, the server MUST close the TLS session by sending a
   fatal error in the case where the client sends a tls_sign_on_off
   protocol message not followed by a certificate with a type equals to

   - The client or the server MAY use a certificate different to these
   being used by TLS Handshake. This MAY happen when the server agrees
   in getting non-repudiation data from the client and that the type of
   the client certificate used by TLS Handshake and the type selected by
   the server from the list in ExtendedClientHello.sign_method are
   different, or when the ExtendedServerHello.cipher_suite does not
   require client and/or server certificates. In these cases, the client
   or the server sends a new message called certificate_sign, right
   after sending the tls_sign_on_off protocol messages. The new message
   contains the sender's certificate in which the type is the same type
   selected by the server from the list in
   ExtendedClientHello.sign_method. The certificate_sign is therefore
   used to generate signed data. It is defined as follows:

      opaque ASN.1Cert<2^24-1>;

      struct {
              ASN.1Cert certificate_list<1..2^24-1>;
           } CertificateSign;

   The certificate_list, as defined in [TLS], is a sequence (chain) of
   certificates. The sender's certificate MUST come first in the list.
   If the server has no interest in getting non-repudiation data from
   the client, it replays with an ordinary TLS ServerHello or return a
   handshake failure alert and close the connection [TLS].

       Client                                               Server

       ClientHello             -------->

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                              <--------            ServerHelloDone
       Finished                -------->
                            <--------                     Finished
       TLSSignOnOff   <------------------------->     TLSSignOnOff
       CertificateSign* <---------------------->  CertificateSign*
       (Signed) Application Data <-----> (Signed) Application Data

   * Indicates optional or situation-dependent messages that are not
   always sent.

2.1. tls sign on off protocol

   To manage the generation of evidence, new sub-protocol is added by
   this document, called tls_sign_on_off. This protocol consists of a
   single message that is encrypted and compressed under the established
   connection state. This message can be sent at any time after the TLS
   session has been established. Thus, no man in the middle can replay
   or inject this message. It consists of a single byte of value 1
   (tls_sign_on) or 0 (tls_sign_off).

      enum {
            change_cipher_spec(20), alert(21), handshake(22),
            application_data(23), tls_sign(TBC), (255)
         } ContentType;

      struct {
              enum { tls_sign_off(0), tls_sign_on(1), (255) } type;
           } TLSSignOnOff;

   The tls_sign_on_off message is sent by the client and/or server to
   notify the receiving party that subsequent records will carry data
   signed under the negotiated parameters.

   Note: TLSSignOnOff is an independent TLS Protocol content type, and
   is not actually a TLS handshake message.

   2.1.1 TLS sign packet format

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   This document defines a new packet format that encapsulates signed
   data, the TLSSigntext. The packet format is shown below. The fields
   are transmitted from left to right.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Content-Type  |     Flag      |     Version                   |
   |              Length           |  Signed Data ...


      Same as TLSPlaintext.type.


      0 1 2 3 4 5 6 7 8
      |A R R R R R R R|

   A = acknowledgement of receipt
   R = Reserved.

   When the whole signed data is delivered to the receiver, the TLS Sign
   will check the signature. If the signature is valid and that the
   sender requires a proof of receipt, the receiver MUST generate a
   TLSSigntext packet with the bit A set to 1 (acknowledgement of
   receipt). This helps the receiver of the acknowledgment of receipt in
   storing the data-field for later use (see section 2.2). The data
   field of that message contains the digest of the whole data receiver
   by the generator of the acknowledgement of receipt. The digest is
   signed before sending the result to the other side.

2.1.1. bad_sign alert

   This alert is returned if a record is received with an incorrect
   signature. This message is always fatal.

2.2. Storing signed data

   The objective of TLS Sign is to provide both parties with evidence
   that can be stored and later presented to a third party to resolve
   disputes that arise if and when a communication is repudiated by one

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   of the entities involved. This document provides the two basic types
   of non-repudiation service:

   O  Non-repudiation with proof of origin: provides the TLS server with
      evidence proving that the TLS client has sent it the signed data
      at a certain time.

   O  Non-repudiation with proof of delivery: provides the TLS client
      with evidence that the server has received the client's signed
      data at a specific time.

   TLS Handshake exchanges the current time and date according to the
   entities internal clock. Thus, the time and date can be stored with
   the signed data as a proof of communication. For B2C or B2B
   transactions, non-repudiation with proof of origin and non-
   repudiation with proof of receipt are both important. If the TLS
   client requests a non-repudiation service with proof of receipt, the
   server SHOULD verify and send back to client a signature on the hash
   of signed data.

   The following figure explains the different events for proving and
   storing signed data [RFC4949]. RFC 4949 uses the term "critical
   action" to refer to the act of communication between the two
   entities. For a complete non-repudiation deployment, 6 phases should
   be respected:

   --------   --------   --------   --------   --------   . --------
   Phase 1:   Phase 2:   Phase 3:   Phase 4:   Phase 5:   . Phase 6:
   Request    Generate   Transfer   Verify     Retain     . Resolve
   Service    Evidence   Evidence   Evidence   Evidence   . Dispute
   --------   --------   --------   --------   --------   . --------

   Service    Critical   Evidence   Evidence   Archive    . Evidence
   Request => Action  => Stored  => Is      => Evidence   . Is
   Is Made    Occurs     For Later  Tested     In Case    . Verified
              and        Use |          ^      Critical   .     ^
              Evidence       v          |      Action Is  .     |
              Is         +-------------------+ Repudiated .     |
              Generated  |Verifiable Evidence|------> ... . ----+

   1- Requesting explicit transaction evidence before sending data.
   Normally, this action is taken by the SSL/TLS client

   2- If the server accepts, the client will generate evidence by
   signing data using his X.509 authentication certificate. Server will
   go through the same process if the evidence of receipt is requested.

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   3 - The signed data is then sent by the initiator (client or server)
   and stored it locally, or by a third party, for a later use if

   4 - The entity that receive the evidence process to verify the signed

   5- The evidence is then stored by the receiver entity for a later use
   if needed.

   6- In this phase, which occurs only if the critical action is
   repudiated, the evidence is retrieved from storage, presented, and
   verified to resolve the dispute.

   With this method, the stored signed data (or evidence) can be
   retrieved by both parties, presented and verified if the critical
   action is repudiated.

3. Security Considerations

   Security issues are discussed throughout this memo.

4. IANA Considerations

   This document defines a new TLS extension "signature", assigned the
   value TBD from the TLS ExtensionType registry defined in [TLSEXT].

   This document defines one TLS ContentType: tls_sign(TBD). This
   ContentType value is assigned from the TLS ContentType registry
   defined in [TLS].

   This document defines a new handshake message, certificate_sign,
   whose value is to be allocated from the TLS HandshakeType registry
   defined in [TLS].

   The bad_sign alert that is defined in this document is assigned to
   the TLS Alert registry defined in [TLS].

5. References

5.1. Normative References

   [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 TLS Protocol Version
             1.1", RFC 4346, April 2005.

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   [TLSEXT]  Blake-Wilson, S., et. al., "Transport Layer Security TLS)
             Extensions", RFC 4366, April 2006.

   [PKCS7]   RSA Laboratories, "PKCS #7: RSA Cryptographic Message
             Syntax Standard," version 1.5, November 1993.

   [SMIME]   Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
             3851, July 2004.

   [XMLDSIG] Eastlake, D., et. al, "(Extensible Markup Language) XML
             Signature Syntax and Processing", RFC 3275, March 2002.

5.2. Informative References

   [RFC4949] Shirey, R., "Internet Security Glossary", RFC 4949, August

   [TLSSIGN] Hajjeh, I., Serhrouchni, A., "Integrating a signature
             module in SSL/TLS, ICETE2004., ACM/IEEE, First
             International Conference on E-Business and
             Telecommunication Networks, Portugal, August 2004.

Author's Addresses

   Ibrahim Hajjeh

   Email: hajjeh@ineovation.com

   Mohamad Badra
   LIMOS Laboratory - UMR6158, CNRS

   Email: badra@isima.fr

Appendix Changelog

   Changes from -01 to -02:

   o Add an IANA section.

   o Small clarifications to section 2.

   o Add the bad_sign alert and the certificate_sign message.

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   Changes from -00 to -01:

   o Clarifications to the format of the signed data in Section 2.

   o Small clarifications to TLS SIGN negotiation in Section 2.

   o Added Jacques Demerjian and Mohammed Achemlal as

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
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   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The IETF Trust (2007).

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   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.


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

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