TLS Working Group                                   N. Mavrogiannopoulos
Internet-Draft                                               Independent
Expires: December 7, 2006                                   June 5, 2006

               Using OpenPGP keys for TLS authentication

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo proposes extensions to the TLS protocol to support the
   OpenPGP trust model and keys.  The extensions discussed here include
   a certificate type negotiation mechanism, and the required
   modifications to the TLS Handshake Protocol.

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1.  Introduction

   At the time of writing, TLS [TLS] uses the PKIX [PKIX]
   infrastructure, to provide certificate services.  Currently the PKIX
   protocols are limited to a hierarchical key management and as a
   result, applications which follow different - non hierarchical -
   trust models, could not be benefited by TLS.

   OpenPGP keys (sometimes called OpenPGP certificates), provide
   security services for electronic communications.  They are widely
   deployed, especially in electronic mail applications, provide public
   key authentication services, allow distributed key management and can
   be used with a non hierarchical trust model called the "web of trust"

   This document will extend the TLS protocol to support OpenPGP keys
   using the existing TLS cipher suites.  In brief this would be
   achieved by adding a negotiation of the certificate type in addition
   to the normal handshake negotiations.  Then the required
   modifications to the handshake messages, in order to hold OpenPGP
   keys as well, will be described.  The normal handshake procedure with
   X.509 certificates is not altered, to preserve compatibility with
   existing TLS servers and clients.

   This document uses the same notation used in the TLS Protocol
   specification [TLS].

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

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2.  Changes to the Handshake Message Contents

   This section describes the changes to the TLS handshake message
   contents when OpenPGP keys are to be used for authentication.

2.1.  Client Hello

   In order to indicate the support of multiple certificate types
   clients will include an extension of type "cert_type" (see Section 4)
   to the extended client hello message.  The hello extension mechanism
   is described in [TLSEXT].

   This extension carries a list of supported certificate types the
   client can use, sorted by client preference.  This extension MUST be
   omitted if the client only supports X.509 certificates.  The
   "extension_data" field of this extension will contain a
   CertificateTypeExtension structure.

      enum { client, server } ClientOrServerExtension;

      enum { X.509(0), OpenPGP(1), (255) } CertificateType;

      struct {
         select(ClientOrServerExtension) {
            case client:
               CertificateType certificate_types<1..2^8-1>;
            case server:
               CertificateType certificate_type;
      } CertificateTypeExtension;

   No new cipher suites are required to use OpenPGP keys.  All existing
   cipher suites that support a compatible with the key, key exchange
   method can be used in combination with OpenPGP keys.

2.2.  Server Hello

   Servers that receive an extended client hello containing the
   "cert_type" extension, and have chosen a cipher suite that supports
   certificates, they MUST select a certificate type from the
   certificate_types field in the extended client hello, or terminate
   the connection with a fatal alert of type "unsupported_certificate".

   The certificate type selected by the server, is encoded in a
   CertificateTypeExtension structure, which is included in the extended
   server hello message, using an extension of type "cert_type".
   Servers that only support X.509 certificates MAY omit including the

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   "cert_type" extension in the extended server hello.

2.3.  Server Certificate

   The contents of the certificate message sent from server to client
   and vice versa are determined by the negotiated certificate type and
   the selected cipher suite's key exchange algorithm.

   If the OpenPGP certificate type is negotiated then it is required to
   present an OpenPGP key in the Certificate message.  The OpenPGP key
   must contain a public key that matches the selected key exchange
   algorithm, as shown below.

      Key Exchange Algorithm  OpenPGP Key Type

      RSA                     RSA public key which can be used for

      DHE_DSS                 DSS public key.

      DHE_RSA                 RSA public key which can be used for

   An OpenPGP public key appearing in the Certificate message will be
   sent using the binary OpenPGP format.  The term public key is used to
   describe a composition of OpenPGP packets to form a block of data
   which contains all information needed by the peer.  This includes
   public key packets, user ID packets and all the fields described in
   section 10.1 of [OpenPGP].

   The option is also available to send an OpenPGP fingerprint, instead
   of sending the entire key.  The process of fingerprint generation is
   described in section 11.2 of [OpenPGP].  The peer shall respond with
   a "certificate_unobtainable" fatal alert if the key with the given
   key fingerprint cannot be found.  The "certificate_unobtainable"
   fatal alert is defined in section 4 of [TLSEXT].

   If the key is not valid, expired, revoked, corrupt, the appropriate
   fatal alert message is sent from section A.3 of the TLS
   specification.  If a key is valid and neither expired nor revoked, it
   is accepted by the protocol.  The key validation procedure is a local
   matter outside the scope of this document.

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      enum {
           key_fingerprint (0), key (1), (255)
      } PGPKeyDescriptorType;

      opaque PGPKeyFingerprint<16..20>;

      opaque PGPKey<0..2^24-1>;

      struct {
           PGPKeyDescriptorType descriptorType;
           select (descriptorType) {
                case key_fingerprint: PGPKeyFingerprint;
                case key: PGPKey;
      } Certificate;

2.4.  Certificate request

   The semantics of this message remain the same as in the TLS
   specification.  However if this message is sent, and the negotiated
   certificate type is OpenPGP, the "certificate_authorities" list MUST
   be empty.

2.5.  Client certificate

   This message is only sent in response to the certificate request
   message.  The client certificate message is sent using the same
   formatting as the server certificate message and it is also required
   to present a certificate that matches the negotiated certificate
   type.  If OpenPGP keys have been selected, and no key is available
   from the client, then a Certificate that contains an empty PGPKey
   should be sent.  The server may respond with a "handshake_failure"
   fatal alert if client authentication is required.

2.6.  Other Handshake messages

   The rest of the handshake messages such as the server key exchange,
   the certificate verify and the finished messages are identical to the
   TLS specification.

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3.  Security Considerations

   As with X.509 ASN.1 formatted keys, OpenPGP keys need specialized
   parsers.  Care must be taken to make those parsers safe against
   maliciously modified keys, that could cause arbitrary code execution.

   Security considerations about the use of the web of trust or the
   verification procedure are outside the scope of this document and
   they are considered an issue to be handled by local policy.

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4.  IANA Considerations

   This document defines a new TLS extension, "cert_type", assigned a
   value of TBD-BY-IANA (the value 7 is suggested) from the TLS
   ExtensionType registry defined in [TLSEXT].  This value is used as
   the extension number for the extensions in both the client hello
   message and the server hello message.  The new extension type will be
   used for certificate type negotiation.

   The "cert_type" extension contains an 8-bit CertificateType field,
   for which a new registry, named "TLS Certificate Types", is
   established in this document, to be maintained by IANA.  The registry
   is segmented in the following way:

   1.  Values 0 (X.509) and 1 (OpenPGP) are defined in this document.

   2.  Values from 2 through 223 decimal inclusive are assigned via IETF
       Consensus [RFC2434].

   3.  Values from 224 decimal through 255 decimal inclusive are
       reserved for Private Use [RFC2434].

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5.  References

5.1.  Normative References

   [TLS]      Dierks, T. and E. Rescorla, "The TLS Protocol Version
              1.1", RFC 4346, April 2006.

   [OpenPGP]  Callas, J., Donnerhacke, L., Finey, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              October 1998.

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

5.2.  Informative References

   [PKIX]  Housley, R., Ford, W., Polk, W., and D. Solo, "Internet X.509
           Public Key Infrastructure Certificate and Certificate
           Revocation List (CRL) Profile", RFC 3280, April 2002.

   [WOT]   Abdul-Rahman, A., "The PGP Trust Model", EDI Forum: The
           Journal of Electronic Commerce, April 1997.

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Appendix A.  Acknowledgements

   This document was based on earlier work made by Will Price and
   Michael Elkins.

   The author wishes to thank Werner Koch, David Taylor, Timo Schulz and
   Pasi Eronen for their suggestions on improving this document.

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Author's Address

   Nikos Mavrogiannopoulos
   Arkadias 8
   Halandri, Attiki  15234


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