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Versions: 00 01 02 03 04 05 06 07 08 09 rfc6091                         
Network Working Group                               N. Mavrogiannopoulos
Internet-Draft                                               Independent
Obsoletes: rfc5081                                      October 26, 2008
(if approved)
Intended status: Informational
Expires: April 29, 2009

  Using OpenPGP Keys for Transport Layer Security (TLS) Authentication

Status of This Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on April 29, 2009.


   This memo proposes extensions to the Transport Layer Security (TLS)
   protocol to support the OpenPGP key format.  The extensions discussed
   here include a certificate type negotiation mechanism, and the
   required modifications to the TLS Handshake Protocol.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Changes to the Handshake Message Contents . . . . . . . . . . . 3
     3.1.  Client Hello  . . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Server Hello  . . . . . . . . . . . . . . . . . . . . . . . 4
     3.3.  Server Certificate  . . . . . . . . . . . . . . . . . . . . 4
     3.4.  Certificate Request . . . . . . . . . . . . . . . . . . . . 6
     3.5.  Client Certificate  . . . . . . . . . . . . . . . . . . . . 6
     3.6.  Other Handshake Messages  . . . . . . . . . . . . . . . . . 7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The IETF has two sets of standards for public key certificates, one
   set for use of X.509 certificates [RFC5280] and one for OpenPGP
   certificates [RFC4880].  At the time of writing, TLS [RFC4346]
   standards are defined to use only X.509 certificates.  This document
   specifies a way to negotiate use of OpenPGP certificates for a TLS
   session, and specifies how to transport OpenPGP certificates via TLS.
   The proposed extensions are backward compatible with the current TLS
   specification, so that existing client and server implementations
   that make use of X.509 certificates are not affected.

2.  Terminology

   The term "OpenPGP key" is used in this document as in the OpenPGP
   specification [RFC4880].  We use the term "OpenPGP certificate" to
   refer to OpenPGP keys that are enabled for authentication.

   This document uses the same notation and terminology used in the TLS
   Protocol specification [RFC4346].

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

3.  Changes to the Handshake Message Contents

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

3.1.  Client Hello

   In order to indicate the support of multiple certificate types,
   clients MUST include an extension of type "cert_type" to the extended
   client hello message.  The "cert_type" TLS extension is assigned the
   value of 9 from the TLS ExtensionType registry.  This value is used
   as the extension number for the extensions in both the client hello
   message and the server hello message.  The hello extension mechanism
   is described in [RFC4366].

   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 contains a
   CertificateTypeExtension structure.

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      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 certificates.  All
   existing cipher suites that support a compatible, with the key, key
   exchange method can be used in combination with OpenPGP certificates.

3.2.  Server Hello

   If the server receives a client hello that contains the "cert_type"
   extension and chooses a cipher suite that requires a certificate,
   then two outcomes are possible.  The server MUST either select a
   certificate type from the certificate_types field in the extended
   client hello or terminate the session with a fatal alert of type

   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
   "cert_type" extension in the extended server hello.

   It is perfectly legal for a server to ignore this message.  In that
   case the normal TLS handshake should be used.  Other certificate
   types than the default MUST NOT be used.

3.3.  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 certificate in the certificate message.  The
   certificate must contain a public key that matches the selected key
   exchange algorithm, as shown below.

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      Key Exchange Algorithm  OpenPGP Certificate Type

      RSA                     RSA public key that can be used for

      DHE_DSS                 DSS public key that can be used for

      DHE_RSA                 RSA public key that can be used for

   An OpenPGP certificate appearing in the certificate message is sent
   using the binary OpenPGP format.  The certificate MUST contain all
   the elements required by Section 11.1 of [RFC4880].

   OpenPGP certificates to be transferred are placed in the Certificate
   structure and tagged with the OpenPGPCertDescriptorType
   "subkey_cert".  Since those certificates might contain several
   subkeys the subkey to be used for this session is explicitely
   specified in the OpenPGPKeyID field, even if the certificate has a
   single subkey.  The peer once receiving this type has to either use
   the specified subkey or terminate the session with a fatal alert of

   The option is also available to send an OpenPGP fingerprint, instead
   of sending the entire certificate, by using the
   "subkey_cert_fingerprint" tag.  This tag uses the
   OpenPGPSubKeyFingerprint structure and requires the subkey ID to be
   specified as well.  The peer shall respond with a
   "certificate_unobtainable" fatal alert if the certificate with the
   given fingerprint cannot be found.  The "certificate_unobtainable"
   fatal alert is defined in Section 4 of [RFC4366].

   The process of fingerprint generation is described in Section 12.2 of

   The types "cert_fingerprint" and "cert" of OpenPGPCertDescriptorType
   that were defined in [RFC5081] are not used and are marked as
   obsolete by this document.

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      enum {
           empty_cert (1), subkey_cert (2), subkey_cert_fingerprint (3),
      } OpenPGPCertDescriptorType;

      uint24 OpenPGPEmptyCert = 0;

      struct {
          opaque OpenPGPKeyID<1..8>;
          opaque OpenPGPCert<0..2^24-1>;
      } OpenPGPSubKeyCert;

      struct {
          opaque OpenPGPKeyID<1..8>;
          opaque OpenPGPCertFingerprint<16..20>;
      } OpenPGPSubKeyFingerprint;

      struct {
           OpenPGPCertDescriptorType descriptorType;
           select (descriptorType) {
                case empty_cert: OpenPGPEmptyCert;
                case subkey_cert: OpenPGPSubKeyCert;
                case subkey_cert_fingerprint:
      } Certificate;

3.4.  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.

3.5.  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 certificates have been selected and no certificate
   is available from the client, then a certificate structure of type
   "empty_cert" that contains an OpenPGPEmptyCert value MUST be sent.
   The server SHOULD respond with a "handshake_failure" fatal alert if
   client authentication is required.

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3.6.  Other Handshake Messages

   All the other handshake messages are identical to the TLS

4.  Security Considerations

   All security considerations discussed in [RFC4346], [RFC4366], and
   [RFC4880] apply to this document.  Considerations about the use of
   the web of trust or identity and certificate verification procedure
   are outside the scope of this document.  These are considered issues
   to be handled by the application layer protocols.

   The protocol for certificate type negotiation is identical in
   operation to ciphersuite negotiation of the [RFC4346] specification
   with the addition of default values when the extension is omitted.
   Since those omissions have a unique meaning and the same protection
   is applied to the values as with ciphersuites, it is believed that
   the security properties of this negotiation are the same as with
   ciphersuite negotiation.

   When using OpenPGP fingerprints instead of the full certificates, the
   discussion in Section 6.3 of [RFC4366] for "Client Certificate URLs"
   applies, especially when external servers are used to retrieve keys.
   However, a major difference is that although the
   "client_certificate_url" extension allows identifying certificates
   without including the certificate hashes, this is not possible in the
   protocol proposed here.  In this protocol, the certificates, when not
   sent, are always identified by their fingerprint, which serves as a
   cryptographic hash of the certificate (see Section 12.2 of

   The information that is available to participating parties and
   eavesdroppers (when confidentiality is not available through a
   previous handshake) is the number and the types of certificates they
   hold, plus the contents of certificates.

5.  IANA Considerations

   This document uses a registry originally defined in [RFC5081] and no
   new actions are required by IANA.

6.  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,

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   Pasi Eronen, Jon Callas, Stephen Kent, Robert Sparks, and Hilarie
   Orman for their suggestions on improving this document.

7.  References

7.1.  Normative References

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              November 2007.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2.  Informative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

Author's Address

   Nikos Mavrogiannopoulos
   Arkadias 8
   Halandri, Attiki  15234

   EMail: nmav@gnutls.org
   URI:   http://www.gnutls.org/

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