Using OpenPGP Keys for Transport Layer Security (TLS) Authentication
RFC 6091

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Internet Engineering Task Force (IETF)              N. Mavrogiannopoulos
Request for Comments: 6091                                           KUL
Obsoletes: 5081                                               D. Gillmor
Category: Informational                                      Independent
ISSN: 2070-1721                                            February 2011

  Using OpenPGP Keys for Transport Layer Security (TLS) Authentication

Abstract

   This memo defines Transport Layer Security (TLS) extensions and
   associated semantics that allow clients and servers to negotiate the
   use of OpenPGP certificates for a TLS session, and specifies how to
   transport OpenPGP certificates via TLS.  It also defines the registry
   for non-X.509 certificate types.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6091.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   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 ................................................8
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References .....................................8
   Appendix A.  Changes from RFC 5081 .................................9

1.  Introduction

   The IETF has two sets of standards for public key certificates: one
   set for the use of X.509 certificates [RFC5280], and one for OpenPGP
   certificates [RFC4880].  At the time of this writing, TLS [RFC5246]
   standards are defined to use X.509 certificates.  This document
   specifies a way to negotiate the 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.

   These extensions are not backward-compatible with [RFC5081], and the
   major differences are summarized in Appendix A.  Although the OpenPGP
   CertificateType value is being reused by this memo with the same
   number as that specified in [RFC5081] but with different semantics,
   we believe that this causes no interoperability issues because the
   latter was not widely deployed.

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 [RFC5246].

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   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].

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 [RFC5246].

   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.  Note that the
   CertificateTypeExtension structure is being used both by the client
   and the server, even though the structure is only specified once in
   this document.  Reusing a single specification for both client and
   server is common in other specifications, such as the TLS protocol
   itself [RFC5246].

      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 key exchange method compatible
   with the key in the certificate can be used in combination with
   OpenPGP certificates.

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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
   "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
   "cert_type" extension in the extended server hello.

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.

      Key Exchange Algorithm    OpenPGP Certificate Type

      RSA                       RSA public key that can be used for
                                encryption.

      DHE_DSS                   DSA public key that can be used for
                                authentication.

      DHE_RSA                   RSA public key that can be used for
                                authentication.

   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 ID to be used for this session is explicitly

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   specified in the OpenPGPKeyID field.  The key ID must be specified
   even if the certificate has only a primary key.  The peer, upon
   receiving this type, has to either use the specified subkey or
   terminate the session with a fatal alert of
   "unsupported_certificate".

   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 primary key
   fingerprint to be specified, as well as the subkey ID to be used for
   this session.  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 5 of [RFC6066].

   Implementations of this protocol MUST ensure that the sizes of key
   IDs and fingerprints in the OpenPGPSubKeyCert and
   OpenPGPSubKeyFingerprint structures comply with [RFC4880].  Moreover,
   it is RECOMMENDED that the keys to be used with this protocol have
   the authentication flag (0x20) set.

   The process of fingerprint generation is described in Section 12.2 of
   [RFC4880].

   The enumerated types "cert_fingerprint" and "cert" of
   OpenPGPCertDescriptorType that were defined in [RFC5081] are not used
   and are marked as obsolete by this document.  The "empty_cert" type
   has replaced "cert" and is a backward-compatible way to specify an
   empty certificate; "cert_fingerprint" MUST NOT be used with this
   updated specification, and hence that old alternative has been
   removed from the Certificate struct description.

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      enum {
           empty_cert(1),
           subkey_cert(2),
           subkey_cert_fingerprint(3),
           (255)
      } OpenPGPCertDescriptorType;

      uint24 OpenPGPEmptyCert = 0;

      struct {
          opaque OpenPGPKeyID<8..255>;
          opaque OpenPGPCert<0..2^24-1>;
      } OpenPGPSubKeyCert;

      struct {
          opaque OpenPGPKeyID<8..255>;
          opaque OpenPGPCertFingerprint<20..255>;
      } OpenPGPSubKeyFingerprint;

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

4.  Security Considerations

   All security considerations discussed in [RFC5246], [RFC6066], and
   [RFC4880] apply to this document.  Considerations about the use of
   the web of trust or identity and certificate verification procedures
   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 cipher suite negotiation as described in the TLS
   specification [RFC5246], 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 cipher
   suites, it is believed that the security properties of this
   negotiation are the same as with cipher suite negotiation.

   When using OpenPGP fingerprints instead of the full certificates, the
   discussion in Section 5 of [RFC6066] 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
   [RFC4880]).

   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 the certificates.

5.  IANA Considerations

   This document uses a registry and the "cert_type" extension
   originally defined in [RFC5081].  Existing IANA references have been
   updated to point to this document.

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   In addition, the "TLS Certificate Types" registry established by
   [RFC5081] has been updated in the following ways:

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

   2.  Values from 2 through 223 decimal inclusive are assigned via "RFC
       Required" [RFC5226].

   3.  Values from 224 decimal through 255 decimal inclusive are
       reserved for Private Use [RFC5226].

6.  Acknowledgements

   The authors wish to thank Alfred Hoenes and Ted Hardie for their
   suggestions on improving this document.

7.  References

7.1.  Normative References

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

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

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6066]   Eastlake 3rd, D., "Transport Layer Security (TLS)
               Extensions: Extension Definitions", RFC 6066,
               January 2011.

7.2.  Informative References

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

   [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.

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Appendix A.  Changes from RFC 5081

   This document incorporates a major change in the "Server Certificate"
   and "Client Certificate" TLS messages that will make implementations
   following this protocol incompatible with those following [RFC5081].
   This change requires the subkey IDs used for TLS authentication to be
   marked explicitly in the handshake procedure.  This was decided in
   order to place no limitation on the OpenPGP certificates' contents
   that can be used with this protocol.

   [RFC5081] required that an OpenPGP key or subkey be marked with the
   authentication flag; thus, authentication would have failed if this
   flag was not set or if this flag was set in more than one subkey.
   The protocol in this memo has no such limitation.

Authors' Addresses

   Nikos Mavrogiannopoulos
   ESAT/COSIC Katholieke Universiteit Leuven
   Kasteelpark Arenberg 10, bus 2446
   Leuven-Heverlee,   B-3001
   Belgium

   EMail: nikos.mavrogiannopoulos@esat.kuleuven.be

   Daniel Kahn Gillmor
   Independent
   119 Herkimer St.
   Brooklyn, NY  11216-2801
   US

   EMail: dkg@fifthhorseman.net

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