Using OpenPGP Keys for Transport Layer Security (TLS) Authentication
draft-mavrogiannopoulos-rfc5081bis-02

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 aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on April 29, 2009.

Abstract

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.

Table of Contents

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

1.  Introduction

The IETF has two sets of standards for public key certificates, one set for use of X.509 certificates [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,” May 2008.) and one for OpenPGP certificates [RFC4880] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.). At the time of writing, TLS [RFC4346] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) 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] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.). 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] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.).

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.

   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 "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.

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.

   Key Exchange Algorithm  OpenPGP Certificate Type

   RSA                     RSA public key that can be used for
                           encryption.

   DHE_DSS                 DSS 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] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.).

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

The process of fingerprint generation is described in Section 12.2 of [RFC4880] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.).

The types "cert_fingerprint" and "cert" of OpenPGPCertDescriptorType that were defined in [RFC5081] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) are not used and are marked as obsolete by this document.

   enum {
        empty_cert (1), subkey_cert (2), subkey_cert_fingerprint (3),
        (255)
   } 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:
                 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.

3.6.  Other Handshake Messages

All the other handshake messages are identical to the TLS specification.

4.  Security Considerations

All security considerations discussed in [RFC4346] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.), [RFC4366] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.), and [RFC4880] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.) 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] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) 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] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.) 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] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.)).

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] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) 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, Pasi Eronen, Jon Callas, Stephen Kent, Robert Sparks, and Hilarie Orman for their suggestions on improving this document.

7.  References

7.1. Normative References

7.2. Informative References

Author's Address

Full Copyright Statement

Copyright © The IETF Trust (2008).

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.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

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 made any independent effort to identify any such rights. Information 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 assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.