Using OpenPGP Keys for Transport Layer Security (TLS) Authentication
draft-mavrogiannopoulos-rfc5081bis-03

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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. This memo replaces the Experimental [RFC5081] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.).

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
Appendix A.  Changes from RFC 5081
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 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) standards are defined to use 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.

The major changes from [RFC5081] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) are summarized in Appendix A (Changes from RFC 5081).

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

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. Note that the CertificateTypeExtension structure is being used both by the client and the server, although specified once in this document, a practice common in the TLS protocol specification [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).

   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.

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 and, 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                 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] (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 ID to be used for this session is explicitely specified in the OpenPGPKeyID field. The key ID must be specified even if the certificate has only a primary key. 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 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 4 of [RFC4366] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.).

Implementations of this protocol MUST ensure that the sizes, of key IDs and fingerprints, in the OpenPGPSubKeyCert and OpenPGPSubKeyFingerprint structures comply with [RFC4880] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.). 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] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.).

The enumerated 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. The "empty_cert" type has replaced "cert" and is a backwards 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.

   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.

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] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), [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 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) 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.). Existing IANA references should be updated to point to this document.

In addition the "TLS Certificate Types" registry established by [RFC5081] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) has to be updated 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 "RFC Required" [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).
3. Values from 224 decimal through 255 decimal inclusive are reserved for Private Use [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

6.  Acknowledgements

The author wishes to thank Daniel Kahn Gillmor and Alfred Hoenes for their suggestions on improving this document.

Appendix A.  Changes from RFC 5081

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

[RFC5081] (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) required that an OpenPGP key or subkey was marked with the authentication flag and thus would have failed if this flag was not set, or this flag was set in more than one subkeys. The protocol in this memo has no such limitation.

7.  References

7.1. Normative References

7.2. Informative References

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