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rfc:rfc5081

Network Working Group N. Mavrogiannopoulos Request for Comments: 5081 Independent Category: Experimental November 2007

Using OpenPGP Keys for Transport Layer Security (TLS) Authentication

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

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

Mavrogiannopoulos Experimental [Page 1] RFC 5081 Using OpenPGP Keys November 2007

1. Introduction

 The IETF has two sets of standards for public key certificates, one
 set for use of X.509 certificates [PKIX] and one for OpenPGP
 certificates [OpenPGP].  At the time of writing, TLS [TLS] 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 [OpenPGP].  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 [TLS].
 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" (see Section 5)
 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 contains a
 CertificateTypeExtension structure.

Mavrogiannopoulos Experimental [Page 2] RFC 5081 Using OpenPGP Keys November 2007

    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 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
 "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.

Mavrogiannopoulos Experimental [Page 3] RFC 5081 Using OpenPGP Keys November 2007

    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 [OpenPGP].
 The option is also available to send an OpenPGP fingerprint, instead
 of sending the entire certificate.  The process of fingerprint
 generation is described in Section 12.2 of [OpenPGP].  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
 [TLSEXT].
    enum {
         cert_fingerprint (0), cert (1), (255)
    } OpenPGPCertDescriptorType;
    opaque OpenPGPCertFingerprint<16..20>;
    opaque OpenPGPCert<0..2^24-1>;
    struct {
         OpenPGPCertDescriptorType descriptorType;
         select (descriptorType) {
              case cert_fingerprint: OpenPGPCertFingerprint;
              case cert: OpenPGPCert;
         }
    } 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.

Mavrogiannopoulos Experimental [Page 4] RFC 5081 Using OpenPGP Keys November 2007

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 that
 contains an empty OpenPGPCert vector 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 [TLS], [TLSEXT], and
 [OpenPGP] 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 [TLS] 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 [TLSEXT] 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
 [OpenPGP]).
 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.

Mavrogiannopoulos Experimental [Page 5] RFC 5081 Using OpenPGP Keys November 2007

5. IANA Considerations

 This document defines a new TLS extension, "cert_type", assigned a
 value of 9 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 is 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].

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

 [TLS]      Dierks, T. and E. Rescorla, "The TLS Protocol Version
            1.1", RFC 4346, April 2006.
 [OpenPGP]  Callas, J., Donnerhacke, L., Finey, H., Shaw, D., and R.
            Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
 [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.

Mavrogiannopoulos Experimental [Page 6] RFC 5081 Using OpenPGP Keys November 2007

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

7.2. 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.

Author's Address

 Nikos Mavrogiannopoulos
 Independent
 Arkadias 8
 Halandri, Attiki  15234
 Greece
 EMail: nmav@gnutls.org
 URI:   http://www.gnutls.org/

Mavrogiannopoulos Experimental [Page 7] RFC 5081 Using OpenPGP Keys November 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 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.
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Mavrogiannopoulos Experimental [Page 8]

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