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rfc:rfc6091

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.

Mavrogiannopoulos & Gillmor Informational [Page 1] RFC 6091 Using OpenPGP Keys February 2011

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].

Mavrogiannopoulos & Gillmor Informational [Page 2] RFC 6091 Using OpenPGP Keys February 2011

 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.

Mavrogiannopoulos & Gillmor Informational [Page 3] RFC 6091 Using OpenPGP Keys February 2011

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

Mavrogiannopoulos & Gillmor Informational [Page 4] RFC 6091 Using OpenPGP Keys February 2011

 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.

Mavrogiannopoulos & Gillmor Informational [Page 5] RFC 6091 Using OpenPGP Keys February 2011

    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.

Mavrogiannopoulos & Gillmor Informational [Page 6] RFC 6091 Using OpenPGP Keys February 2011

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.

Mavrogiannopoulos & Gillmor Informational [Page 7] RFC 6091 Using OpenPGP Keys February 2011

 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.

Mavrogiannopoulos & Gillmor Informational [Page 8] RFC 6091 Using OpenPGP Keys February 2011

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

Mavrogiannopoulos & Gillmor Informational [Page 9]

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