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Network Working Group                               N. MavrogiannopoulosRequest for Comments: 5081                                   IndependentCategory: Experimental                                     November 2007Using OpenPGP Keys for Transport Layer Security (TLS) AuthenticationStatus 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 Contents1. Introduction ....................................................22. Terminology .....................................................23. Changes to the Handshake Message Contents .......................23.1. Client Hello ...............................................23.2. Server Hello ...............................................33.3. Server Certificate .........................................33.4. Certificate Request ........................................43.5. Client Certificate .........................................53.6. Other Handshake Messages ...................................54. Security Considerations .........................................55. IANA Considerations .............................................66. Acknowledgements ................................................67. References ......................................................67.1. Normative References .......................................67.2. Informative References .....................................7Mavrogiannopoulos             Experimental                      [Page 1]

RFC 5081                   Using OpenPGP Keys              November 20071.  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" (seeSection 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 20073.5.  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 20075.  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.  References7.1.  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 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 inBCP 78 andBCP 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 athttp://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.Mavrogiannopoulos             Experimental                      [Page 8]