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+TLS Working Group                                   N. Mavrogiannopoulos
+Internet-Draft                                    Independent Consultant
+Expires: July 19, 2006                                  January 15, 2006
+
+
+               Using OpenPGP keys for TLS authentication
+                     draft-ietf-tls-openpgp-keys-08
+
+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 July 19, 2006.
+
+Copyright Notice
+
+   Copyright (C) The Internet Society (2006).
+
+Abstract
+
+   This memo proposes extensions to the TLS protocol to support the
+   OpenPGP trust model and keys.  The extensions discussed here include
+   a certificate type negotiation mechanism, and the required
+   modifications to the TLS Handshake Protocol.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 1]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+Table of Contents
+
+   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
+   2.  Extension Type . . . . . . . . . . . . . . . . . . . . . . . .  4
+     2.1.  Certificate Type . . . . . . . . . . . . . . . . . . . . .  4
+   3.  Changes to the Handshake Message Contents  . . . . . . . . . .  5
+     3.1.  Client Hello . . . . . . . . . . . . . . . . . . . . . . .  5
+     3.2.  Server Hello . . . . . . . . . . . . . . . . . . . . . . .  5
+     3.3.  Server Certificate . . . . . . . . . . . . . . . . . . . .  6
+     3.4.  Certificate request  . . . . . . . . . . . . . . . . . . .  7
+     3.5.  Client certificate . . . . . . . . . . . . . . . . . . . .  7
+     3.6.  Server key exchange  . . . . . . . . . . . . . . . . . . .  8
+     3.7.  Certificate verify . . . . . . . . . . . . . . . . . . . .  8
+     3.8.  Finished . . . . . . . . . . . . . . . . . . . . . . . . .  8
+   4.  Cipher suites  . . . . . . . . . . . . . . . . . . . . . . . .  9
+   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
+   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
+     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
+   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 12
+   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
+   Intellectual Property and Copyright Statements . . . . . . . . . . 14
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 2]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+1.  Introduction
+
+   At the time of writing, TLS [1] uses the PKIX [4] infrastructure, to
+   provide certificate services.  Currently the PKIX protocols are
+   limited to a hierarchical key management and as a result,
+   applications which follow different - non hierarchical - trust
+   models, like the "web of trust" model, could not be benefited by TLS.
+
+   OpenPGP keys (sometimes called OpenPGP certificates), provide
+   security services for electronic communications.  They are widely
+   deployed, especially in electronic mail applications, provide public
+   key authentication services, and allow distributed key management.
+
+   This document will extend the TLS protocol to support OpenPGP keys
+   and trust model using the existing TLS cipher suites.  In brief this
+   would be achieved by adding a negotiation of the certificate type in
+   addition to the normal handshake negotiations.  Then the required
+   modifications to the handshake messages, in order to hold OpenPGP
+   keys as well, will be described.  The normal handshake procedure with
+   X.509 certificates will not be altered, to preserve compatibility
+   with existing TLS servers and clients.
+
+   This document uses the same notation used in the TLS [1] Protocol
+   specification.
+
+   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 RFC 2119.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 3]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+2.  Extension Type
+
+   A new value, "cert_type(7)", is added to the enumerated
+   ExtensionType, defined in TLSEXT [3].  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 will be used for certificate type negotiation
+   and contains an indicator of the type of the certificate to be used.
+
+2.1.  Certificate Type
+
+   As it can be seen from the CertificateType type at Section 3.1 below,
+   the allowed certificate types are up to 256.  These are segmented in
+   the following way:
+
+   1.  Values 0 (zero) and 1 are defined in this document.
+
+   2.  Values from 2 through 223 decimal (0xDF) inclusive are reserved
+       to be defined by other documents or a future version of this
+       document.
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+   3.  Values from 224 decimal (0xE0) through 255 decimal (0xFF)
+       inclusive are reserved for private use.  Interoperability with
+       these certificate types is a local matter.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 4]
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+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+3.  Changes to the Handshake Message Contents
+
+   This section describes the changes to the TLS handshake message
+   contents when OpenPGP keys are to be used for authentication.
+
+3.1.  Client Hello
+
+   In order to indicate the support of multiple certificate types
+   clients will include an extension of type "cert_type" to the extended
+   client hello message.  The hello extension mechanism is described in
+   TLSEXT [3].
+
+   This extension carries a list of supported certificate types the
+   client can use, sorted by client preference.  This extension SHOULD
+   be omitted if the client only supports X.509 certificates.  The
+   "extension_data" field of this extension will contain 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;
+
+3.2.  Server Hello
+
+   Servers that receive an extended client hello containing the
+   "cert_type" extension, and have chosen a cipher suite that supports
+   certificates, then they MUST 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.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 5]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+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 key in the Certificate message.  The OpenPGP key
+   must contain a public key that matches the selected key exchange
+   algorithm, as shown below.
+
+
+      Key Exchange Algorithm  OpenPGP Key Type
+
+      RSA                     RSA public key which can be used for
+                              encryption.
+
+      DHE_DSS                 DSS public key.
+
+      DHE_RSA                 RSA public key which can be used for
+                              signing.
+
+   An OpenPGP public key appearing in the Certificate message will be
+   sent using the binary OpenPGP format.  The term public key is used to
+   describe a composition of OpenPGP packets to form a block of data
+   which contains all information needed by the peer.  This includes
+   public key packets, user ID packets and all the fields described in
+   "Transferable Public Keys" section in OpenPGP [2].
+
+   The option is also available to send an OpenPGP fingerprint, instead
+   of sending the entire key.  The process of fingerprint generation is
+   described in OpenPGP [2].  The peer shall respond with a
+   "certificate_unobtainable" fatal alert if the key with the given key
+   fingerprint cannot be found.  The "certificate_unobtainable" fatal
+   alert is defined in section 4 of TLSEXT [3].
+
+   If the key is not valid, expired, revoked, corrupt, the appropriate
+   fatal alert message is sent from section A.3 of the TLS
+   specification.  If a key is valid and neither expired nor revoked, it
+   is accepted by the protocol.  The key validation procedure is a local
+   matter outside the scope of this document.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 6]
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+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+      enum {
+           key_fingerprint (0), key (1), (255)
+      } PGPKeyDescriptorType;
+
+      opaque PGPKeyFingerprint<16..20>;
+
+      opaque PGPKey<0..2^24-1>;
+
+      struct {
+           PGPKeyDescriptorType descriptorType;
+           select (descriptorType) {
+                case key_fingerprint: PGPKeyFingerprint;
+                case key: PGPKey;
+           }
+      } Certificate;
+
+3.4.  Certificate request
+
+   The semantics of this message remain the same as in the TLS
+   specification.  However the structure of this message has been
+   modified for OpenPGP keys.  The PGPCertificateRequest structure will
+   only be used if the negotiated certificate type is OpenPGP.
+
+
+     enum {
+         rsa_sign(1), dss_sign(2), (255)
+     } ClientCertificateParamsType;
+
+      struct {
+         ClientCertificateParamsType certificate_params_types<1..2^8-1>;
+     } PGPCertificateRequest;
+
+   The certificate_params_types is a list of accepted client certificate
+   parameter types, sorted in order of the server's preference.
+
+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 keys have been selected, and no key is available
+   from the client, then a Certificate that contains an empty PGPKey
+   should be sent.  The server may respond with a "handshake_failure"
+   fatal alert if client authentication is required.  This transaction
+   follows the TLS specification.
+
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 7]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+3.6.  Server key exchange
+
+   The server key exchange message for OpenPGP keys is identical to the
+   TLS specification.
+
+3.7.  Certificate verify
+
+   The certificate verify message for OpenPGP keys is identical to the
+   TLS specification.
+
+3.8.  Finished
+
+   The finished message for OpenPGP keys is identical to the description
+   in the specification.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 8]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+4.  Cipher suites
+
+   No new cipher suites are required to use OpenPGP keys.  OpenPGP keys
+   can be combined with existing cipher suites defined in TLS [1],
+   except the ones marked as "Exportable".  Exportable cipher suites
+   SHOULD NOT be used with OpenPGP keys.
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+Mavrogiannopoulos         Expires July 19, 2006                 [Page 9]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
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+5.  Security Considerations
+
+   As with X.509 ASN.1 formatted keys, OpenPGP keys need specialized
+   parsers.  Care must be taken to make those parsers safe against
+   maliciously modified keys, that could cause arbitrary code execution.
+
+   Security considerations about the use of the web of trust or the
+   verification procedure are outside the scope of this document and
+   they are considered an issue to be handled by local policy.
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+Mavrogiannopoulos         Expires July 19, 2006                [Page 10]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+6.  References
+
+6.1.  Normative References
+
+   [1]  Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246,
+        January 1999.
+
+   [2]  Callas, J., Donnerhacke, L., Finey, H., and R. Thayer, "OpenPGP
+        Message Format", RFC 2440, November 1998.
+
+   [3]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
+        T. Wright, "TLS Extensions", RFC 3546, June 2003.
+
+6.2.  Informative References
+
+   [4]  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.
+
+   [5]  "Recommendation X.509: The Directory - Authentication
+        Framework", 1988.
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+Mavrogiannopoulos         Expires July 19, 2006                [Page 11]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+Appendix A.  Acknowledgements
+
+   This document was based on earlier work made by Will Price and
+   Michael Elkins.
+
+   The author wishes to thank Werner Koch, David Taylor and Timo Schulz
+   for their suggestions on improving this document.
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+Mavrogiannopoulos         Expires July 19, 2006                [Page 12]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+Author's Address
+
+   Nikos Mavrogiannopoulos
+   Independent Consultant
+   Arkadias 8
+   Halandri, Attiki  15234
+   Greece
+
+   Email: nmav@gnutls.org
+   URI:   http://www.gnutls.org/
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+Mavrogiannopoulos         Expires July 19, 2006                [Page 13]
+
+Internet-Draft  Using OpenPGP keys for TLS authentication   January 2006
+
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+Intellectual Property Statement
+
+   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
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+Disclaimer of Validity
+
+   This document and the information contained herein are provided on an
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+
+Copyright Statement
+
+   Copyright (C) The Internet Society (2006).  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.
+
+
+Acknowledgment
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
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