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+TLS Working Group N. Mavrogiannopoulos
+Internet-Draft Independent
+Expires: December 7, 2006 June 5, 2006
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+ Using OpenPGP keys for TLS authentication
+ draft-ietf-tls-openpgp-keys-10
+
+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 December 7, 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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+1. Introduction
+
+ At the time of writing, TLS [TLS] uses the PKIX [PKIX]
+ 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, 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, allow distributed key management and can
+ be used with a non hierarchical trust model called the "web of trust"
+ [WOT].
+
+ This document will extend the TLS protocol to support OpenPGP keys
+ 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 is not altered, to preserve compatibility with
+ existing TLS servers and clients.
+
+ This document uses the same notation 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].
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+2. 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.
+
+2.1. Client Hello
+
+ In order to indicate the support of multiple certificate types
+ clients will include an extension of type "cert_type" (see Section 4)
+ 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 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;
+
+ No new cipher suites are required to use OpenPGP keys. All existing
+ cipher suites that support a compatible with the key, key exchange
+ method can be used in combination with OpenPGP keys.
+
+2.2. Server Hello
+
+ Servers that receive an extended client hello containing the
+ "cert_type" extension, and have chosen a cipher suite that supports
+ certificates, 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
+
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+ "cert_type" extension in the extended server hello.
+
+2.3. 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
+ section 10.1 of [OpenPGP].
+
+ The option is also available to send an OpenPGP fingerprint, instead
+ of sending the entire key. The process of fingerprint generation is
+ described in section 11.2 of [OpenPGP]. 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].
+
+ 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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+ 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;
+
+2.4. 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.
+
+2.5. 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.
+
+2.6. Other Handshake messages
+
+ The rest of the handshake messages such as the server key exchange,
+ the certificate verify and the finished messages are identical to the
+ TLS specification.
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+3. 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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+4. IANA Considerations
+
+ This document defines a new TLS extension, "cert_type", assigned a
+ value of TBD-BY-IANA (the value 7 is suggested) 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 will be
+ 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].
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+ 3. Values from 224 decimal through 255 decimal inclusive are
+ reserved for Private Use [RFC2434].
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+5. References
+
+5.1. 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., and R. Thayer,
+ "OpenPGP Message Format", RFC 2440, November 1998.
+
+ [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.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", RFC 2119, March 1997.
+
+5.2. 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.
+
+ [WOT] Abdul-Rahman, A., "The PGP Trust Model", EDI Forum: The
+ Journal of Electronic Commerce, April 1997.
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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, Timo Schulz and
+ Pasi Eronen for their suggestions on improving this document.
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+Author's Address
+
+ Nikos Mavrogiannopoulos
+ Independent
+ Arkadias 8
+ Halandri, Attiki 15234
+ Greece
+
+ Email: nmav@gnutls.org
+ URI: http://www.gnutls.org/
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+Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
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+Disclaimer of Validity
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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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