< draft-ietf-tls-openpgp-keys-10.txt		draft-ietf-tls-openpgp-keys-11.txt >
	TLS Working Group N. Mavrogiannopoulos		TLS Working Group N. Mavrogiannopoulos
	Internet-Draft Independent		Internet-Draft Independent
	Expires: December 7, 2006 June 5, 2006		Expires: February 1, 2007 July 31, 2006
	Using OpenPGP keys for TLS authentication		Using OpenPGP keys for TLS authentication
	draft-ietf-tls-openpgp-keys-10		draft-ietf-tls-openpgp-keys-11
	Status of this Memo		Status of this Memo
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	Copyright Notice		Copyright Notice
	Copyright (C) The Internet Society (2006).		Copyright (C) The Internet Society (2006).
	Abstract		Abstract
	This memo proposes extensions to the TLS protocol to support the		This memo proposes extensions to the TLS protocol to support the
	OpenPGP trust model and keys. The extensions discussed here include		OpenPGP key format. The extensions discussed here include a
	a certificate type negotiation mechanism, and the required		certificate type negotiation mechanism, and the required
	modifications to the TLS Handshake Protocol.		modifications to the TLS Handshake Protocol.
			Table of Contents
			1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
			2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 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. Other Handshake messages . . . . . . . . . . . . . . . . . 7
			4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
			5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
			6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
			6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
			6.2. Informative References . . . . . . . . . . . . . . . . . . 10
			Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
			Intellectual Property and Copyright Statements . . . . . . . . . . 13
	1. Introduction		1. Introduction
	At the time of writing, TLS [TLS] uses the PKIX [PKIX]		The IETF has two sets of standards for public key certificates, one
	infrastructure, to provide certificate services. Currently the PKIX		set for use of X.509 certificates [PKIX] and one for OpenPGP
	protocols are limited to a hierarchical key management and as a		certificates [OpenPGP]. At the time of writing, the TLS [TLS]
	result, applications which follow different - non hierarchical -		standards are defined to use only X.509 certificates. This document
	trust models, could not be benefited by TLS.		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.
	OpenPGP keys (sometimes called OpenPGP certificates), provide		2. Terminology
	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		The term ``OpenPGP key'' is used in this document as in the OpenPGP
	using the existing TLS cipher suites. In brief this would be		specification [OpenPGP]. We use the term ``OpenPGP certificate'' to
	achieved by adding a negotiation of the certificate type in addition		refer to OpenPGP keys that are enabled for authentication.
	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		This document uses the same notation and terminology used in the TLS
	specification [TLS].		Protocol specification [TLS].
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	2. Changes to the Handshake Message Contents		3. Changes to the Handshake Message Contents
	This section describes the changes to the TLS handshake message		This section describes the changes to the TLS handshake message
	contents when OpenPGP keys are to be used for authentication.		contents when OpenPGP certificates are to be used for authentication.
	2.1. Client Hello		3.1. Client Hello
	In order to indicate the support of multiple certificate types		In order to indicate the support of multiple certificate types
	clients will include an extension of type "cert_type" (see Section 4)		clients MUST include an extension of type "cert_type" (see Section 5)
	to the extended client hello message. The hello extension mechanism		to the extended client hello message. The hello extension mechanism
	is described in [TLSEXT].		is described in [TLSEXT].
	This extension carries a list of supported certificate types the		This extension carries a list of supported certificate types the
	client can use, sorted by client preference. This extension MUST be		client can use, sorted by client preference. This extension MUST be
	omitted if the client only supports X.509 certificates. The		omitted if the client only supports X.509 certificates. The
	"extension_data" field of this extension will contain a		"extension_data" field of this extension contains a
	CertificateTypeExtension structure.		CertificateTypeExtension structure.
	enum { client, server } ClientOrServerExtension;		enum { client, server } ClientOrServerExtension;
	enum { X.509(0), OpenPGP(1), (255) } CertificateType;		enum { X.509(0), OpenPGP(1), (255) } CertificateType;
	struct {		struct {
	select(ClientOrServerExtension) {		select(ClientOrServerExtension) {
	case client:		case client:
	CertificateType certificate_types<1..2^8-1>;		CertificateType certificate_types<1..2^8-1>;
	case server:		case server:
	CertificateType certificate_type;		CertificateType certificate_type;
	}		}
	} CertificateTypeExtension;		} CertificateTypeExtension;
	No new cipher suites are required to use OpenPGP keys. All existing		No new cipher suites are required to use OpenPGP certificates. All
	cipher suites that support a compatible with the key, key exchange		existing cipher suites that support a compatible, with the key, key
	method can be used in combination with OpenPGP keys.		exchange method can be used in combination with OpenPGP certificates.
	2.2. Server Hello		3.2. Server Hello
	Servers that receive an extended client hello containing the		If the server receives a client hello that contains the "cert_type"
	"cert_type" extension, and have chosen a cipher suite that supports		extension and chooses a cipher suite that requires a certificate,
	certificates, they MUST select a certificate type from the		then two outcomes are possible. The server MUST either select a
	certificate_types field in the extended client hello, or terminate		certificate type from the certificate_types field in the extended
	the connection with a fatal alert of type "unsupported_certificate".		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		The certificate type selected by the server is encoded in a
	CertificateTypeExtension structure, which is included in the extended		CertificateTypeExtension structure, which is included in the extended
	server hello message, using an extension of type "cert_type".		server hello message using an extension of type "cert_type". Servers
	Servers that only support X.509 certificates MAY omit including the		that only support X.509 certificates MAY omit including the
	"cert_type" extension in the extended server hello.		"cert_type" extension in the extended server hello.
	2.3. Server Certificate		3.3. Server Certificate
	The contents of the certificate message sent from server to client		The contents of the certificate message sent from server to client
	and vice versa are determined by the negotiated certificate type and		and vice versa are determined by the negotiated certificate type and
	the selected cipher suite's key exchange algorithm.		the selected cipher suite's key exchange algorithm.
	If the OpenPGP certificate type is negotiated then it is required to		If the OpenPGP certificate type is negotiated then it is required to
	present an OpenPGP key in the Certificate message. The OpenPGP key		present an OpenPGP certificate in the Certificate message. The
	must contain a public key that matches the selected key exchange		certificate must contain a public key that matches the selected key
	algorithm, as shown below.		exchange algorithm, as shown below.
	Key Exchange Algorithm OpenPGP Key Type		Key Exchange Algorithm OpenPGP Certificate Type
	RSA RSA public key which can be used for		RSA RSA public key which can be used for
	encryption.		encryption.
	DHE_DSS DSS public key.		DHE_DSS DSS public key which can be used for
			authentication.
	DHE_RSA RSA public key which can be used for		DHE_RSA RSA public key which can be used for
	signing.		authentication.
	An OpenPGP public key appearing in the Certificate message will be		An OpenPGP certificate appearing in the Certificate message is sent
	sent using the binary OpenPGP format. The term public key is used to		using the binary OpenPGP format. The certificate MUST contain all
	describe a composition of OpenPGP packets to form a block of data		the elements required by Section 10.1 of [OpenPGP].
	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		The option is also available to send an OpenPGP fingerprint, instead
	of sending the entire key. The process of fingerprint generation is		of sending the entire certificate. The process of fingerprint
	described in section 11.2 of [OpenPGP]. The peer shall respond with		generation is described in section 11.2 of [OpenPGP]. The peer shall
	a "certificate_unobtainable" fatal alert if the key with the given		respond with a "certificate_unobtainable" fatal alert if the
	key fingerprint cannot be found. The "certificate_unobtainable"		certificate with the given fingerprint cannot be found. The
	fatal alert is defined in section 4 of [TLSEXT].		"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.
	enum {		enum {
	key_fingerprint (0), key (1), (255)		cert_fingerprint (0), cert (1), (255)
	} PGPKeyDescriptorType;		} OpenPGPCertDescriptorType;
	opaque PGPKeyFingerprint<16..20>;		opaque OpenPGPCertFingerprint<16..20>;
	opaque PGPKey<0..2^24-1>;		opaque OpenPGPCert<0..2^24-1>;
	struct {		struct {
	PGPKeyDescriptorType descriptorType;		OpenPGPCertDescriptorType descriptorType;
	select (descriptorType) {		select (descriptorType) {
	case key_fingerprint: PGPKeyFingerprint;		case cert_fingerprint: OpenPGPCertFingerprint;
	case key: PGPKey;		case cert: OpenPGPCert;
	}		}
	} Certificate;		} Certificate;
	2.4. Certificate request		3.4. Certificate request
	The semantics of this message remain the same as in the TLS		The semantics of this message remain the same as in the TLS
	specification. However if this message is sent, and the negotiated		specification. However if this message is sent, and the negotiated
	certificate type is OpenPGP, the "certificate_authorities" list MUST		certificate type is OpenPGP, the "certificate_authorities" list MUST
	be empty.		be empty.
	2.5. Client certificate		3.5. Client certificate
	This message is only sent in response to the certificate request		This message is only sent in response to the certificate request
	message. The client certificate message is sent using the same		message. The client certificate message is sent using the same
	formatting as the server certificate message and it is also required		formatting as the server certificate message and it is also required
	to present a certificate that matches the negotiated certificate		to present a certificate that matches the negotiated certificate
	type. If OpenPGP keys have been selected, and no key is available		type. If OpenPGP certificates have been selected and no certificate
	from the client, then a Certificate that contains an empty PGPKey		is available from the client, then a Certificate structure that
	should be sent. The server may respond with a "handshake_failure"		contains an empty OpenPGPCert vector MUST be sent. The server SHOULD
	fatal alert if client authentication is required.		respond with a "handshake_failure" fatal alert if client
			authentication is required.
	2.6. Other Handshake messages		3.6. Other Handshake messages
	The rest of the handshake messages such as the server key exchange,		All the other handshake messages are identical to the TLS
	the certificate verify and the finished messages are identical to the		specification.
	TLS specification.
	3. Security Considerations		4. Security Considerations
	As with X.509 ASN.1 formatted keys, OpenPGP keys need specialized		All security considerations discussed in [TLS], [TLSEXT] as well as
	parsers. Care must be taken to make those parsers safe against		[OpenPGP] apply to this document. Considerations about the use of
	maliciously modified keys, that could cause arbitrary code execution.		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.
	Security considerations about the use of the web of trust or the		The protocol for certificate type negotiation is identical in
	verification procedure are outside the scope of this document and		operation to ciphersuite negotiation of the [TLS] specification with
	they are considered an issue to be handled by local policy.		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.
	4. IANA Considerations		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 while the "client_certificate_url"
			extension allows to identify 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 11.2 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.
			5. IANA Considerations
	This document defines a new TLS extension, "cert_type", assigned a		This document defines a new TLS extension, "cert_type", assigned a
	value of TBD-BY-IANA (the value 7 is suggested) from the TLS		value of TBD-BY-IANA (the value 7 is suggested) from the TLS
	ExtensionType registry defined in [TLSEXT]. This value is used as		ExtensionType registry defined in [TLSEXT]. This value is used as
	the extension number for the extensions in both the client hello		the extension number for the extensions in both the client hello
	message and the server hello message. The new extension type will be		message and the server hello message. The new extension type is used
	used for certificate type negotiation.		for certificate type negotiation.
	The "cert_type" extension contains an 8-bit CertificateType field,		The "cert_type" extension contains an 8-bit CertificateType field,
	for which a new registry, named "TLS Certificate Types", is		for which a new registry, named "TLS Certificate Types", is
	established in this document, to be maintained by IANA. The registry		established in this document, to be maintained by IANA. The registry
	is segmented in the following way:		is segmented in the following way:
	1. Values 0 (X.509) and 1 (OpenPGP) are defined in this document.		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		2. Values from 2 through 223 decimal inclusive are assigned via IETF
	Consensus [RFC2434].		Consensus [RFC2434].
	3. Values from 224 decimal through 255 decimal inclusive are		3. Values from 224 decimal through 255 decimal inclusive are
	reserved for Private Use [RFC2434].		reserved for Private Use [RFC2434].
	5. References		6. References
	5.1. Normative References		6.1. Normative References
	[TLS] Dierks, T. and E. Rescorla, "The TLS Protocol Version		[TLS] Dierks, T. and E. Rescorla, "The TLS Protocol Version
	1.1", RFC 4346, April 2006.		1.1", RFC 4346, April 2006.
	[OpenPGP] Callas, J., Donnerhacke, L., Finey, H., and R. Thayer,		[OpenPGP] Callas, J., Donnerhacke, L., Finey, H., Shaw, D., and R.
	"OpenPGP Message Format", RFC 2440, November 1998.		Thayer, "OpenPGP Message Format",
			draft-ietf-openpgp-rfc2440bis-18 (work in progress),
			May 2006.
	[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,		[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
	and T. Wright, "Transport Layer Security (TLS)		and T. Wright, "Transport Layer Security (TLS)
	Extensions", RFC 4366, April 2006.		Extensions", RFC 4366, April 2006.
	[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an		[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
	IANA Considerations Section in RFCs", RFC 2434,		IANA Considerations Section in RFCs", RFC 2434,
	October 1998.		October 1998.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", RFC 2119, March 1997.		Requirement Levels", RFC 2119, March 1997.
	5.2. Informative References		6.2. Informative References
	[PKIX] Housley, R., Ford, W., Polk, W., and D. Solo, "Internet X.509		[PKIX] Housley, R., Ford, W., Polk, W., and D. Solo, "Internet X.509
	Public Key Infrastructure Certificate and Certificate		Public Key Infrastructure Certificate and Certificate
	Revocation List (CRL) Profile", RFC 3280, April 2002.		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.
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	This document was based on earlier work made by Will Price and		This document was based on earlier work made by Will Price and
	Michael Elkins.		Michael Elkins.
	The author wishes to thank Werner Koch, David Taylor, Timo Schulz and		The author wishes to thank Werner Koch, David Taylor, Timo Schulz,
	Pasi Eronen for their suggestions on improving this document.		Pasi Eronen, Jon Callas, Stephen Kent, Robert Sparks and Hilarie
			Orman for their suggestions on improving this document.
	Author's Address		Author's Address
	Nikos Mavrogiannopoulos		Nikos Mavrogiannopoulos
	Independent		Independent
	Arkadias 8		Arkadias 8
	Halandri, Attiki 15234		Halandri, Attiki 15234
	Greece		Greece
	Email: nmav@gnutls.org		Email: nmav@gnutls.org
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