wdiff draft-ietf-tls-openpgp-keys-10.txt draft-ietf-tls-openpgp-keys-11.txt

TLS Working Group N. Mavrogiannopoulos
Internet-Draft Independent
Expires: December 7, 2006 June 5, February 1, 2007 July 31, 2006

Using OpenPGP keys for TLS authentication
draft-ietf-tls-openpgp-keys-10
draft-ietf-tls-openpgp-keys-11

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Abstract

This memo proposes extensions to the TLS protocol to support the
OpenPGP trust model and keys. key format. The extensions discussed here include a
certificate type negotiation mechanism, and the required
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

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, the TLS [TLS] uses the PKIX [PKIX]
infrastructure, to provide certificate services. Currently the PKIX
protocols
standards are limited defined to use only X.509 certificates. This document
specifies 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 way to negotiate use of OpenPGP certificates), provide
security services certificates 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
session, and specifies how to support transport OpenPGP keys
using certificates via TLS.
The proposed extensions are backward compatible with the existing current TLS cipher suites. In brief this would be
achieved by adding a negotiation
specification, so that existing client and server implementations
that make use of the certificate type X.509 certificates are not affected.

2. Terminology

The term ``OpenPGP key'' is used in this document as in addition
to the normal handshake negotiations. Then OpenPGP
specification [OpenPGP]. We use the required
modifications term ``OpenPGP certificate'' to the handshake messages, in order
refer 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. 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 keys certificates are to be used for authentication.

2.1.

3.1. Client Hello

In order to indicate the support of multiple certificate types
clients will MUST include an extension of type "cert_type" (see Section 4) 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 will contain contains 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. certificates. All
existing cipher suites that support a compatible compatible, with the key, key
exchange method can be used in combination with OpenPGP keys.

2.2. certificates.

3.2. Server Hello

Servers that receive an extended

If the server receives a client hello containing that contains the "cert_type" extension,
extension and have chosen chooses a cipher suite that supports
certificates, they 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, hello or terminate the connection with a fatal alert of type
"unsupported_certificate".

The certificate type selected by the server, server is encoded in a
CertificateTypeExtension structure, which is included in the extended
server hello message, 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.

2.3.

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 certificate in the Certificate message. The OpenPGP key
certificate must contain a public key that matches the selected key
exchange algorithm, as shown below.

Key Exchange Algorithm OpenPGP Key Certificate Type

RSA RSA public key which can be used for
encryption.

DHE_DSS DSS public key. key which can be used for
authentication.

DHE_RSA RSA public key which can be used for
signing.
authentication.

An OpenPGP public key certificate appearing in the Certificate message will be is 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 certificate MUST contain all
the fields described in
section elements required by Section 10.1 of [OpenPGP].

The option is also available to send an OpenPGP fingerprint, instead
of sending the entire key. certificate. 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
certificate 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.

enum {
key_fingerprint
cert_fingerprint (0), key cert (1), (255)
} PGPKeyDescriptorType; OpenPGPCertDescriptorType;

opaque PGPKeyFingerprint<16..20>; OpenPGPCertFingerprint<16..20>;

opaque PGPKey<0..2^24-1>; OpenPGPCert<0..2^24-1>;

struct {
PGPKeyDescriptorType
OpenPGPCertDescriptorType descriptorType;
select (descriptorType) {
case key_fingerprint: PGPKeyFingerprint; cert_fingerprint: OpenPGPCertFingerprint;
case key: PGPKey; cert: OpenPGPCert;
}
} Certificate;

2.4.

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.

2.5.

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 certificates have been selected, selected and no key certificate
is available from the client, then a Certificate structure that
contains an empty PGPKey
should OpenPGPCert vector MUST be sent. The server may SHOULD
respond with a "handshake_failure" fatal alert if client
authentication is required.

2.6.

3.6. Other Handshake messages

The rest of

All the other handshake messages such as the server key exchange,
the certificate verify and the finished messages are identical to the TLS
specification.

4. 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

All security considerations discussed in [TLS], [TLSEXT] as well as
[OpenPGP] apply to this document. Considerations about the use of
the web of trust or the identity and certificate verification procedure
are outside the scope of this document and
they document. These are considered an issue issues
to be handled by local policy.

4. 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 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
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 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. References

5.1.

6.1. 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 2440, November 1998.
draft-ietf-openpgp-rfc2440bis-18 (work in progress),
May 2006.

[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.

6.2. 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.

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 Schulz,
Pasi Eronen Eronen, Jon Callas, Stephen Kent, Robert Sparks and Hilarie
Orman for their suggestions on improving this document.

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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