TLS Working Group N. Mavrogiannopoulos
Internet-Draft Independent
Expires: December 7, 2006 June 5, 2006
Using OpenPGP keys for TLS authentication
draft-ietf-tls-openpgp-keys-10
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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].
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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