Network Working Group N. Mavrogiannopoulos
Request for Comments: 5081 Independent
Category: Experimental November 2007
Using OpenPGP Keys for Transport Layer Security (TLS) Authentication
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
This memo proposes extensions to the Transport Layer Security (TLS)
protocol to support the OpenPGP 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 ....................................................2
2. Terminology .....................................................2
3. Changes to the Handshake Message Contents .......................2
3.1. Client Hello ...............................................2
3.2. Server Hello ...............................................3
3.3. Server Certificate .........................................3
3.4. Certificate Request ........................................4
3.5. Client Certificate .........................................5
3.6. Other Handshake Messages ...................................5
4. Security Considerations .........................................5
5. IANA Considerations .............................................6
6. Acknowledgements ................................................6
7. References ......................................................6
7.1. Normative References .......................................6
7.2. Informative References .....................................7
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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, TLS [TLS] standards
are defined to use only X.509 certificates. This document 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.
2. Terminology
The term "OpenPGP key" is used in this document as in the OpenPGP
specification [OpenPGP]. We use the term "OpenPGP certificate" to
refer to OpenPGP keys 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 certificates are to be used for authentication.
3.1. Client Hello
In order to indicate the support of multiple certificate types,
clients MUST include an extension of type "cert_type" (see Section 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 contains a
CertificateTypeExtension structure.
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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 certificates. All
existing cipher suites that support a compatible, with the key, key
exchange method can be used in combination with OpenPGP certificates.
3.2. Server Hello
If the server receives a client hello that contains the "cert_type"
extension and chooses a cipher suite that 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 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.
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 certificate in the certificate message. The
certificate must contain a public key that matches the selected key
exchange algorithm, as shown below.
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Key Exchange Algorithm OpenPGP Certificate Type
RSA RSA public key that can be used for
encryption.
DHE_DSS DSS public key that can be used for
authentication.
DHE_RSA RSA public key that can be used for
authentication.
An OpenPGP certificate appearing in the certificate message is sent
using the binary OpenPGP format. The certificate MUST contain all
the elements required by Section 11.1 of [OpenPGP].
The option is also available to send an OpenPGP fingerprint, instead
of sending the entire certificate. The process of fingerprint
generation is described in Section 12.2 of [OpenPGP]. The peer shall
respond with a "certificate_unobtainable" fatal alert if the
certificate with the given fingerprint cannot be found. The
"certificate_unobtainable" fatal alert is defined in Section 4 of
[TLSEXT].
enum {
cert_fingerprint (0), cert (1), (255)
} OpenPGPCertDescriptorType;
opaque OpenPGPCertFingerprint<16..20>;
opaque OpenPGPCert<0..2^24-1>;
struct {
OpenPGPCertDescriptorType descriptorType;
select (descriptorType) {
case cert_fingerprint: OpenPGPCertFingerprint;
case cert: OpenPGPCert;
}
} Certificate;
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.
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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 certificates have been selected and no certificate
is available from the client, then a certificate structure that
contains an empty OpenPGPCert vector MUST be sent. The server SHOULD
respond with a "handshake_failure" fatal alert if client
authentication is required.
3.6. Other Handshake Messages
All the other handshake messages are identical to the TLS
specification.
4. Security Considerations
All security considerations discussed in [TLS], [TLSEXT], and
[OpenPGP] apply to this document. Considerations about the use of
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.
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 although the
"client_certificate_url" extension allows identifying 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 12.2 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.
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5. IANA Considerations
This document defines a new TLS extension, "cert_type", assigned a
value of 9 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 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. 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,
Pasi Eronen, Jon Callas, Stephen Kent, Robert Sparks, and Hilarie
Orman for their suggestions on improving this document.
7. References
7.1. 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 4880, November 2007.
[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.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
7.2. 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.
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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