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TLS Working Group N. Mavroyanopoulos
Internet-Draft February 15, 2002
Expires: August 14, 2002
Using OpenPGP keys for TLS authentication
<draft-ietf-tls-openpgp-keys-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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.
Abstract
This document 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.
This document uses the same notation used in the TLS Protocol draft.
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 RFC 2119.
N. Mavroyanopoulos Expires August 14, 2002 [Page 1]
Internet-Draft Using OpenPGP keys for TLS Authentication January 2002
1. Introduction
At the time of writing, TLS [TLS] uses the X.509 protocols, to
provide certificate services. Currently the X.509 protocols are
limited to a hierarchical key management. As a result, applications
which follow different - non hierarchical - trust models, like the
"web of trust" model, 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, and allow distributed key
management. This document will update the TLS protocol to support
OpenPGP trust model and keys using the existing TLS cipher suites.
2. OpenPGP keys for TLS authentication
The X.509 [X509] certificates recommended for use with TLS will not
be used in conjunction with OpenPGP keys. An implementation SHOULD
be able to support both TLS with X.509 certificates and TLS with
OpenPGP keys. Implementations are not required to support both. The
"peer certificate" in the session state of TLS MAY refer to either
X.509 or OpenPGP.
2.1 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.
2.1.1 Hello Messages
2.1.1.1 Extension Type
A new value, "cert_type(7)", is added to the enumerated
ExtensionType, 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.
2.1.1.2 Client Hello
In order to indicate the support of multiple certificate types
clients will include an extension of type "cert_type" to the
extended client hello message. The 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
SHOULD be omitted if the client supports only X.509 certificates.
N. Mavroyanopoulos Expires August 14, 2002 [Page 2]
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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;
2.1.1.3 Server Hello
Servers that receive an extended client hello containing the
"cert_type" extension 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 "cert_type" extension in the extended server hello.
2.1.2 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.
N. Mavroyanopoulos Expires August 14, 2002 [Page 3]
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An OpenPGP key appearing in the Certificate message will be sent
in binary OpenPGP format. The option is also available to send an
OpenPGP fingerprint, instead of sending the entire key. The
process of fingerprint generation is described in [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 ouside the scope of this document.
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;
2.1.3 Certificate request
The semantics of this message remain the same as in the TLS
specification. However the structure of this message has been
modified for OpenPGP keys. the PGPCertificateRequest structure
will only be used if the negotiated certificate type is OpenPGP.
enum {
rsa_sign(1), dss_sign(2), (255)
} ClientCertificateParamsType;
struct {
ClientCertificateParamsType certificate_params_types<1..2^8-1>;
} PGPCertificateRequest;
certificate_params_types is a list of accepted client certificate
parameter types, sorted in order of the server's preference.
N. Mavroyanopoulos Expires August 14, 2002 [Page 4]
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2.1.4 Client certificate
The client certificate message is sent using the same formatting as
the server certificate message. This message is only sent in response
to the certificate request message. If no OpenPGP key is available
from the client, then a certificate that contains an empty PGPKey is
sent. The server may respond with a "handshake_failure" fatal alert
if client authentication is required. This transaction follows the
TLS specification.
2.1.5 Server key exchange
The server key exchange message for OpenPGP keys is identical to the
TLS specification.
2.1.6 Certificate verify
The certificate verify message for OpenPGP keys is identical to the
TLS specification.
2.1.7 Finished
The finished message for OpenPGP keys is identical to the description
in the specification.
3. Cipher suites
No new cipher suites are required to use OpenPGP keys. OpenPGP keys
can be combined with existing cipher suites defined in [TLS], except
the ones marked as "Exportable". Exportable cipher suites SHOULD NOT
be used with OpenPGP keys.
3.1 New cipher suites
Some additional cipher suites are defined here in order to support
algorithms which are defined in [OpenPGP] but are not present in
[TLS].
CipherSuite TLS_DHE_DSS_WITH_CAST_128_CBC_SHA = { 0x00, 0x70 };
CipherSuite TLS_DHE_DSS_WITH_CAST_128_CBC_RMD = { 0x00, 0x71 };
CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_RMD = { 0x00, 0x72 };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_RMD = { 0x00, 0x73 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_RMD = { 0x00, 0x74 };
CipherSuite TLS_DHE_RSA_WITH_CAST_128_CBC_SHA = { 0x00, 0x75 };
CipherSuite TLS_DHE_RSA_WITH_CAST_128_CBC_RMD = { 0x00, 0x76 };
CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_RMD = { 0x00, 0x77 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_RMD = { 0x00, 0x78 };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_RMD = { 0x00, 0x79 };
N. Mavroyanopoulos Expires August 14, 2002 [Page 5]
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CipherSuite TLS_RSA_WITH_CAST_128_CBC_SHA = { 0x00, 0x7A };
CipherSuite TLS_RSA_WITH_CAST_128_CBC_RMD = { 0x00, 0x7B };
CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_RMD = { 0x00, 0x7C };
CipherSuite TLS_RSA_WITH_AES_128_CBC_RMD = { 0x00, 0x7D };
CipherSuite TLS_RSA_WITH_AES_256_CBC_RMD = { 0x00, 0x7E };
All of the above cipher suites use either the CAST [CAST],
AES [AES], or 3DES block ciphers in CBC mode. The choice of hash
is either SHA-1 or RIPEMD-160. Implementations are not required
to support the above cipher suites.
References
[TLS] T. Dierks, and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[OpenPGP] Callas, J., Donnerhacke, L., Finney, H., Thayer, R.,
"OpenPGP Message Format", RFC 2440, November 1998.
[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.
and Wright, T., "TLS Extensions", work in progress,
December 2001.
[X509] CCITT. Recommendation X.509: "The Directory - Authentication
Framework". 1988.
[CAST] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
May 1997.
[AES] J. Daemen, V. Rijmen, "The Rijndael Block Cipher"
http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf
3rd September 1999.
Author's Address
Nikos Mavroyanopoulos
nmav@gnutls.org
N. Mavroyanopoulos Expires August 14, 2002 [Page 6]
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Full Copyright Statement
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Acknowledgement
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N. Mavroyanopoulos Expires August 14, 2002 [Page 7]
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