TLS Working Group N. Mavroyanopoulos
Internet-Draft January 22, 2002
Expires: July 21, 2002
Using OpenPGP keys for TLS authentication
<draft-ietf-tls-openpgp-keys-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document builds upon the TLS Protocol Specification [TLS]. The
extensions described herein are intended to apply to Version 1.0 of
the TLS specification.
The purpose of this document is to update the TLS protocol with
extensions to support the certificates, public key algorithms,
symmetric ciphers, hash algorithms, and trust model used by OpenPGP
[OpenPGP].
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.
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1. Introduction
At the time of writing, TLS [TLS] uses X.509 digital certificiates,
or Kerberos, for authentication.
OpenPGP keys (sometimes called OpenPGP certificates), provide
security services for electronic communications. This document will
will update the TLS protocol with extensions to support these keys.
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)", has been 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. This value was chosen based on the
version of defined in [TLSEXT] that was current at the time of
writing, so may be changed in future.
2.1.1.2 Client Hello
An extension of type CertificateTypeExtension is appended to the
standard client hello message using the client hello extension
mechanism defined in [TLSEXT].
This extension carries a list of supported Certificate types the
client can use, sorted by client preference. This extension
SHOULD NOT be used if the client supports only X.509 certificates.
enum { client, server } ClientOrServerExtension;
enum { X.509(0), OpenPGP(1), (255) } CertificateType;
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struct {
select(ClientOrServerExtension) {
case client:
CertificateType certificate_type<1..2^8-1>;
case server:
CertificateType certificate_type;
}
} CertificateTypeExtension;
2.1.1.3 Server Hello
The certificate type selected by the server (certificate_type),
is appended to the server hello message using the hello
extension mechanism defined in [TLSEXT].
The certificate type selected by the server (certificate_type),
is encoded in an CertificateTypeExtension structure, which is
sent in an extended server hello message, using an extension of
type "cert_type".
The CertificateTypeExtension structure may be omited if the server
only supports X.509 certificates. In case the server does not
support any of the certificate types sent by the client, the
server should terminate the connection with a fatal alert of
"unsupported_certificate" type.
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.
In case 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 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
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peer will 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 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.
enum {
rsa_sign(1), dss_sign(2), (255)
} ClientCertificateType;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
} CertificateRequest;
certificate_types is a list of the types of certificates requested,
sorted in order of the server's preference.
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
returned. The server may respond with a "handshake_failure" fatal
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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 key types is identical to
the TLS specification.
2.1.7 Finished
The finished message for OpenPGP key types 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 };
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
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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@gnu.org
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