TLS Working Group N. Mavroyanopoulos
Internet-Draft November 10, 2003
Expires: May 9, 2004
Using OpenPGP keys for TLS authentication
<draft-ietf-tls-openpgp-keys-04.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 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.
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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. 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.
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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, and have chosen a cipher suite that supports
certificates, then 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 "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.
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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 "Transferable Public Keys"
section in [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 [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.
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;
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certificate_params_types is a list of accepted client certificate
parameter types, 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
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 };
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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
is either SHA-1 or RIPEMD-160. Implementations are not required
to support the above cipher suites.
4. Security considerations
This document adds support for OpenPGP keys in TLS 1.0. Since the
OpenPGP keys can be viewed as an other format to hold public key
parameters, no additional threats are introduced to the TLS protocol.
All the security considerations in the TLS [TLS] and the OpenPGP
[OpenPGP] specifications still apply.
5. Acknowledgments
The author wishes to thank Werner Koch, David Taylor and Timo Schulz for
their suggestions on improving this document.
5. References
[TLS] T. Dierks, and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[OpenPGP] J. Callas, L. Donnerhacke, H. Finney, R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[TLSEXT] S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
and T. Wright, "TLS Extensions", RFC3546, June 2003.
[X509] CCITT. Recommendation X.509: "The Directory - Authentication
Framework". 1988.
[PKIX] R. Housley, W. Ford, W. Polk, D. Solo, "Internet X.509
Public Key Infrastructure Certificate and CRL Profile",
RFC 2459, January 1999.
[CAST] C. Adams, "The CAST-128 Encryption Algorithm", RFC 2144,
May 9997.
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[AES] P. Chown, "Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
Author's Address
Nikos Mavroyanopoulos
8 Arkadias Street
Chalandri 15234
Greece
Email: nmav@gnutls.org
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