TLS Working Group N. Mavroyanopoulos
Internet-Draft April 2, 2004
Expires: October 1, 2004
Using OpenPGP keys for TLS authentication
draft-ietf-tls-openpgp-keys-05
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Extension Type . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Changes to the Handshake Message Contents . . . . . . . . . . 5
3.1 Client Hello . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Server Hello . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Server Certificate . . . . . . . . . . . . . . . . . . . . . . 6
3.4 Certificate request . . . . . . . . . . . . . . . . . . . . . 7
3.5 Client certificate . . . . . . . . . . . . . . . . . . . . . . 7
3.6 Server key exchange . . . . . . . . . . . . . . . . . . . . . 8
3.7 Certificate verify . . . . . . . . . . . . . . . . . . . . . . 8
3.8 Finished . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Cipher suites . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Internationalization Considerations . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
Normative References . . . . . . . . . . . . . . . . . . . . . 12
Informative References . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
At the time of writing, TLS [1] uses the PKIX [6] 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, 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 extend the TLS protocol to support OpenPGP keys
and trust model 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 the normal handshake procedure
with X.509 certificates will not be altered, to preserve
compatibility with existing TLS servers and clients.
This document uses the same notation used in the TLS Protocol
specification.
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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2. Extension Type
A new value, "cert_type(7)", is added to the enumerated
ExtensionType, defined in TLSEXT [3]. This value is used as the
extension number for the extensions in both the client hello message
and the server hello message. This new extension type will be used
for certificate type negotiation.
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3. 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.
3.1 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 hello extension mechanism is described in
TLSEXT [3].
This extension carries a list of supported certificate types the
client can use, sorted by client preference. This extension MAY 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;
3.2 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.
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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 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
"Transferable Public Keys" section in OpenPGP [2].
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 [2]. 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 [3].
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;
3.4 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;
The certificate_params_types is a list of accepted client certificate
parameter types, sorted in order of the server's preference.
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 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. This transaction
follows the TLS specification.
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3.6 Server key exchange
The server key exchange message for OpenPGP keys is identical to the
TLS specification.
3.7 Certificate verify
The certificate verify message for OpenPGP keys is identical to the
TLS specification.
3.8 Finished
The finished message for OpenPGP keys is identical to the description
in the specification.
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4. Cipher suites
No new cipher suites are required to use OpenPGP keys. OpenPGP keys
can be combined with existing cipher suites defined in TLS [1],
except the ones marked as "Exportable". Exportable cipher suites
SHOULD NOT be used with OpenPGP keys.
Some additional cipher suites are defined here in order to support
algorithms which are defined in OpenPGP [2], and are always available
in OpenPGP implementations but are not present in TLS [1].
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_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_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 AES [5] and 3DES block
ciphers in CBC mode. The choice of hash is the RIPEMD-160 [4]
algorithm. Implementations are not required to support the above
cipher suites.
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5. Internationalization Considerations
All the methods defined in this document are represented as machine
readable structures. As such issues of human internationalization and
localization are not introduced.
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6. 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 may crash or modify the application's
memory.
Security considerations about the use of the web of trust or the
verification procedure are outside the scope of this document, since
they are considered a local policy matter.
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Normative References
[1] Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246, January
1999.
[2] Callas, J., Donnerhacke, L., Finey, H. and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[3] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T.
Wright, "TLS Extensions", RFC 3546, June 2003.
[4] Dobbertin, H., Bosselaers, A. and B. Preneel, "RIPEMD-160: A
Strengthened Version of RIPEMD", April 1996.
[5] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
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Informative References
[6] 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.
[7] "Recommendation X.509: The Directory - Authentication
Framework", 1988.
Author's Address
Nikos Mavroyanopoulos
EMail: nmav@gnutls.org
URI: http://www.gnutls.org/
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Appendix A. Acknowledgements
The author wishes to thank Werner Koch, David Taylor and Timo Schulz
for their suggestions on improving this document.
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