Using OpenPGP Keys for Transport Layer Security (TLS) Authentication

The IETF has two sets of standards for public key certificates, one set for use of X.509 certificates and one for OpenPGP certificates . At the time of writing, TLS standards are defined to use 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. The major changes from are summarized in .

The term "OpenPGP key" is used in this document as in the OpenPGP specification . 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 . 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 .

This section describes the changes to the TLS handshake message contents when OpenPGP certificates are to be used for authentication.

In order to indicate the support of multiple certificate types, clients MUST include an extension of type "cert_type" to the extended client hello message. The "cert_type" TLS extension is assigned the value of 9 from the TLS ExtensionType registry. This value is used as the extension number for the extensions in both the client hello message and the server hello message. The hello extension mechanism is described in . 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. Note that the CertificateTypeExtension structure is being used both by the client and the server, although specified once in this document, a practice common in the TLS protocol specification .

; case server: CertificateType certificate_type; } } CertificateTypeExtension; ]]> No new cipher suites are required to use OpenPGP certificates. All existing cipher suites that support a key exchange method compatible with the key in the certificate can be used in combination with OpenPGP certificates.

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 session 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. It is perfectly legal for a server to ignore this message. In that case the normal TLS handshake should be used and, other certificate types than the default MUST NOT be used.

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.

Key Exchange Algorithm OpenPGP Certificate Type RSA RSA public key that can be used for encryption. DHE_DSS DSA 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 certificates to be transferred are placed in the Certificate structure and tagged with the OpenPGPCertDescriptorType "subkey_cert". Since those certificates might contain several subkeys the subkey ID to be used for this session is explicitely specified in the OpenPGPKeyID field. The key ID must be specified even if the certificate has only a primary key. The peer once receiving this type has to either use the specified subkey or terminate the session with a fatal alert of "unsupported_certificate". The option is also available to send an OpenPGP fingerprint, instead of sending the entire certificate, by using the "subkey_cert_fingerprint" tag. This tag uses the OpenPGPSubKeyFingerprint structure and requires the primary key fingerprint to be specified, as well as the subkey ID to be used for this session. 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 . Implementations of this protocol MUST ensure that the sizes, of key IDs and fingerprints, in the OpenPGPSubKeyCert and OpenPGPSubKeyFingerprint structures comply with . Moreover it is RECOMMENDED that the keys to be used with this protocol have the authentication flag (0x20) set. The process of fingerprint generation is described in Section 12.2 of . The enumerated types "cert_fingerprint" and "cert" of OpenPGPCertDescriptorType that were defined in are not used and are marked as obsolete by this document. The "empty_cert" type has replaced "cert" and is a backwards compatible way to specify an empty certificate; cert_fingerprint" MUST NOT be used with this updated specification, and hence that old alternative has been removed from the Certificate struct description.

; opaque OpenPGPCert<0..2^24-1>; } OpenPGPSubKeyCert; struct { opaque OpenPGPKeyID<8..255>; opaque OpenPGPCertFingerprint<20..255>; } OpenPGPSubKeyFingerprint; struct { OpenPGPCertDescriptorType descriptorType; select (descriptorType) { case empty_cert: OpenPGPEmptyCert; case subkey_cert: OpenPGPSubKeyCert; case subkey_cert_fingerprint: OpenPGPSubKeyCertFingerprint; } } Certificate; ]]>

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.

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 of type "empty_cert" that contains an OpenPGPEmptyCert value MUST be sent. The server SHOULD respond with a "handshake_failure" fatal alert if client authentication is required.

All the other handshake messages are identical to the TLS specification.

All security considerations discussed in , , and 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 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 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 ). 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.

This document uses a registry originally defined in . Existing IANA references should be updated to point to this document. In addition the "TLS Certificate Types" registry established by has to be updated in the following way: Values 0 (X.509) and 1 (OpenPGP) are defined in this document. Values from 2 through 223 decimal inclusive are assigned via "RFC Required" . Values from 224 decimal through 255 decimal inclusive are reserved for Private Use .

The author wishes to thank Daniel Kahn Gillmor and Alfred Hoenes for their suggestions on improving this document.

This document incorporates a major and incompatible change in the "Server Certificate" and "Client Certificate" TLS messages. This change requires the subkey IDs used for TLS authentication to marked explicitely in the handshake procedure. This was decided in order to place no limitation on the OpenPGP certificates' contents that can be used with with this protocol. required that an OpenPGP key or subkey was marked with the authentication flag and thus would have failed if this flag was not set, or this flag was set in more than one subkeys. The protocol in this memo has no such limitation.