TLS Working Group                                     N. Mavroyanopoulos
Internet-Draft                                         November 10, 2003
Expires: May 9, 2004

                Using OpenPGP keys for TLS authentication


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
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   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",
   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 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. 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; 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

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

   DHE_DSS                 DSS public key.

   DHE_RSA                 RSA public key which can be used for

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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

   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


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