Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                             V. Dukhovni
Expires: 25 September 2022                                     Two Sigma
                                                               A. Wilson
                                                           24 March 2022

                 TLS Extension for DANE Client Identity


   This document specifies a TLS and DTLS extension to convey a DNS-
   Based Authentication of Named Entities (DANE) Client Identity to a
   TLS or DTLS server.  This is useful for applications that perform TLS
   client authentication via DANE TLSA records.

Status of This Memo

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  DANE Client Identity Extension  . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This document specifies a Transport Layer Security (TLS) extension
   [RFC6066] to convey a DANE [RFC6698] Client Identity to the TLS
   server.  This is useful for applications that perform TLS client
   authentication via DANE TLSA records, as described in [DANECLIENT].
   The extension could be empty to indicate to the server that the
   client has a DANE record and that the server can perform DANE
   authentication of the client with the identity extracted from the
   client certificate.  Or the extension can contain the full client
   identity, in the form of the DNS domain name that is expected to have
   a DANE TLSA record published for it.

   This extension supports both TLS [RFC5246] [RFC8446] and DTLS
   [RFC6347], and the term TLS in this document is used generically to
   describe both protocols.

2.  Overview

   When TLS clients use X.509 client certificates or raw public keys
   that are authenticated via DANE TLSA records, it is useful for them
   to convey their intent to be authenticated via DANE, or even to
   convey their complete DANE identity to the server.  The TLS extension
   defined in this document is used to accomplish this.

   In the case of X.509 client certificates, a TLS server can learn the
   client's identity by examining subject alternative names included in
   the certificate itself.  However, without a mechanism such as the one
   defined in this extension, the TLS server cannot know apriori that

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   the client has a published TLSA record, and thus may unnecessarily
   issue DNS queries for DANE TLSA records in-band with the TLS
   handshake even in cases where the client has no TLSA record
   associated with it.  When multiple identities are present in the
   certificate, a client must use this extension to specify exactly
   which one the server should use.  An additional situation in which
   this extension helps is where some TLS servers may need to
   selectively prompt for client certificate credentials only for
   clients that are equipped to provide certificates.

   When TLS raw public keys [RFC7250] are being used to authenticate the
   client, the client uses this extension to explicitly indicate to the
   server what its domain name identity is (since there is no X.509
   certificate from which the identity can be extracted).

   Detailed protocol behavior of TLS clients and servers is described in

3.  DANE Client Identity Extension

   The DANE Client Identity Extension type, "dane_clientid", will have a
   value assigned and registered in the IANA TLS Extensions registry.
   Its extension data (if not empty) has the following format:

   opaque ClientName<1..2^8-1>;

   The ClientName field contains the single domain name of the client in
   textual presentation format, as described in RFC 1035 [RFC1035],
   omitting the trailing dot.

   A TLS server implementing this specification MUST send an empty
   extension of type "dane_clientid" to indicate that it understands the
   extension and is capable of performing DANE client authentication.
   In TLS 1.2, the empty extension is sent in the ServerHello message.
   In TLS 1.3, it is sent in the CertificateRequest message.

   A TLS client implementing this specification SHOULD send an extension
   of type "dane_clientid".  If the client only needs to indicate that
   it has a DANE record and that the client's domain name identity can
   be obtained from its certificate, then the extension sent can be
   empty.  If the client needs to send its domain name identity, then
   the "extension_data" field of the extension MUST contain a
   "ClientName" data structure populated with the domain name.

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   In TLS 1.2, the client extension is sent in the ClientHello message.
   In TLS 1.3, it is sent in the Certificate message.  Additionally, in
   TLS 1.3, the client is only permitted to send the extension if it
   sees the corresponding empty extension in the server's
   CertificateRequest message.

4.  Security Considerations

   In TLS 1.3, this extension is sent in the CertificateRequest and
   Certificate messages, which are encrypted.

   In TLS 1.2, this extension cannot be encrypted.  When used with TLS
   1.2, to prevent unnecessary privacy leakage of the client's name in
   cleartext, a TLS client implementing this specification should be
   configured to only send this extension to TLS servers it intends to
   perform client authentication with.

5.  IANA Considerations

   This extension requires the registration of a new value in the TLS
   ExtensionsType registry.

6.  Normative References

              Huque, S., Dukhovni, V., and A. Wilson, "TLS Client
              Authentication via DANE TLSA Records", 2 May 2021,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

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   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

Authors' Addresses

   Shumon Huque

   Viktor Dukhovni
   Two Sigma

   Ash Wilson

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