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A well-known URI for publishing ECHConfigList values.

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Author Stephen Farrell
Last updated 2022-07-24
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TLS                                                           S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Experimental                               24 July 2022
Expires: 25 January 2023

         A well-known URI for publishing ECHConfigList values.


   We propose use of a well-known URI at which web servers can publish
   ECHConfigList values as a way to help get those published in the DNS.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 25 January 2023.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Example use of the well-known URI for ECH . . . . . . . . . .   3
   4.  The ech well-known URI  . . . . . . . . . . . . . . . . . . .   3
   5.  The JSON structure for ECHConfigList values . . . . . . . . .   4
   6.  Zone factory behaviour  . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Encrypted ClientHello (ECH) [I-D.ietf-tls-esni] for TLS1.3 [RFC8446]
   defines a confidentiality mechanism for server names and other
   ClientHello content in TLS.  For many applications, that requires
   publication of ECHConflgList data structures in the DNS.  An
   ECHConfigList structure contains a list of ECHConfig values.  Each
   ECHConfig value contains the public component of a key pair that will
   typically be periodically (re-)generated by a web server.  Many web
   infrastructures will have an API that can be used to dynamically
   update the DNS RR values containing ECHConfigList values.  Some
   deployments however, will not, so web deployments could benefit from
   a mechanism to use in such cases.

   We define such a mechanism here.  Note that this is not intended for
   universal deployment, but rather for cases where the web server
   doesn't have write access to the relevant zone file (or equivalent).
   That zone file will eventually include an HTTPS or SVCB RR
   [I-D.ietf-dnsop-svcb-https] containing an ECHConfigList.

   We use the term "zone factory" for the entity that does have write
   access to the zone file.  We assume the zone factory (ZF) can also
   make HTTPS requests to the web server with the ECH keys.

   We propose use of a well-known URI [RFC8615] on the web server that
   allows ZF to poll for changes to ECHConfigList values.  For example,
   if a web server generates new ECHConfigList values hourly and
   publishes those at the well-known URI, ZF can poll that URI.  When ZF
   sees new values, it can check if those work, and if they do, then
   update the zone file and re-publish the zone.

   [[The source for this is in PRs are
   welcome there too.]]

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Example use of the well-known URI for ECH

   An example deployment could be as follows:

   *  Web server generates new ECHConfigList values hourly at N past the
      hour via a cronjob
   *  ECHConfigList values are "current" for an hour, published with a
      TTL of 1800, and remain usable for 3 hours from the time of
   *  Web server has a set of "backend" sites - the DNS name for each
      such site is here represented as $BACKEND, which will end up as an
      SNI value to be encrypted inside an ECH extension
   *  Web server has a "front-end" site ($FRONT), where $FRONT will
      typically be the DNS name used in the ECHConfigList public_name
      field for ECHConfig version 0xff0d
   *  A cronjob creates creates a JSON file for each backend site at
   *  Each JSON file contains an array with the ECHConfigList values
      values for that particular $BACKEND as shown in Figure 1 - the
      values in Figure 1 with ellipses are the values we want to
      eventually see in the DNS
   *  On the zone factory, a cronjob runs at N+3 past the hour, it knows
      all the names involved and checks to see if the content at those
      well-known URIs has changed or not
   *  If the content has changed the cronjob attempts to use the
      ECHConfigList values, and for each $BACKEND where that works, it
      updates the zone file and re-publishes the zone containing only
      the new ECHConfigList values

4.  The ech well-known URI

   When a web server ($FRONT) wants to publish ECHConfigList information
   for a backend site ($BACKEND) then it provides the JSON content
   defined in Section 5 at: https://$FRONT/.well-known/ech/$BACKEND.json

   The well-known URI defined here MUST be an https URL and therefore
   the zone factory verifies the correct $FRONT is being accessed.  If
   there is any failure in accessing the well-known URI, then the zone
   factory MUST NOT modify the zone.

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5.  The JSON structure for ECHConfigList values

   [[Since the specifics of the JSON structure in Figure 1 are very
   likely to change, this is mostly TBD.  What is here for now, is what
   the author has currently implemented simply because it worked ok and
   was easy to do:-) One issue raised as a result of the dispatch
   presentation is whether or not anything beyond the ECHConfigList
   might make sense to represent in the JSON response.  One example
   could be the inner ClientHello ALPN extension, if that might somehow
   be useful to the TLS cilent (which really should know in that case).
   The scope in that respect and the correct level of generality to
   cover here is something to consider as this evolves.]]

               "desired-ttl": 1800,
               "ports": [ 443, 8413 ],
               "echconfiglist": "AD7+DQA65wAgAC..AA=="
               "desired-ttl": 1800,
               "ports": [ 443, 8413 ],
               "echconfiglist": "AD7+DQA65wAgAC..AA=="

                           Figure 1: Sample JSON

   The JSON file at the well-known URI MUST contain an array with one or
   more elements.  Each element of the array MUST have these fields:

   *  desired-ttl: contains a number indicating the TTL that the web
      server would like to see used for this RR.  The zone factory MUST
      NOT use a longer TTL.
   *  ports: this has a list of the TCP ports on which the web server
      with the relevant key pair will listen (needed to produce the
      correct zone file).
   *  ECHConfigList: contains the value to be used as a base64 encoded

   The JSON file contains an array for a couple of reasons:

   *  As TLS authentication doesn't really distinguish ports, servers on
      the same host could in any case cheat on one another, so we may as
      well just read one JSON file per name.
   *  Different ports could map to different sets of ECHConfig values

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   *  As ECHConfigList is (regrettably:-) an extensible structure, it
      may be necessary to publish different ECHConfigList values to get
      best interoperability.

6.  Zone factory behaviour

   The zone factory SHOULD check that the presented ECHConfigList values
   work with the $BACKEND server before publication.  A "special" TLS
   client may be needed for this check, that does not require the
   ECHConfigList value to have already been published in the DNS.  [[I
   guess that calls for the zone factory to know of a "safe" URL on
   $BACKEND to try, or maybe it could use HTTP HEAD?  Figuring that out
   is TBD.  The ZF could also try a GREASEd ECH and see if the retry-
   configs it gets back is one of the ECHConfig values in the

   A careful zone factory could explode the ECHConfigList value
   presented into "singleton" values with one public key in each and
   test each for each port claimed.

   The zone factory SHOULD publish all the ECHConfigList values that are
   presented in the JSON file, and that pass the check above.

   The zone factory SHOULD only publish ECHConfigList values that are in
   the latest version of the JSON file.  This leaves the control of
   "expiry" with the web server, so long as the ECHConfigList values
   presented actually work.  [[An alternative could be to have the new
   values just be appended to the zone, but that'd require some form of
   "notAfter" value in the JSON file which seems unnecessary and more

   The SCVB and HTTPS RR specification [I-D.ietf-dnsop-svcb-https]
   defines how and where the ECHConfigList values for $BACKEND needs to
   be published in the DNS.  The zone factory is assumed to be in
   control of how ECHConfigList values are included in such RRs.

   A possibly interesting (unintended) consequence of this design is
   that once a TLS client has first gotten an ECHConfigList from the DNS
   for $BACKEND with the ECHConfigList structure containing the
   public_name field, the TLS client would know both $FRONT and $BACKEND
   and so could later probe for this .well-known as an alternative to
   doing so via DoT/DoH.  Probably not something a web browser might do,
   but could be fun for other applications maybe.

   [[The extent to which retry-configs could be used for a similar
   purpose might be worth considering.  But the JSON stuff here may
   still be needed if implementations (such as mine:-) tend to only
   return one ECHConfig in retry-configs.]]

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7.  Security Considerations

   This document defines another way to publish ECHConfigList values.
   If the wrong keys were read from here and published in the DNS, then
   clients using ECH would do the wrong thing, likely resulting in
   denial of service, or a privacy leak, or worse, when TLS clients
   attempt to use ECH with a backend web site.  So: Don't do that:-)

8.  Acknowledgements

   Thanks to Niall O'Reilly for a quick review of -00.

9.  IANA Considerations

   [[TBD: IANA registration of a .well-known.  Also TBD - how to handle
   I18N for $FRONT and $BACKEND within such a URL.]]

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", DOI 10.17487/RFC8615, RFC 8615, May 2019,

              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-14, 13 February 2022,

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              Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-https-10, 24 May 2022,

Appendix A.  Change Log

   [[RFC editor: please remove this before publication.]]

   The -00 WG draft replaces draft-farrell-tls-wkesni-03.

Author's Address

   Stephen Farrell
   Trinity College Dublin
   Phone: +353-1-896-2354

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