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A well-known URI for publishing ECHConfigList values.
draft-ietf-tls-wkech-02

Document Type Active Internet-Draft (tls WG)
Authors Stephen Farrell , Rich Salz , Benjamin M. Schwartz
Last updated 2023-04-21
Replaces draft-farrell-tls-wkesni
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draft-ietf-tls-wkech-02
TLS                                                           S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Experimental                                    R. Salz
Expires: 23 October 2023                             Akamai Technologies
                                                             B. Schwartz
                                                              Google LLC
                                                           21 April 2023

         A well-known URI for publishing ECHConfigList values.
                        draft-ietf-tls-wkech-02

Abstract

   We propose use of a well-known URI at which an HTTP origin can inform
   an authoritative DNS server, or other interested parties, about this
   origin's Service Bindings, i.e. its "HTTPS" DNS records.  These
   instructions can include Encrypted ClientHello (ECH) configurations,
   allowing the origin to publish and rotate its own ECH keys.

   AUTHORS NOTE: This version proposes changing from the highly
   ECHConfig specific approach of -00 to a much more generic approach.
   The authors are seeking feedback from the Working Group as to which
   of these approaches may be more likely to garner rough consensus.  If
   the WG feel this is worse than -00 we're fine with reverting.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 23 October 2023.

Copyright Notice

   Copyright (c) 2023 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Example use of the well-known URI for ECH . . . . . . . . . .   3
   4.  The origin-svcb well-known URI  . . . . . . . . . . . . . . .   4
   5.  The JSON structure for origin service binding info  . . . . .   4
   6.  Zone factory behaviour  . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.  This
   mechanism is extensible to deliver other kinds of information about
   the origin, but in this specification it only provides the
   functionality necessary to configure ECH.

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   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 https://github.com/sftcd/wkesni/ PRs are
   welcome there too.]]

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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:

   1.  Web server generates new ECHConfigList values hourly at N past
       the hour via some regular, automated process (e.g. a cronjob)
   2.  ECHConfigList values are "current" for an hour, and remain usable
       for 3 hours from the time of generation
   3.  The cronjob updates the ECHConfigList values in a JSON resource
       at at https://$ORIGIN/.well-known/origin-svcb, as shown in
       Figure 1.
   4.  On the zone factory, an HTTP client retrieves this JSON resource.
       It attempts to connect to the origin using these values and
       confirms that they are working.
   5.  The zone factory observes that the JSON resource has an HTTP
       freshness lifetime of 3600 seconds, and chooses a DNS TTL of
       1800.  It updates the zone file for $ORIGIN and re-publishes the
       zone containing only the new ECHConfigList values.
   6.  When the TTL of the DNS records approaches the remaining
       freshness lifetime of the JSON resource, the zone factory
       attempts to refresh its cached copy of the JSON resource.  If the
       resource has changed, it repeats this process.

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4.  The origin-svcb well-known URI

   If a web server ($ORIGIN) wants to convey information to the Zone
   Factory, it publishes the JSON content defined in Section 5 at:
   https://$ORIGIN/.well-known/origin-svcb

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

5.  The JSON structure for origin service binding info

   [[The JSON structure is a work in progress.]]

       {
           "endpoints": [{
               "priority": 1,
               "target": "cdn.example.",
               "ech": "AD7+DQA65wAgAC..AA=="
           }, {
               "priority": 1,
               "port": 8413,
               "ech": "AD7+DQA65wAgAC..AA=="
           }]
       }

               Figure 1: Sample JSON for ECH without aliases

           {
               "alias": "cdn.example.net:443"
           }

                    Figure 2: Sample JSON with aliasing

   The JSON file at the well-known URI MUST contain an object with
   either an "endpoints" key or an "alias" key.  If the "endpoints" key
   is present, its value is an array whose elements represent HTTPS
   records in ServiceMode.  Each element MAY contain one or more keys
   from the JSON HTTP Origin Info registry (see IANA Considerations).
   The initial registry entries are:

   *  priority: The value is a positive integer corresponding to the
      SvcPriority.  If omitted, the zone factory SHOULD infer
      numerically increasing SvcPriority from the order of the endpoints
      array.

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   *  target: The value is a string containing a fully qualified domain
      name, corresponding to the HTTPS record's TargetName.  The default
      value is ".".
   *  port: The value is a non-negative integer, corresponding to the
      value of the "port" SvcParamKey.
   *  ech: The value is a string containing an ECHConfigList encoded in
      Base64 [RFC4648], corresponding to the value of the "ech"
      SvcParamKey.

   An empty endpoint object corresponds to an HTTPS record with inferred
   SvcPriority, TargetName=".", and no ECH support.  An empty record of
   this kind can be useful as a simple way to make use of the HTTPS RR
   type's HSTS behavior.

   [[TODO: What does the zone factory do if it encounters an
   unrecognized field?]]

   If the object contains an "alias" key, its value MUST be an
   "authority" (Section 3.2 of [RFC3986]).  This indicates that $ORIGIN
   is hosted on the same endpoints as this target, and is equivalent to
   an HTTPS AliasMode record.  A zone factory might implement this
   directive by publishing an AliasMode record, publishing a CNAME
   record, copying HTTPS records from the target zone, or fetching
   https://$TARGET/.well-known/origin-svcb" (if it exists).

   This arrangement provides the following important properties:

   *  Origins can indicate that different ECHConfigs are used on
      different ports.
   *  Origins can indicate that multiple CDNs are in use, each with its
      own ECHConfig.
   *  Origins that simply alias to a single target can indicate this
      without copying the ECHConfig and other parameters, which can
      interfere with key rotation and other maintenance.
   *  "port" and "target" are generally sufficient to uniquely identify
      a ServiceMode record, so zone factories can use the endpoint list
      to add ECH to pre-existing ServiceMode records that may have other
      SvcParams.

6.  Zone factory behaviour

   The zone factory SHOULD check that the presented endpoints work and
   provide access to $ORIGIN before publication.  A bespoke 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 $ORIGIN 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

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   back is one of the ECHConfig values in the ECHConfigList.]]

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

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

   The zone factory MUST set a DNS TTL short enough that any generated
   records expire from DNS caches before the JSON object's HTTP cache
   lifetime expires.  The zone factory MUST refresh the JSON object and
   regenerate the zone before it expires each time.  This ensures that
   ECHConfigs are not used longer than intended by the origin, while
   permitting the zone factory to limit the TTL if desired.

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:-)

   Although this configuration resource MAY be publicly accessible,
   general HTTP clients SHOULD NOT attempt to use this resource in lieu
   of HTTPS records queries through their preferred DNS server:

   *  The bootstrap connection would not be able to use ECH, so it would
      reveal all the information that ECH seeks to protect.
   *  The origin could serve the user with a uniquely identifying
      configuration, potentially resulting in an unexpected tracking
      vector.

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.]]

   If approved, this specification requests the creation of an IANA
   registry named "JSON HTTP Origin Info" with a Standards Action
   registration policy, containing a field named "Name" whose value is a
   UTF-8 string.

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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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/info/rfc8615>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-16, 6 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-16>.

   [I-D.ietf-dnsop-svcb-https]
              Schwartz, B. M., 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-12, 11 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
              svcb-https-12>.

Appendix A.  Change Log

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

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

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   Version 01 changed from a special-purpose design, carrying only
   ECHConfigs and port numbers, to a more general approach based on
   Service Bindings.

   Version 02 is just a keep-alive

Authors' Addresses

   Stephen Farrell
   Trinity College Dublin
   Dublin
   2
   Ireland
   Phone: +353-1-896-2354
   Email: stephen.farrell@cs.tcd.ie

   Rich Salz
   Akamai Technologies
   Email: rsalz@akamai.com

   Benjamin Schwartz
   Google LLC
   Email: bemasc@google.com

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