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TLS                                                           S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Experimental                               July 3, 2019
Expires: January 4, 2020

                A well-known URI for publishing ESNIKeys


   We propose use of a well-known URI at which web servers can publish
   ESNIKeys 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 January 4, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Example Use of .well-known for ESNIKeys . . . . . . . . . . .   3
   4.  The JSON structure for ESNIKeys . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Encrypted Server Name Indication (ESNI) [I-D.ietf-tls-esni] for
   TLS1.3 [RFC8446] defines a confidentiality mechanism for server names
   in TLS.  That requires publication of an ESNIKEys data structure in
   the DNS.

   Many web servers will have an API that can be used to dynamically
   update ESNIKeys in the DNS.  Some implementations/deployments
   however, will not, so web server implementers 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 just for
   cases where the zone file (or equivalent) that includes the ESNIKeys
   RR is on some machine, which we here call a "zone factory," to which
   the web server doesn't have write access.

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

   [[This idea could: a) wither on the vine, b) be published as it's own
   RFC, or c) end up as a PR for [I-D.ietf-tls-esni].  There is no
   absolute need for this to be in the RFC that defines ESNI, so (b)
   seems feasible it there's enough interest, hence this draft.  The
   source for this is in https://github.com/sftcd/wkesni/ 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 .well-known for ESNIKeys

   An example deployment could be as follows:

   o  Web server generates new ESNIKeys hourly at N past the hour via a
   o  ESNIKeys are "current" for an hour, and published with a TTL of
   o  Old ESNIKeys are still usable for 3 hours from time of generation
   o  Web server has a set of "hidden" sites (each represented as a
      $HIDDEN value) and a "cover" site ($COVER)
   o  $COVER will typically be the value used in the ESNIKeys
      public_name field for ESNIKeys version 0xff02
   o  The cronjob creates creates a JSON file for each hidden site at
   o  Each JSON file contains an array with the ESNIKeys RR values for
      that particular $HIDDEN as shown in Figure 1 - the values in
      Figure 1 with ellipses are the RR values we want to eventually see
      in the DNS
   o  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
   o  If the content has changed the cronjob attempts to use the
      ESNIKeys, and for each $HIDDEN where that works, it updates the
      zone file and re-publishes the zone containing only the new
      ESNIKeys RR values

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               "ESNIKeys.version": 0xff01,
               "desired-ttl": 1800,
               "ESNIKeys": "/wH5QHc...="
               "ESNIKeys.version": 0xff02,
               "desired-ttl": 1800,
               "ESNIKeys": "FF02897...OA"

                           Figure 1: Sample JSON

4.  The JSON structure for ESNIKeys

   [[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:-)]]

   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:

   o  ESNIKeys.version: contains a number with the value of the version
      field of the ESNIKeys.  This is needed (today) as different
      versions are published in the DNS differently.  (Draft-02 used a
      TXT RR and is still all that is usable with some early test
      deployments, draft-03 uses a new RRTYPE from the experimental
   o  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.
   o  ESNIKeys: contains the RRVALUE value to be used, either as a
      base64 encoded string (for ESNIKeys.version of 0xff01) or as an
      ASCII-HEX string (for ESNIKeys.version of 0xff02).

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

   o  While ESNI is still in draft form, it may be necessary to publish
      different versions of the ESNIKeys structure.
   o  For some deployments, the same $HIDDEN could be accessible, using
      ESNI, via different $COVER (or public_name) web servers.
   o  As ESNIKeys is (regrettably:-) an extensible structure, it may be
      necessary to publish different ESNIKeys values to get best

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   The zone factory SHOULD check that the presented ESNIKeys values work
   with the $HIDDEN server before publication.  A "special" TLS client
   may be needed for this check, that does not require the ESNIKeys
   value to have already been published in the DNS.  [[I guess that
   could call for the zone factory to know of a "safe" URL on $HIDDEN to
   use, or maybe it could use HTTP HEAD?  Figuring that out is TBD.]]

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

   The zone factory SHOULD only publish ESNIKeys 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 ESNIKeys 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 complex.]]

   The ESNI specification [I-D.ietf-tls-esni] defines how and where the
   ESNIKeys RR for $HIDDEN needs to be published in the DNS.

   A possibly interesting (unintended) consequence of this design is
   that once a TLS client has first gotten ESNIKeys from the DNS for
   $HIDDEN with the draft-03 ESNIKeys structure containing the
   public_name field, the TLS client would know both $COVER and $HIDDEN
   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.

5.  Security Considerations

   This document defines another way to publish ESNIKeys.  If the wrong
   keys were read from here and publised in the DNS, then clients using
   ESNI would do the wrong thing.  So: Don't do that:-)

6.  Acknowledgements

   [[Your name here.]]

7.  IANA Considerations

   [[TBD: IANA registration of a .well-known]]

8.  Normative References

              Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
              "Encrypted Server Name Indication for TLS 1.3", draft-
              ietf-tls-esni-03 (work in progress), March 2019.

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   [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, <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,

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Appendix A.  Change Log

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

   Nothing yet.

Author's Address

   Stephen Farrell
   Trinity College Dublin
   Dublin  2

   Phone: +353-1-896-2354
   EMail: stephen.farrell@cs.tcd.ie

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