TLS                                                           S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Experimental                               July 5, 2019
Expires: January 6, 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.

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   This Internet-Draft will expire on January 6, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Example use of the well-known URI for ESNI  . . . . . . . . .   3
   4.  The esni well-known URI . . . . . . . . . . . . . . . . . . .   3
   5.  The JSON structure for ESNIKeys . . . . . . . . . . . . . . .   4
   6.  Zone factory behaviour  . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   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.  An ESNIKeys structure contains the public component of a
   key pair that will typically be periodically (re-)generated by a web
   server.  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 [RFC8615] 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 if there's enough interest, hence this draft.  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 ESNI

   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, published with a TTL of 1800,
      and remain usable for 3 hours from the time of generation
   o  Web server has a set of "hidden" sites - the DNS name for each
      hidden web site is here represented as $HIDDEN, which will end up
      as a realSNI value to be encrypted inside an ESNI extension
   o  Web server has a "cover" site ($COVER), where $COVER will
      typically be the DNS name 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

4.  The esni well-known URI

   When a web server ($COVER) wants to publish ESNIKeys information for
   a hidden site ($HIDDEN) then it provides the JSON content defined in
   Section 5 at: https://$COVER/.well-known/esni/$HIDDEN.json

   The well-known URI defined here MUST be an https URL and therefore
   the zone factory verifies the correct $COVER 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 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:-)]]

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

                           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:

   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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6.  Zone factory behaviour

   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.]]

   From the point of view of the zone factory, the KeyShareEntry values
   within each element of the JSON array are entirely independent.  The
   zone factory MUST NOT assume that there is any specific relationship
   between the ESNIKeys values in one JSON structure, nor between the
   set of JSON structures for the set of $HIDDEN sites that share a

   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.

7.  Security Considerations

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

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8.  Acknowledgements

   Thanks to Niall O'Reilly for a quick review.

9.  IANA Considerations

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

10.  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.

   [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)", RFC 8615, DOI 10.17487/RFC8615, May 2019,

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Appendix A.  Change Log

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

   From -00 to -01:

   o  Re-structured a bit after re-reading rfc8615

Author's Address

   Stephen Farrell
   Trinity College Dublin
   Dublin  2

   Phone: +353-1-896-2354

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