Re: [TLS] publishing ESNIKeys under a .well-known URI...

Ben Schwartz <bemasc@google.com> Fri, 22 November 2019 13:05 UTC

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From: Ben Schwartz <bemasc@google.com>
Date: Fri, 22 Nov 2019 21:05:01 +0800
Message-ID: <CAHbrMsANNKk52hCX63sTY6sYDYdAUXz2uSft+dSia+SMhq1zLA@mail.gmail.com>
To: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Cc: Stephen Farrell <stephen.farrell@cs.tcd.ie>, Christian Huitema <huitema@huitema.net>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] publishing ESNIKeys under a .well-known URI...
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On Fri, Nov 22, 2019, 3:29 AM Daniel Kahn Gillmor <dkg@fifthhorseman.net>;
wrote:

> On Wed 2019-07-03 16:01:21 +0100, Stephen Farrell wrote:
> > It doesn't take much to encourage me so I just
> > pushed out that idea in I-D form:-) [1]
>  […]
> > [1] https://tools.ietf.org/html/draft-farrell-tls-wkesni-00
>
> Thanks for this (and for the -01 update for the draft).  I like this
> work, and i think we should pursue it in the WG.
>
> A couple notes/questions:
>
>  - Clients might use this, not just "zone factories".  For instance,
>    consider a client with limited access to the DNS that makes an
>    initial direct connection to the hidden host, leaking SNI.  If, in
>    that connection, it also fetches this record, it could use that to
>    bootstrap future connections to the host, right?
>
>    The draft currently contemplates this briefly for followup queries
>    for some clients, but it doesn't go into it in more detail.


Hopefully this use case is covered by the "esniconfig" Alt-Svc parameter (
https://tools.ietf.org/html/draft-ietf-dnsop-svcb-httpssvc-01#section-8).


>  - Why is it hosted on the cover server, instead of on the hidden
>    server?  is that just so that the zone factory doesn't leak $HIDDEN
>    to the network?
>
>    Surely on a zone factory update, the zone factory already knows the
>    eSNI for $HIDDEN so it could make the request with eSNI to
>    https://$HIDDEN/.well-known/esni/$HIDDEN.json rather than to
>    https://$COVER/.well-known/esni/$HIDDEN.json
>
>    At the same time, for $COVER to publish this information potentially
>    puts $COVER at more risk, right?  And, a $COVER could *claim* to be a
>    cover for $HIDDEN without approval of the $HIDDEN site by publishing
>    these records; if anyone believes that claim, it could cause traffic
>    to be re-routed through the ersatz $COVER.  If it's going to be
>    hosted at $COVER and not $HIDDEN, we should be explicit about what
>    defends against such an attack.
>
>    There could be an "obvious" reason for the choice of hosting it at
>    $COVER instead of at $HIDDEN, but it should be spelled out in the
>    draft.
>
>  - If this is treated as a separate/independent source of authority
>    about ESNI data for a host from the DNS (e.g. in the client examples
>    contemplated in my first point above, not just the "zone factory"),
>    then the draft probably needs some text discussing what to do when
>    discovering a discrepancy between what's in the DNS and what's found
>    at .well-known.
>
> Regards,
>
>         --dkg
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