[TLS] ESNIKeys.public_name vs. cleartext SNI

Stephen Farrell <stephen.farrell@cs.tcd.ie> Sun, 28 July 2019 12:25 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Sun, 28 Jul 2019 13:25:14 +0100
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Subject: [TLS] ESNIKeys.public_name vs. cleartext SNI
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Hiya,

ESNI drafts -03 and -04 envisage that the ESNIKeys.public_name
and the TLS h/s cleartext SNI are the same. I'd expect that
they mostly will be.

But they don't have to be. A client could provide a way to
use a client-chosen cleartext SNI when using ESNI to encrypt
the real SNI.

For example, my openssl build allows this for s_client. I've
a colleague who's got a first build of curl using that that
also allows such a local over-ride. I think other command-line
clients might be likely to also support that kind of thing, if
for no other reason than the existence of the draft-02 version
that's deployed and has no public_name. But I guess being able
to do a local over-ride may also be generally useful, while
not being so useful for web browsers. For example, if a given
public_name value is being censored, clients might at that
point want to use something else.

On the server side, my code also make no checks that the
cleartext SNI used was the same as the public_name. And I
don't think that causes any problems, so maybe ought be
allowed too.

I don't think allowing this (if the WG decide to) requires
any major change to the spec, but there are a couple of
language changes needed in section 5.1.3 and I guess we
should add a sentence to the effect that using the public_name
value from ESNIKeys in the cleartext SNI is recommended
if you've no other value but is not required.

Cheers,
S.