Re: [TLS] draft-ietf-tls-esni feedback

Stephen Farrell <stephen.farrell@cs.tcd.ie> Wed, 23 October 2019 01:30 UTC

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To: "Salz, Rich" <rsalz@akamai.com>, Rob Sayre <sayrer@gmail.com>
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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Wed, 23 Oct 2019 02:30:05 +0100
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Subject: Re: [TLS] draft-ietf-tls-esni feedback
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Unstylishly quoting myself...

On 22/10/2019 18:49, Stephen Farrell wrote:
> Me too. I'd go for multiples of 32 octets, with a SHOULD
> to add an extra block or two randomly, but anything of
> that kind should work.

Stylishly however, it seems I'm in the happy position to
also admit I was wrong there (thanks to Christian Huitema
for spelling this out to me offlist).

In most cases, when using ESNI, DNS queries will need to
be made for the A/AAAA and ESNIKeys (or HTTPSSSSSSSSSVC:-)
and hopefully those DNS queries will also be padded.

RFC8467 [1] defines a bunch of ways of doing that, and has
one recommended way.

While it is likely too early to say that the recommendation
in RFC8467 is really a best practice, I think it is the
case that padding of the DNS query and of the SNI within
the ESNI extension to the CH ought be "commensurate" in
the sense that observing both ought not provide more
information to someone who only observes one of those.

So, at minimum, that'd mean s/32/128/ in my quoted text
above, and likely more. (Plus, of course, doing the kind
of due-diligence that lead to [1].)

Bottom line is that I think I'd modify my starting out
position for what to suggest for draft-05 to be to copy
(and modify as needed) what's recommended in [1] for
padding the query.

Cheers,
S.

[1] https://tools.ietf.org/html/rfc8467