Re: [TLS] ESNIKeys.public_name vs. cleartext SNI

Stephen Farrell <stephen.farrell@cs.tcd.ie> Sun, 28 July 2019 19:05 UTC

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To: Dmitry Belyavsky <beldmit@gmail.com>
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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Sun, 28 Jul 2019 20:05:25 +0100
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Subject: Re: [TLS] ESNIKeys.public_name vs. cleartext SNI
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Hiya,

On 28/07/2019 19:07, Dmitry Belyavsky wrote:
> Dear Stephen,
> 
> I published a draft describing "Fake SNI" designed for the same purpose. It
> allows (at least in theory) to cheat those who blocks all the handshakes
> containing ESNI.

Yes, I recall that. The question below was only really about
draft-ietf-tls-esni-04 though.

Cheers,
S.

> 
> вс, 28 июля 2019 г., 15:25 Stephen Farrell <stephen.farrell@cs.tcd.ie>ie>:
> 
>>
>> Hiya,
>>
>> ESNI drafts -03 and -04 envisage that the ESNIKeyublic_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.
>> _______________________________________________
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>> TLS@ietf.org
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>>
>