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Re: [TLS] [ECH] Reverting the config ID change

Jonathan Hoyland <jonathan.hoyland@gmail.com> Wed, 17 February 2021 17:50 UTC

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From: Jonathan Hoyland <jonathan.hoyland@gmail.com>
Date: Wed, 17 Feb 2021 17:51:55 +0000
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To: Carrick Bartle <cbartle891@icloud.com>
Cc: Stephen Farrell <stephen.farrell@cs.tcd.ie>, "TLS@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] [ECH] Reverting the config ID change
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Being totally indistinguishable is probably impossible, but all else being
equal more resistance is better than less, no?

Regards,

Jonathan

On Wed, 17 Feb 2021 at 17:41, Carrick Bartle <cbartle891@icloud.com> wrote:

> Numerous ways a client can "stick out" have been identified, to the point
> where it's trivial to block connections using real ECH, regardless of the
> length of the config_id, which was why I thought we'd largely dropped the
> attempt not to stick out.
>
>
>
> On Feb 17, 2021, at 8:35 AM, Jonathan Hoyland <jonathan.hoyland@gmail.com>
> wrote:
>
> I know that ECH doesn't provide security against probing attackers, but
> such an attacker could easily maintain a list of active keys, and drop
> connections using them.
> If the key ID is very long, this would be highly effective at allowing
> grease ECH connections, but blocking real ECH connections.
>
> An adversary using this strategy against a one byte id would have high
> collateral damage, but against an eight byte id would virtually none.
>
> Providing some resistance to probing adversaries is a nice-to-have, even
> if we can't provide complete protection.
>
> My preference would be for a shorter id.
>
> Regards,
>
> Jonathan
>
> On Wed, 17 Feb 2021 at 16:25, Stephen Farrell <stephen.farrell@cs.tcd.ie>
> wrote:
>
>>
>>
>> On 17/02/2021 16:00, Eric Rescorla wrote:
>> > On Tue, Feb 16, 2021 at 4:44 PM Stephen Farrell <
>> stephen.farrell@cs.tcd.ie>
>> > wrote:
>> >
>> >>
>> >>
>> >> On 17/02/2021 00:34, Eric Rescorla wrote:
>> >>> How is it any harder to manage a multi-octet server-chosen value than
>> a
>> >>> single-octet server-chosen value?
>> >>
>> >> Easier for the library on the server side. If it's >1 octet
>> >> then someone will want some semantics. If ==1 then they'll
>> >> have to accept none and possible collisions so it can be
>> >> handled independently inside the library.
>> >>
>> >
>> > The server is free to enforce 1 byte.
>>
>> A server operator would be free to do that. The person
>> writing the code likely would not be as some server
>> operator would also be free to try impose semantics
>> on a multibyte field.
>>
>> S.
>>
>>
>> >
>> > -Ekr
>> >
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