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

Carrick Bartle <cbartle891@icloud.com> Wed, 17 February 2021 17:41 UTC

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From: Carrick Bartle <cbartle891@icloud.com>
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Date: Wed, 17 Feb 2021 09:41:18 -0800
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Cc: Stephen Farrell <stephen.farrell@cs.tcd.ie>, "TLS@ietf.org" <tls@ietf.org>
To: Jonathan Hoyland <jonathan.hoyland@gmail.com>
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Subject: Re: [TLS] [ECH] Reverting the config ID change
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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 <mailto: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 <mailto: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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