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

Eric Rescorla <ekr@rtfm.com> Mon, 21 October 2019 19:30 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 21 Oct 2019 12:29:25 -0700
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
Cc: Rob Sayre <sayrer@gmail.com>, "TLS@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] draft-ietf-tls-esni feedback
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On Mon, Oct 21, 2019 at 11:41 AM Stephen Farrell <stephen.farrell@cs.tcd.ie>;
wrote:

>
> Hiya,
>
> On 21/10/2019 19:08, Eric Rescorla wrote:
> > No. You want padding to be set to be the longest size that you would send
> > to any origin in the anonymity group, and the server knows this.
>
> In the past, I've argued that this is not necessarily true
> and hence the current padding scheme is not a good plan.
>
> Two main reasons:
>
> - a server may not have visibility of all names that can
> be in certificate SANs
>

The question is not the server, but the operator.


- even if a server does have such visibility at time t, a
> CA re-issuing certs can change the situation during the
> lifetime of one ESNIKeys value
>
> I believe it is not unknown for servers to have a DB
> of TLS server certs/private keys that is queried based
> on the SNI (which could've arrived as ESNI). I don't
> know how common that is.
>
> My guess is that all of the above will lead to everyone
> always using 260 for this value, making it pointless
> and somewhat wasteful.
>

I don't agree that 260 is pointless, though perhaps wasteful.




> I would argue for an algorithmic method of padding and
> to not require servers to include any value in ESNIKeys at
> all. I proposed one such algorithm on the list before.
>

Yes, but I do not believe that it obtained the consensus of the WG.
Obviously, if the chairs call consensus on this point, we will change the
draft.

-Ekr


> Cheers,
> S.
>
> PS: I think 260 is the right max, didn't look it up just
> now but I did some time ago and it seemed correct.
>