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

Stephen Farrell <stephen.farrell@cs.tcd.ie> Mon, 21 October 2019 18:41 UTC

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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Date: Mon, 21 Oct 2019 19:41:39 +0100
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Subject: Re: [TLS] draft-ietf-tls-esni feedback
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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

- 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 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.

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.