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

Rob Sayre <sayrer@gmail.com> Mon, 21 October 2019 19:14 UTC

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From: Rob Sayre <sayrer@gmail.com>
Date: Mon, 21 Oct 2019 12:14:40 -0700
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
Cc: Eric Rescorla <ekr@rtfm.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 12:10 PM Stephen Farrell <stephen.farrell@cs.tcd.ie>;
wrote:

>
> Hiya,
>
> On 21/10/2019 20:01, Rob Sayre wrote:
> > On Mon, Oct 21, 2019 at 11:41 AM Stephen Farrell <
> stephen.farrell@cs.tcd.ie>;
> > wrote:
> >
> >> My guess is that all of the above will lead to everyone
> >> always using 260 for this value, making it pointless
> >> and somewhat wasteful.
> >>
> >
> > Whether it's wasteful depends on how big typical ClientHello (without
> early
> > data) messages are. If they'll easily fit in one packet, 260 doesn't
> matter.
>
> I don't think we ought be so confident of that. TLS is
> so broadly used that there may be other circumstances
> now or in future where this would be a problem that'd
> cause ESNI to not be used. It seems prudent to use fewer
> bytes when that's possible (so long as we don't expose
> the actual SNI length).
>

I have seen MTUs under 1500 many times, but nothing under 1200. Is there
data on this? (I honestly haven't seen any)


>
> Removing the padding_length field also removes a way
> in which server configurations can be broken (if some
> server admin sets a too-low value), which is also a
> more prudent design than we currently have.
>

I think padding_length makes sense as a minimum. As a maximum, it could
actually be an attack as well.

thanks,
Rob