Re: [TLS] Finished stuffing/PSK Binders

Eric Rescorla <ekr@rtfm.com> Sun, 09 October 2016 14:11 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 9 Oct 2016 07:10:59 -0700
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Finished stuffing/PSK Binders
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On Sun, Oct 9, 2016 at 6:58 AM, Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Fri, Oct 07, 2016 at 08:01:43AM -0700, Eric Rescorla wrote:
> > After the discussion on PR #615, I took another pass at this with some
> > help from the research community. Please see:
> >
> >    https://github.com/tlswg/tls13-spec/pull/672
> >
>
> Also, an observation: This seems to interact in somewhat annoying way
> with stateless HRR.
>
> Basically, CH reconstruction no longer works properly, so one needs to
> have a  freezeable PRF hash (and most implementations of hashes can not
> be frozen).
>

I've been coming to the conclusion that CH reconstruction is a bad idea.
It's
tricky to get right and in the common case involves a lot of bloat in the CH
(because of duplicating the Key Shares). I think we would be better off just
removing it and replacing (rather than appending to ) KeyShares in HRR.
This was primarily intended as an attempt to avoid the need to continue
the hash in any case.

Best,
-Ekr


And server not supporting PSK does not help here.
>
>
> (BTW: Simlar thing comes up if you try to freeze an established TLS
> session: Currently you need to freeze a hash due to post-handshake
> authentication, even if you don't support it. Nothing else in TLS
> 1.2 or 1.3 needs hash freezing for established session).
>
>
> -Ilari
>