Re: [TLS] Finished stuffing/PSK Binders

Eric Rescorla <ekr@rtfm.com> Tue, 11 October 2016 13:52 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 11 Oct 2016 06:51:26 -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 7:10 AM, Eric Rescorla <ekr@rtfm.com>; wrote:

>
>
> 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.
>

See:
https://github.com/tlswg/tls13-spec/pull/678

-Ekr


> 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
>>
>
>