Re: [TLS] Finished stuffing/PSK Binders

Eric Rescorla <ekr@rtfm.com> Sun, 09 October 2016 15:51 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 9 Oct 2016 08:50:35 -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 8:44 AM, Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Sun, Oct 09, 2016 at 07:10:59AM -0700, Eric Rescorla 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.
> >
>
> That creates a bit of a edge case, where the server might need to pull
> client's share accross retry.
>
> (Since if group is OK, the group is not included in HRR, which presumably
> causes the client to blank its KeyShare).
>

I think we would change that rule, probably.

-Ekr


>
>
> Also, doing some size calculations:
>
> ClientHello with "smallest group" rule is ~200 bytes with reasonable
> parameters (I got a dump of one TLS 1.3 draft-16 CH from my TLS stack,
> it is 265 bytes, but would be 196 with "smallest group" rule).
>
> The amount of space needed to freeze (octet) SHA-256 is 32+63+8=103
> octets. And the space needed to freeze (octet) SHA-384 is 64+127+16=207
> bytes (for full SHA-256 and SHA-384, add one more byte).
>
> So if selecting ciphersuite using SHA-384, the state size might be
> comparable to ClientHello size.
>
>
> -Ilari
>