Re: [TLS] Finished stuffing

Eric Rescorla <ekr@rtfm.com> Sat, 17 September 2016 21:44 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sat, 17 Sep 2016 14:43:49 -0700
Message-ID: <CABcZeBM7J8r2P2MoqLJ0UTKg7M1JDJ-_YN_KA-Tk=rz=KbgMaw@mail.gmail.com>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Finished stuffing
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On Tue, Sep 13, 2016 at 12:15 PM, Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Tue, Sep 13, 2016 at 12:04:40PM -0500, Benjamin Kaduk wrote:
> >
> >
> > On 09/09/2016 03:19 PM, Ilari Liusvaara wrote:
> > > On Fri, Sep 09, 2016 at 02:50:59PM -0500, Benjamin Kaduk wrote:
> >
> > >> I have a slight (i.e., unjustified) preference for doing
> > >> ClientHello-with-block-of-zeros rather than prefix-of-ClientHello.
> (Is
> > >> there a reason to require this extension to be the last one with
> > >> block-of-zeros?  Clearly there is for prefix-of-ClientHello.)
> > > What about the case where client tries DHE-PSK and gets attempt
> > > rejected because of missing group (or because address verification)?
> > > 0-RTT is gone yes, but the PSK attempt isn't.
> > >
> > > What happens to the hash in this case?
> > >
> > >
> >
> > I feel like I must be missing something, but I don't really understand
> > the question.  (Sadly, waiting in the hope that someone else did
> > understand and would respond didn't work.)  The 0-RTT failed, so the
> > full handshake will have an actual Finished message, with a different
> > hash calculated (including over the "hello_finished" extension).  The
> > most plausible way I could interpret the question seems to be asking
> > about the lack of Hash(resumption_context) in the 1-RTT Finished, but
> > the security properties of that should be the same as for the
> > hello_finished, so I'm still puzzled.
> >
> > Sorry for being dense...
>
> I mean the following case (perhaps bit misconfigured server):
>
> Client: ClientHello(groups=23,24,29;PSK=foo;shares=23:bar,29:baz,.
> ..,finished=zot)
> Server: HelloRetryRequest(group=24)
> Client: ClientHello(groups=23,24,29;PSK=foo;shares=23:bar,29:baz,
> 24:quux,...,finished=???)
>
>
> What is the finished data calculated over in the second case?
>

In this case, I believe that the finished is computed over
"ClientHello(groups=23,24,29;PSK=foo;shares=23:bar,29:baz,24:quux,..."

But that the handshake transcript is computed over all of:
"Client: ClientHello(groups=23,24,29;PSK=foo;shares=23:bar,29:baz,.
..,finished=zot)
Server: HelloRetryRequest(group=24)
Client: ClientHello(groups=23,24,29;PSK=foo;shares=23:bar,29:baz,
24:quux,...,finished=???)"

-Ekr

-Ilari
>
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