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Re: [TLS] PR#962:CertificateRequest in post-handhake phase

Eric Rescorla <ekr@rtfm.com> Tue, 18 April 2017 20:47 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 18 Apr 2017 16:46:20 -0400
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] PR#962:CertificateRequest in post-handhake phase
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On Tue, Apr 18, 2017 at 4:44 PM, Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:

> On Tue, Apr 18, 2017 at 03:51:53PM -0400, Eric Rescorla wrote:
> > On Tue, Apr 18, 2017 at 3:41 PM, Ilari Liusvaara <
> ilariliusvaara@welho.com>
> > wrote:
> > >
> > > On topic of PSKs, I noticed that TLS 1.3 makes it very easy to mount
> > > dictionary attacks against PSK, regardless of DHE-PSK (especially to
> > > recover the client PSK). I assumed that the document already documents
> > > this, but I couldn't find any remark that using low-entropy PSKs is
> very
> > > bad idea.
> > >
> >
> > Good point. As far as I can tell...
> >
> > 1. You can search the binder.
> > 2. Because we forbid PSK with server authentication, you can also
> > impersonate the server and then mount a dictionary attack (even w/o the
> > binder).
>
> AFAICT, if I wanted to extract low-entropy client PSK, I wouldn't even
> bother with 2, since 1 seems so much more efficient (all passive too!)..
>

I agree. I just meant that if we were to dispense with the binder it would
still be trivial.

-Ekr


>
>
> Also, wonder if there are efficient ways of probing server PSKs (I
> didn't compe up with one, even in pure-PSK, due to binders making
> such probing nontrivial).
>
>
> -Ilari
>

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