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

Ilari Liusvaara <ilariliusvaara@welho.com> Tue, 18 April 2017 20:44 UTC

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Date: Tue, 18 Apr 2017 23:44:02 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Eric Rescorla <ekr@rtfm.com>
Cc: "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] PR#962:CertificateRequest in post-handhake phase
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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!)..


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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