Re: [TLS] Rizzo claims implementation attach, should be interesting

Martin Rex <mrex@sap.com> Wed, 21 September 2011 00:08 UTC

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From: Martin Rex <mrex@sap.com>
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To: nico@cryptonector.com (Nico Williams)
Date: Wed, 21 Sep 2011 02:10:24 +0200 (MEST)
In-Reply-To: <CAK3OfOhBcrRG6AanVmqmXtXoZcPA5s0iyXH5bu2r2TrnUg4egA@mail.gmail.com> from "Nico Williams" at Sep 20, 11 06:33:17 pm
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Cc: asteingruebl@paypal-inc.com, tls@ietf.org
Subject: Re: [TLS] Rizzo claims implementation attach, should be interesting
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Nico Williams wrote:
> 
> Martin Rex wrote:
> > > data sent over SSL might be untrusted?  On what grounds would it have
> > > been OK to say "no untrusted data, please"?  And where was this
> > > restriction documented?
> >
> > It is a design limit that should be obvious to the conscious reader
> > with background knowledge about cryptography.
> 
> It's been roughly two decades, and only now did this get noticed.  And
> there are many experts who've worked on TLS and apps that use it.
> 
> Undocumented design limitations are not obvious, even if in retrospect
> they should have been.

You seem to be approaching the issue from the wrong end.

Rely only on those design properties that have been explicitly specified
_and_ independently verified.  NEVER assume capabilities / properties
that are neither specified nor independently verified.

Of course, taking short cuts is much easier and cheaper, but don't
complain later if you get burnt.


> 
> > But when you start multiplexing adaptively chosen plaintext from
> > an attacker with data from a victim, then you're creating an oracle.
> 
> Multiplexing?  Just mixing will do.  IMAP lets you... read emails from
> untrusted sources (e.g., spam), as well as ones from trusted sources.
> Oops.  This is silly.

No, this isn't silly.  Welcome to the real world.

Your 2^112 bits symmetric crypto strength for 3DES-EDE-CBC
and 2^128 bits symmetric crypto strength for AES128-CBC can be
significantly reduced by your usage scenarios.  In a highly automated
online world up to the point of non-existence, as in Rizzo's last
years demonstration for ASP.NET:

http://www.schneier.com/blog/archives/2010/09/new_attack_agai_1.html

Such flaws are _not_ limited to propriatary misuse of crypto.
There is more to come later this year.


-Martin