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

Martin Rex <mrex@sap.com> Wed, 21 September 2011 01:40 UTC

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From: Martin Rex <mrex@sap.com>
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To: mrex@sap.com
Date: Wed, 21 Sep 2011 03:42:22 +0200 (MEST)
In-Reply-To: <201109202314.p8KNERej029946@fs4113.wdf.sap.corp> from "Martin Rex" at Sep 21, 11 01:14:27 am
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Cc: asteingruebl@paypal-inc.com, geoffk@geoffk.org, tls@ietf.org
Subject: Re: [TLS] Rizzo claims implementation attach, should be interesting
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Martin Rex wrote:
> 
> Geoffrey Keating wrote:
> > 
> > Martin Rex <mrex@sap.com> writes:
> > 
> > > SSL was NEVER designed with a promise that you could multiplex
> > > data from an evil attack with data from a victim over the very same
> > > SSL connection and be secure against adaptive chose plaintext
> > > attacks trying to recover data from the victim.
> > 
> > This would imply that you should not use SSL for downloading e-mail
> > using POP or IMAP, because that's multiplexing evil data (mail sent to
> > you by the attacker) with trusted data (credentials and non-evil
> > mail).
>  
> You are quite correct in recognizing that you might be helping
> the attacker in a known-plaintext or chosen-plaintext attack (and when
> you have an automatically polling IMAP client, maybe even an adaptive
> chosen-plaintext attack).  So it would be sensible to limit the number
> of guesses for the attacker, and to not re-use the same crypto keys
> on subsequent accesses to the same IMAP server (TLS does the latter for
> you when you create a new TLS connection state for each subsequent access,
> i.e. TLS session resume or full handshake, both will do).

Ooops -- what I forgot:

Your IMAP cleartext logon password travels  Client->Server and the
attackers EMail on download travels Server->Client.  Since TLS uses
distinct traffic encryption and mac keys for both directions,
there is no chosen plaintext or adaptive chosen plaintext attack
for the particular situation of IMAP Email download, I believe.

But that would also suggest that BEAST is not attacking the
Cookie from the Server response, but instead the cookie from a
client request.  (If the browser automatically inserts the cookie into
arbitrary requests issued by the attackers malware, then this would 
mean that a serious Cross-Site-Request-Forgery problem in the browser
is a prerequisite for the BEAST attack to succeed.


-Martin