full-disclosure - Re: Possible issues with encrypted Linux filesystems?

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Message-ID: <4D064F88.7000900@gmail.com>
Date: Mon, 13 Dec 2010 11:53:28 -0500
From: Steve Pinkham <steve.pinkham@...il.com>
To: full-disclosure@...ts.grok.org.uk
Subject: Re: Possible issues with encrypted Linux
	filesystems?

Disclaimer: I'm not a cryptographer.  I don't even play one on TV.

> I'm now worried that if an attacker knows, or "guesses" that you are
> using, say, CentOS Linux 5.5, (or at least some mutation of Red Hat),
> he might use this knowledge of "known artefacts" to his advantage, by
> starting out from the data he knows "must be there", and looking for
> it's "patterns". I don't know... This may be a longshot, wishful
> thinking or both, but somehow it feels to me like it's a lot easier
> to break a code when you already know exactly what the decrypted data
> is, and what it looks like.

These sorts of attacks were the design criteria for modern disk
encryption modes.

http://en.wikipedia.org/wiki/Disk_encryption_theory

XTS is the current best mode for FDE.  CBC-ESSIV seems "strong enough"
against most reasonable attacks, which is why it is still the default in
some software.  If an attacker has access to your computer twice,
they're going to install a keylogger, not image your data and hope you
change the sectors with sensitive data and rewrite them in the same place.

The strength and protection of your key material should be your primary
concern. (Actually, I'd say your primary concern should be keeping a
clean conscience, but that's out of the scope of this discussion.  ;-)

You are much more likely to be compromised by malware, a keylogger, a
hidden camera pointed at your keyboard, or "thermorectal cryptanalysis"
then any of the things you are worried about.  Physical security trumps
other concerns.

Obligatory XKCD:
http://xkcd.com/538/
-- 
 | Steven Pinkham, Security Consultant    |
 | http://www.mavensecurity.com           |
 | GPG public key ID CD31CAFB             |

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