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Re: RFC: backporting GEOM to the 4.x branch

From: Roland Dowdeswell <elric@xxxxxxxxxx>
Date: Sun, 27 Feb 2005 16:30:32 -0500

[ cc'ing tech-security@xxxxxxxxxx, because there has been talk
  of GBDE there in the past.]

Well, I thought that since I saw this:

ALeine wrote a while ago:
>df@xxxxxx wrote:
>> Wouldn't be easier porting cgd* from NetBSD ?
>> * http://www.netbsd.org/guide/en/chap-cgd.html
>Perhaps, but I believe GBDE to be superior to CGD for a number
>of reasons, one of the most important being that with GBDE you
>can change the passphrase without re-encrypting the entire disk,
>which is not the case with CGD, AFAIK. From Poul-Henning Kamp's
>paper on GBDE:

That, as the author of CGD, I should respond to some common
misconceptions about my work which seem to be percolating around.

First, on the capability front, you can:

	1.  change the passphrase on a disk without re-encrypting it,
	2.  have as many passphrases as you would like to configure,
	3.  use n-factor authentication with arbitrary large n.

Also, GBDE has a number of serious drawbacks.  All of which would
be show-stoppers if I were considering using it for serious security
work, or even use in a production environment.

There is no protection _at_all_ against dictionary attacks.  Where
CGD uses PKCS#5 in a completely standard way to frustrate dictionary
attacks, GBDE does exactly nothing.  In fact, worse than nothing.
It is possible to conduct half of the dictionary attack offline,
so the actual online portion of the attack is something that my
laptop could make about 2^30 guesses in a couple of hours.  So, it
is insecure from the start.

GBDE has no facility for using different encryption algorithms than
the rather...  interesting one that it comes with.  There is no
way to trade speed and security for different use cases, and the
only algorithm that it comes with is very slow.  Less than half
the performance of CGD's most secure algorithm (AES256).

So, now that we've touched on the security problems...  Let's think
about using GBDE in production.  Please reference


And read Section 7.5, and refer to figure 2.

Each disk write involves two writes to the disk.  Where is the
journal?  I do not see any talk about a journal in the paper, or
the GBDE source code.  Hence, if the OS crashes or if a removable
disk is removed at the wrong time, etc. etc. it is possible that
only one of those writes would succeed.  I think that we can all
see where this is going.

    Roland Dowdeswell                      http://www.Imrryr.ORG/~elric/

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