Subject: Re: FUD about CGD and GBDE
To: Poul-Henning Kamp <>
From: Thor Lancelot Simon <>
List: tech-security
Date: 03/03/2005 10:48:47
On Thu, Mar 03, 2005 at 01:18:45PM +0100, Poul-Henning Kamp wrote:
> In message <>, Bernd Walter writes:
> >No matter what disk you take - writes never have been atomic.
> >The major difference I see is that you get a read error back in
> >the disk failure case, while such a crypto failure produces more or
> >less random data without any error.
> >Mounting unclean filesystems rw for bg_fsck can be considered
> >dangerous with such unexpected data corruption.
> >And how would you know that a restore from backup is required for
> >a damaged file?
> 100% true.
> The trouble is that it would cost a lot in performance and a doubling
> in metadata to protect yourself against this.

No, it would not.  What it _would_ take would be an abandonment of the
adamant position that your home-grown cryptosystem is superior to
simply encrypting the disk with 256-bit AES.  After all, cgd doesn't
diminish the atomicity of writes; it's only your key-key blocks that
create this problem.

Generally, complexity is not considered a desirable property in
cryptosystems.  GBDE violates this rule in spades.  There are _reasons_
why complexity is not good: to begin with, a very complex cryptographic
construct will require detailed analysis (which it does not appear
GBDE has had by anyone but its author until Roland started looking at
it) in order that we may know that it is even as secure as the underlying
algorithmic building blocks it uses.

Furthermore, with a very complex cryptographic construct it is easy to
persuade oneself that the complexity actually has security benefits
which it does not, and thus go forth into the dangerous world unprotected
from _other_ attacks that one overconfidently ignores.  I'd say that is
certainly going on here as well (e.g. the continual minimization of how
GBDE's implementation makes dictionary attacks easier).

I have often observed the phenomenon that engineers who build extremely
complicated software systems insist vigorously that, due to their very
complexity (or perhaps due to the amount of time they've spent building
them) they _must_ be better than simpler systems that are easier to
implement and maintain.  One might characterize this as the love of
one's own big ideas.

The best antidote to this is to ask not "do I believe that all this
complexity could plausibly have some benefit" but rather "is there
some shortcoming in the simple system that could plausibly justify
all this complexity"; in other words, to begin from the assumption
that complexity is bad.

In the case of CGD and GBDE I think the answer is plainly "no, there
is not".  To believe that I wanted something better, I'd essentially
have to believe that AES256 were vulnerable to, at best, a chosen-
plaintext attack.  The key derivation is solid (and standard); the
encryption is solid (and standard); and it is a strong point in the
system's favor that it does not _try_ to give me more than I want
from it.  Compare to GBDE, where a massively complicated construct
using double-encryption -- and MD5! -- is used to splat keys all
over the disk but no attention is paid to key derivation at all.

That this conversation has degenerated to the point that GBDE's
proponents are claiming that it _would be_ more secure _if_ only
someone knew how to break 256-bit AES is to me a pretty good
indicator of why GBDE is not encryption software that I want to

 Thor Lancelot Simon	                            

"The inconsistency is startling, though admittedly, if consistency is to be
 abandoned or transcended, there is no problem."		- Noam Chomsky