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List:       linux-crypto
Subject:    Re: Vulnerability in encrypted loop device for Linux
From:       Marc Mutz <Marc () Mutz ! com>
Date:       2001-12-19 13:59:42
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On Wednesday 19 December 2001 04:20, Jerome Etienne wrote:
<snip>
> The text attached describes what i believe to be a security hole in
> the encrypted loop device for linux. Because of it an
> attacker is able to modify the content of the encrypted device
> without being detected. This text proposes to fix the hole by
> authenticating the device.
<snip>

IMO, this is nothing new. What you try to tell us is that encryption 
doesn't guarantee integrety. It doesn't, encryption provides only 
confidentiality. This is well-known and it's probably the first thing 
that's discussed in any cryptography textbook, chaper "modes of 
encryption". It's also well-known that CBC in the form used in kerneli, 
doesn't provide intergrety. No-one I know ever claimed that encrypted 
loop devices are immune to undetected alteration.

Given, nobody ever explicitly told the user, either.

I also strongly agree that having the encrypted filesystem MAC'ed would 
be a nice thing to have and you are the first (to my knowledge) to come 
forward with this idea.

But to call this a vulnerability is a bit far-featched, I think.

As a workaround, users could use GnuPG (or PGP if you want) to create a 
detached signature of the /dev/loop device after unmounting and to 
check this signature before mounting. If I'm not mistaken you can even 
leave the file lying around, since the MAC is then signed with your 
secret PGP key anyway.

Marc

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Linux-crypto:  cryptography in and on the Linux system
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