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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 14:18:42
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On Wednesday 19 December 2001 14:29, Jerome Etienne wrote:
> On Wed, Dec 19, 2001 at 11:51:43AM +0100, Gisle S{lensminde wrote:
> > Yes, this is a problem with loopback crypto. The problem is that
> > the loopback interface assume that it's length preserving,
>
> can you explain the rationnal behind such assumption ?
<snip>

Loopback transforms are block transformations. They act upon the blocks 
of a block device. You cannot alter the size of blocks

Well, theoretically, you could make the input block size different from 
the output block size, but since both the HD's and the linux kernel 
have hard-coded (a minimum of) 512 byte blocks, you could only increase 
or decrease those sizes by an amount of at least 512 bytes. Worse: 
block sizes are assumed to be powers of 2 (except in special cases like 
audio CD's which aren't handled by the normal block device layers, 
AFAIK). So you can't go and take e.g. 32k clusters and append to each 
one a 512 byte block to hold the MAC.

Marc

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