Re: Encrypted backups

[Klaus Holler <kho@gmx.at>]

On Tuesday 13 January 2004 17:17, Marcus Williams wrote:
> On 13/01/2004, Bill wrote:
> > - make sure your kernel support encrypted loopback & your
> > encryption of choice - configure a crypto loop to your tape
> > device
> >
> > losetup -e twofish /dev/loop0 /dev/nst0
>
> Well, you learn a new thing every day. Never thought you could do
> that! What happens with media errors on the tape when you read it
> back?

depends on the cipher mode you use:

if using ECB (electronic codebook mode) every block of size 
AES_BLOCK_SIZE (e.g. for AES) is encrypted independently from each 
other - then media defects in one block should be a problem _only_ 
for the current block. Caveat: Statistical analysis are easier 
though if well-defined (known plaintext) values occur; refer to [1] 
for details.

if using CBC (cipher block chaining mode) you will be more dependent 
from media errors because this mode adds a feedback mechanism - 
encryption results from previous blocks are taken into account when 
calculating the current one (plaintext block is XORed with previous 
ciphertext block). I guess that you will lose all blocks starting 
from the block having the media defect ;-(

Instead of specifying /dev/nst0 as loopback target device just 
specify a local file myfile created with dd if=/dev/zero 
of=/tmp/myfile bsize=1024,count=...,skip=...), then you can 
simulate tape errors manually by changing the file using a hex 
editor - just try to recover afterwards ;-)

Regards, 
Klaus

[1] Schneier Bruce: Applied Cryptography. 2nd edition. Chapter 9. 

-- 
Dipl.-Ing. Klaus Holler <gmx.at after kho@>