AW: methods of filling encrypted disks

[Kuehn, Ulrich]

> Von: Travis H. [mailto:solinym at gmail.com] 
> 
> So on this page:
> http://www.saout.de/tikiwiki/tiki-index.php?page=EncryptedDevice
> there is a suggestion that people fill the encrypted image of 
> a dm-crypt target with random data.  Why?
> 
[...] 
> I found the suggestion of using /dev/urandom to be far too 
> slow, as it produces 160 bits of output per SHA-1 
> computation.  I want to know if the fourth paragraph is 
> correct, that copying /dev/zero to the upper layer before 
> creating a file system would indeed provide the same 
> protection against whatever attack the "fill with random bits"
> protects against.

What about using /dev/zero to fill the drive? Assuming that you 
configure dm-crypt to use a secure cipher and a reasonably good 
mode of operation, of course. Maybe use a key different from that 
you will use finally for the device.

However, make sure that you do that before mkfs, otherwise all the
non-user-writeable parts of the device (inode tables etc) will not
be filled.

Are there any problems with this? I would assume that when the 
crypto is good enough for my data, it should also hide all-zeroes,
shouldn't it?

Ulrich

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