[dm-crypt] minimal LUKS container size

Arno Wagner arno at wagner.name
Fri Dec 16 08:14:10 CET 2011

On Wed, Dec 07, 2011 at 01:03:26PM +0100, Milan Broz wrote:
> Minimal size depends on two things:
>  - volume key size
>  - alignment of data area
> Cryptsetup 1.3 by default aligns data to 1MB offset multiple, that's why
> you see this change.
> You can switch to old alignment by using data alignment override, e.g.
> cryptsetup luksFormat -s 128 --align-payload=8
> (So here forcing keysize to 128bit and forcing alignment to 4k offset.
> - vaule is in 512 byte sectors.
> This is perhaps the smallest header possible while still using
> reasonable key size - 1032 x 512 bytes sectors ~ 520kB)
> (But also note that misalignment to flash memory block can have
> some performance effects.)
> Anyway, some example in FAQ would be nice.

Just added this. Tested with 1.3.1 and also 1.4.1 for arc4.

To my surprise, giving alignment --align-payload=1 (or = 2) results 
in a data-area staring at 0x101000, while --align-payload=0 gives 
2MiB offset. Is this rounded up to multiples of 8? I had a brief 
look into the sources but did not find the relevant code. 
The full behaviour should go into the man-page under the 
explanation for "--align-payload". 

Data area size can be 512 bytes (verified with loop-device), but
not 0 bytes, as this gives an ioctl-error on opening.

For cipher, the most extreme I found is RC4 with an 8 bit key.
This can be luksFormat-ed, but fails on opening. This is 
probably a bug when using arc4, as it fails for 128 bit as well.
(tested with 1.4.1). 

Blowfish with 64 bits works though, but is insecure, so added with
a warning. 

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
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