cryptsetup can't destroy last key of a LUKS partition under Ubuntu/Debian

Hello everyone,

I noticed last week that the Debian packaged version of cryptsetup has a
little limitation, which could be a security issue for people who have to
destroy their data forever.

It is impossible to destroy a keyslot when you used it to unlock the master
key.

I reported the bug to debian (etch and lenny are affected as far as I tested):
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=513596
and to ubuntu (tested on hardy):
https://bugs.launchpad.net/cryptsetup/+bug/324871

It's not a major security problem, but people who were planning to run
'cryptsetup luksDelKey /dev/sda1 0' on their installation when the police
comes to wake them up should be adviced that it won't work out of the box.

Cheers,

Pierre Dinh-van