The cipher_null is no-encryption, it can be used for testing
or temporarily when encrypting device (cryptsetup-reencrypt).
Accepting only empty password prevents situation when you replace
a LUKS header on an unlocking device with the faked header using
null cipher (and the same UUID).
Here a system could think that the device was properly unlocked
(with any entered password) and will try to use this unencrypted
partition instead.
(IOW it prevents situation when attacker intentionaly forces
an user to boot into dirrerent system just by LUKS header manipulation.)
Properly configured systems should have an additional integrity protection
in place here (LUKS here provides only confidentiality) but it is better
to not not allow this situation in the first place.
(Despite the fact that once you allow physical tampering of your system
it cannot be properly secured anymore.)
If reencrypt removes encryption (or adds encryption to not yet encrypted system)
there is a temporary header using null (none) cipher.
We do not need to pass through password when unlocking these devices.
The patch adds the two options
--perf-same_cpu_crypt
--perf-submit_from_crypt_cpus
that set the same named options inside dmcrypt
(available in Linux kernel 3.20 and later).
For historic reasons, in the plain mode the hashing is not used
if keyfile is used (with exception of --key-file=-).
Print warning if the parameters are ignored.
For other cases, uses keyfile offset, keyfile size and hash
as psecified on commandline.
Partially fixes issue#243
If AddKey was called with master key argument, the code always asked
for a passphrase ignoring the keyfile argument.
Now it is properly processed as the same as if no master key is specified.
The --header always takes precedence over positional device argument.
Also allow specify UUID= for luksSuspend and luksResume if used with
detached header.
Apparently there are some people using ECB.
This mode by design do not use any IV, unfortunately
kernel dmcrypt allows to specify them (but userspace crypto api don't).
Let support activation as it was in previous version.
Should fix issue#238.
