[dm-crypt] cryptsetup: luksFormat: requires <device> as arguments

Hello,

I am trying to create a new detached luks header, but the command fails:


   cryptsetup -v --type luks1 --cipher aes-xts-plain64 --key-size 512
--hash sha512 --use-random luksFormat /dev/null --header LUKS-HEADER
--align-payload=0

   WARNING!
   ========
   This will overwrite data on LUKS-HEADER irrevocably.

   Are you sure? (Type uppercase yes): YES
   Enter passphrase for LUKS-HEADER:
   Verify passphrase:
   Device /dev/null is not compatible.
   Command failed with code -3 (out of memory).


And when I ommit /dev/null, then I get another error:


   cryptsetup: luksFormat: requires <device> [<new key file>] as arguments


Why does cryptsetup require a device, when I am formatting the detached
luks header? No device is involved at this point. All I want is to
create header file.

How can I create a header, without specifying the device ?

thanks,