[dm-crypt] unlock via password and device

["chiasa.men" <chiasa.men@web.de>]

For the purpose of full disc encryption the tutorials usually suggest to
decrypt luks via usbkey.
The keyscript then often waits the key to appear for several seconds, after
that they fall back to the password prompt.

If the key appears after the waiting time it is ignored.

Is there a way to "step back" after the password prompt appeared?

The current workaround is another process that loops for the key in the
background, decrypts the disk and kills "askpass" several times (resulting in
"wrong password" in the main process)

What would be a clean way to implement the wanted behavior?

As I understood, the keyscript has to print the key to stdout.

I would expect the following keyscript construct to work as well:

---
while luksNotOpened
  if [ -e key ]
    dd if=key
  sleep 1
done &

askpass
---
The background process should still print to stdout and I guess askpass would
do the same when enter is pressed?
However that does not work