$ dd if=/dev/zero of=verysecret.loop bs=52428800 count=1
1+0 records in
1+0 records out
$ losetup /dev/loop0 verysecret.loop
$ cryptsetup -c aes-cbc-essiv:sha256 -y -s 256 luksFormat /dev/loop0

WARNING!
========
This will overwrite data on /dev/loop0 irrevocably.

Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase: ******
Verify passphrase: ******
$ cryptsetup luksOpen /dev/loop0 verysecret
Enter LUKS passphrase: ******
key slot 0 unlocked.
$ mkfs.xfs /dev/mapper/verysecret

[...]

$ mount /dev/mapper/verysecret /mnt
$ umount /mnt
$ cryptsetup luksClose verysecret
$ cryptsetup luksAddKey /dev/loop0
Enter any LUKS passphrase: ******
key slot 0 unlocked.
Enter new passphrase for key slot: ******
$ cryptsetup luksDelKey /dev/loop0 0
losetup -d /dev/loop0