# cryptsetup status usb
/dev/mapper/usb is inactive.
# cryptsetup open --key-file /etc/usbkey
/dev/disk/by-uuid/98b904aa-5b37-476e-a690-5c73cc93a5b7 usb
# cryptsetup status usb
/dev/mapper/usb is active.
  type:    LUKS1
  cipher:  aes-xts-plain64
  keysize: 512 bits
  key location: dm-crypt
  device:  /dev/sda
  sector size:  512
  offset:  4096 sectors
  size:    30027776 sectors
  mode:    read/write
# cryptsetup close usb
# systemctl daemon-reload
# systemctl start systemd-cryptsetup@cr_usb.service 
Please enter passphrase for disk Cruzer_Fit (cr_usb): ^C
# cat /etc/crypttab 
cr_mmcblk0p3  UUID=c0587206-aaa9-4e93-98be-d7ee30d6b12c
cr_usb  UUID=98b904aa-5b37-476e-a690-5c73cc93a5b7 /etc/usbkey nofail

       Each line is in the form

           name encrypted-device password options

       The third field specifies the encryption password. If the field is not
present or the password is set to "none" or "-", the
       password has to be manually entered during system boot. Otherwise, the
field is interpreted as an absolute path to a file
       containing the encryption password.