$ echo "bv:<last13ofcookie>" > trash
$ echo "<mypass>" > trash
$ john -w=trash userpass 
Loaded 1 password hash (Traditional DES [128/128 BS SSE2])
<mypass>         (bv)
guesses: 1  time: 0:00:00:00 100%  c/s: 25.00  trying: <mypass>