the key is quite easily :) computed from the lock
key[x]= ns(lock[x]^lock[x-1])
  ns is a nibble swap (switch the upper 4 bits with the lower 4 bits)
exception:
ey[0] is a bit different
let's name A and B the 2 last bytes of the lock
 key[0]= ns(lock[0]^A^B^0x05)         ; 0x05 is a kind of magic nibble

##</code><code>##

  $key_arr[0] = pack 'h2', unpack 'H2',
      ( $lock_arr[0] ^ $lock_arr[@lock_arr - 1]
        ^ $lock_arr[@lock_arr - 2] );
  for (my $i=1; $i<@lock_arr; $i++) {
    $key_arr[$i] = pack 'h2', unpack 'H2',
        ($lock_arr[$i] ^ $lock_arr[$i-1]);
  }