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the key is quite easily :) computed from the lock
key[x]= ns(lock[x]^lock[x-1])
...
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

  $key_arr[0] = pack 'h2', unpack 'H2',
      ( $lock_arr[0] ^ $lock_arr[@lock_arr - 1]
...
    $key_arr[$i] = pack 'h2', unpack 'H2',
        ($lock_arr[$i] ^ $lock_arr[$i-1]);
  }