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