When Alice receives the box with the two padlocks , she removes her padlock and sends it back to Bob .
When Bob receives the box , he adds his own padlock to the box , and sends it back to Alice .
When Alice receives the box with the two padlocks , Alice removes Alice 's padlock and sends it back Bob Bob .
When Bob receives the box , he adds his own padlock to the box , and sends it back to Alice .