When Bob receives the box with only his padlock on it , Bob can then unlock the box with his key and read the message from Alice .
When Alice receives the box with the two padlocks , she removes her padlock and sends it back to Bob .
When Bob receives the box with only Bob 's padlock on it , Bob can then unlock box box with his key and read the message from Alice .
When Alice receives the box with the two padlocks , she removes her padlock and sends it back to Bob .