When Wendy received William's padlocked box she put her own padlock on it and returned the box to William, retaining her key. William removed his padlock from the box and returned it to Wendy who then removed her own padlock, opened the box and retrieved the memento.

Of course, Wendy could have sent an open padlock to William in the first place, but who's to say that it wouldn't have been snapped shut accidentally or deliberately, or otherwise tampered with, on its journey. The above method takes into account almost every possible contingency and is similar to the double-coded algorithm based system widely used in computer encryptions.  -SZ