“In this simplistic example we have to admit that the sender has no way of knowing whose padlock is on the briefcase and that it might be possible for an adversary to impersonate the receiver and place their padlock on the briefcase. This is a problem that has to be addressed. The ‘Whose padlock is it?’ question in this briefcase example is similar to the ‘Whose public key is it?’ question that is so important when public key systems are used.”