Taking the analogy further, imagine that Alice designs a padlock and key. She guards the key, but she manufactures thousands of replica padlocks and distributes them to post offices all over the world. If Bob wants to send a message, he puts it in a box, goes to the local post office, asks for an “Alice padlock” and padlocks the box. Now he is unable to unlock the box, but when Alice receives it she can open it with her unique key.
