Each person creates a key and hundreds of locks that can only be opened using that key. Everyone keeps their key in a secure place in their house, but they distribute their locks to hardware stores like the Home Depot around the country. Say you live in this country and want to send a box to your friend Maria. You grab one of Maria’s locks from your local hardware store and attach it to your box. When you mail the box to her, the postal service intercepts it. Of course, without the key, they can’t open it! But when Maria gets the box, she can open it because she has the sole key that can
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