Valuable object

Question:

You want to send a valuable object to a friend securely. You have a box that is more than large enough to contain the object. You have several locks with keys. The box has a locking ring that is more than large enough to have a lock attached. But your friend does not have the key to any lock that you have. How do you send the object securely?

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Solution: This puzzle has a direct application in cryptography. Let’s say you want to send a secret message to your friend. Because you don’t trust commercial and freeware encryption methods, you use a secret cipher of your own. Only you know the key to the cipher. Not even your friend knows it. So you send the encrypted message; your friend encrypts it further with his or her favorite secret cipher and sends it back to you. You remove your cipher and send it back to your friend; and he or she removes the second secret cipher and reads the clear text. This process has the benefit of being very quick and easy by e-mail.

The above story of the locked box is often told as an introduction to public key cryptography. The ideas are perhaps related in some ways. But the story may just be told to show that really secure ciphers (ones where the decryption keys are not spread around to many people) are possible.

Answer: Put the valuable object into the box, secure it with one of your locks, and send the box to your friend. Your friend should then attach one of his own locks and return it. When you receive it again, remove your lock and send it back. Now your friend can unlock his own lock and retrieve the object.

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