How do I send a gift without someone seeing what's inside?

Question. Mr. A wants to send a present to Mr. B by package delivery service. However, he does not want someone to see the contents of the gift on the way. The present comes in a box and the box can be padlocked. Mr. A and Mr. B both have their own padlocks and their own keys. Neither has the key to the other's padlock. How can these padlocks be used so that Mr. A can safely send the gift to Mr. B and Mr. B can open the box and see the gift? Please think about it, if you like. This is not a trick question.

The answers are

1.Mr. A puts his own padlock on the present and sends it to Mr. B.

2.Mr. B puts his own padlock on it and sends it back to Mr. A.

3.Mr. A unlocks his own padlock with a key and sends it back to Mr. B.

4.Mr. B can unlock his own padlock with the key and open the present.

In this way, even if both parties have only the key to their own padlocks, they can safely send presents to each other. Whoever thought of this is very clever. When I saw the answer, I thought "I see! I thought, "I see.

References

• he Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography by Simon Singh