Boris and Natasha
Boris and Natasha live in different cities in a country with a corrupt postal service. Every box sent by mail is opened by the postal service, the contents stolen, and the box never delivered. Except: if the box is locked, then the postal service won’t bother trying to open it (since there are so many other boxes whose contents are so much easier to steal) and the box is delivered unharmed.
Boris and Natasha each has a large supply of boxes of different sizes, each capable of being locked by padlocks. Also, Boris and Natasha each has a large supply of padlocks with matching keys. The padlocks have unique keys. Finally, Boris has a ring that he would like to send to Natasha. How can Boris send the ring to Natasha so that she can wear it (without either of them destroying any locks or boxes)?
Boris puts the ring in a box and locks it with one of his padlocks, and sends the box to Natasha. Natasha attaches one of her padlocks (so now the box is locked with two locks) and sends the box back to Boris. Boris removes his padlock and sends the box back to Natasha. Natasha removes her padlock, opens the box, and removes the ring.
Of course the corrupt postman will withhold the box Boris sent, add one of his (the postman’s) own padlocks, send it back to Boris, who will unlock his padlock so that the postman can open the box with his key.
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