A man is on the bank of a river. He has a small box, a chain, a lock (like a masterlock), the key to that lock, and a diamond. He wants to get the diamond to a woman who is on the other side of the river.
She has an identical box to the man, a chain, a lock, and the key to that lock. (Note: each key opens only it’s respective lock and the locks are not the same.) The river is too big to swim across and the boxes do not float.
The only way to get the diamond across the river is put it on the ferry, which will only carry cargo. However, neither person trusts the ferry driver, so in order to send anything across the river, it must be in a locked box. (ex: They can not just send the key across.) Also, anytime a box is sent across it must be locked even if the box is empty.
Without killing the ferry driver, how does the man get the diamond to the woman?
The man puts the diamond into the box. He then puts his chain around the box and locks it. He sends the box over to the woman. She then puts her chain around the same box, locks it, and sends it to the man. The man uses his key to take off his lock and chain and sends the box back to the women. The woman can then use her key to take off her lock and chain and get the diamond.