You need to check that your friend, Bob, has your correct phone number…

…, but you cannot ask him directly. You must write the question on a card which and give it to Eve who will take the card to Bob and return the answer to you. What must you write on the card, besides the question, to ensure Bob can encode the message so that Eve cannot read your phone number?

 

Answer: Since you are just "checking," you ask him to call you at a certain time. If he doesn't, he doesn't have your number.

Too simple? A reader suggest: "In that case you need a check-sum. Have Bob add all the digits of your phone number together, write down the total, and pass that back to you."