You need to check that your friend, Bob, has your correct phone number, but you cannot ask him directly. You must write a question on a card which and

You need to check that your friend, Bob, has your correct phone number, but you cannot ask him directly. You must write a 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?
ask bob the difference between my number and his number in decimal.
Or
Give bob your public key, and ask bob encrypt his phone no. by this public key and send back. You use your private key to decrypt the message