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 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 the phone number?
I found it on internet and it's cool. At first glance you'll try to encrypt. But this is just about checking and Eve cannot change the message (this is not an hypothesis). So you can leverage this property
a XOR b XOR b = a
and send this message "XOR the number with this string". Cool XOR isn't it?