Suppose that you obtained a certificate of your own. Explain how you could use that certificate to deal with situations in which an intruder might steal your credit card number.

What will be an ideal response?

You use a protocol that requires that a merchant accept a credit card only if it comes with a certificate and that the credit card number is always encrypted with your private key. Thus only you, and not an intruder, could have sent it. Effectively, you are signing your credit card number.