Whenever someone uses a credit card on the internet, prime numbers spring into action.
Before the card number is sent over the internet, it must be encrypted (put into code) for security and once the code is received by the store, it must be decrypted (decoded).
The code that is used the most is called RSA and it is based on prime numbers. It uses a "public key", information that is available to anyone and a "private key" information that only the store has.
The "public key" is a large number that is the product of two large primes and the "private key" is the two large primes themselves.
It is very difficult to factor a given large number into primes, which is what you would have to do to break the code. For example, it took researchers two years to factor a 232-digit number, even using hundreds of parallel computers.
So when a credit card is used, only the "public key" is sent from the store, the message is coded using the key, and sent to the store. The store already has the answer to the key and can easily uncode the message.