You’re at the checkout screen after an online shopping spree, ready to enter your credit card number. You type it in and ...

All mainstream credit card numbers obey a mathematical trick designed to catch the most common typos. It’s called the Luhn ...

The Luhn algorithm—also known as the “Modulus 10 algorithm”—is a formula used to determine the accuracy of credit card numbers.

Using terms that sound like something out of a spy novel — such as MIIs and the Luhn algorithm — you can decode parts of a credit card number and even tell whether it’s fake or valid.