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

What's a verification code, and why are they so important? Six-digit verification codes are a form of two-factor authentication, a process that helps keep your important online accounts secure.

While the six number passcode is not mandatory, it is highly recommended. If you would rather take your chances, though, and go with a four digit code, you can make that selection in your settings ...

Enter a four-digit passcode, verify it, and you’re there: you’re back to the happy days of four-digit passcodes. Read next: How to remove or bypass a forgotten iPhone passcode ...