Here’s the problem. When you enter a passcode into your iPhone, the processor has to make a calculation to check if your code is correct. But Apple has made the math so complicated that it takes about 80 milliseconds — roughly 1/12 of a second — for the phone to crunch the numbers.

“This means it would take more than 5 ½ years to try all combinations of a six-character alphanumeric passcode with lowercase letters and numbers,” Apple security guide says.

How did they come up with 5.5 years?

Well, that’s assuming, first, that the phone has been disabled from locking you out after a few bad tries. Also we’re assuming that you have some way to enter your guesses electronically instead of tapping out billions of numbers by hand. (Basically, we’re assuming that Apple has chosen to help you guess and doesn’t have another backdoor into the system we don’t know about.)