If you’ve got anything even remotely private on your phone (and who doesn’t? Your phone has access to your email, and thereby access to everything else), you’ve hopefully got a security PIN on the lockscreen. But which numbers should you use? 4 unique numbers would be most secure, right? Not quite, and here’s why: we’ve got nasty, nasty fingers.
While 4 unique digits (each an unknown digit from 0-9) might seem like the most secure option for a 4 digit pin (with 10,000 possibilities), our greasy mitts introduce a bit of an issue: our fingerprints...
Before we dive in: Cheers to Skeleton Key Security for bringing this up and Presh Talwalker for having done the math a bit earlier. I’d highly recommend either for a deeper dive into the subject.
So, what are you to do? Use one of the numbers twice.
By using one digit twice, you’re introducing two challenges: first, the intruder must guess whether you used 4 unique digits (and one of the digits just didn’t smudge clearly), or 3 digits with 1 repeated. If they go with the latter, they’ll need to figure out which of the 3 digits was used twice. This increases the number of permutations from 24 to 36. It’s not at all intuitive, but using one less unique digit actually makes things marginally more secure.
“But wait,” you say. “24 to 36 isn’t a huge jump. If they’ve got the time to try 24 possibilities, they’ve got the time to try 36.”
Side note: Apple [or anyone else with a touchscreen-based PIN input system] could actually make the 4-digit system exponentially more secure with an option to randomize the placement of each digit on the keypad each time. This would negate the fingerprint issue, spiking the possible count back up to 10,000. Of course, this would also be terribly confusing and definitely shouldn’t be enabled by default =)