Three Is The Magic Number For Smartphone Security
If you have a smartphone you might also have a PIN code to unlock it. Well and good, but the usual four digits isn’t terribly secure. Why not? Because of finger smudges. It turns out it isn’t hard to read the finger smudges on a phone and figure out the PIN.
An article at Mind Your Decisions and Skeleton Key Security both outline the math, but the simplest way to boost security on a four-digit PIN is to use three different numbers instead of four. That’s because with four different numbers -- 1234, for instance -- there are 24 possible permutations. But repeat a digit, for example by using only 1, 2, and 3, and that number goes up to 36.
This is because the repeated number can go anywhere in the sequence, and the would-be hacker has to try all the sequences in which a number is repeated. Since there are 12 for each digit that ads up to 36 combinations. As it turns out three is the “magic number” in a four-digit PIN, because using two reduces the number of permutations too much. There are only two finger smudges, but only 14 possibilities.
Of course, the simplest way to secure a phone is to either clean smudges off the screen, or use a PIN with more than four digits. But not every phone lets you change the number of digits in the PIN, and this method gives a slight improvement in the case of a stolen phone. Crucially, it buys the owner more time to do a remote wipe.
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