If you have an Apple or an Android phone—and you probably do—you have the option of locking your phone using a PIN code. With most of these phones, you can choose a four-digit PIN or a six-digit PIN.

Android offers a number of ways to lock down your device. There is the classic pattern lock, PINs, passwords, and the gimmicky face unlock. These options (mostly) get the job done, but a passcode that ...