[Brett's] girlfriend is very concerned about cell phone security — So much so that she used a PIN so secure, even she couldn’t remember it.
Beyond forgetting the PIN, the phone also had encryption enabled, the bootloader locked, and zero permissions for the Android Device Manager to change the PIN. Lucky for her, [Brett] had purchased an STM32F4Discovery Development Board a few months ago, and was itching for a suitable project for it.
Now unfortunately, Android allows you to pick a PIN of anywhere between 4 and 8 digits, which as you can guess, results in a massive number of possible permutations. She was pretty sure it was only 6 digits, and that she didn’t use a 1, 2, or 3… and she thought it started with a 4 or a 7… and she didn’t think any of the digits were repeated… This helped narrow it down a bit, from 1 million possibilities to about 5,000 — assuming all of the boundary conditions she remembers are in fact correct.
[Brett] started by writing a C library to generate permutations of the PIN, testing the board on his own phone to make sure it works with a known PIN, and boom, they were in business.
28,250 PIN attempts later, they decided they were not. Did we mention you can only enter 5 PINs in every 30 seconds?