Hacking microcontroller firmware through a USB

[harlan4096]

In this article, I want to demonstrate extracting the firmware from a secure USB device running on the Cortex M0.

Who hacks video game consoles?

The manufacture of counterfeit and unlicensed products is widespread in the world of video game consoles. It’s a multi-billion dollar industry in which demand creates supply. You can now find devices for almost all the existing consoles that allow you to play copies of licensed video game ‘backups’ from flash drives, counterfeit gamepads and accessories, various adapters, some of which give you an advantage over other players, and devices for the use of cheats in online and offline video games. There are even services that let you buy video game achievements without having to spend hours playing. Of course, this is all sold without the consent of the video game console manufacturers.

Modern video game consoles, just like 20 years ago, are proprietary systems where the rules are set by the hardware manufacturers, and not by the millions of customers using those devices. A variety of protective measures are included in their design to ensure these consoles only run signed code, so they only play licensed and legally acquired video games and all players have equal rights and only play with officially licensed accessories. In some countries it’s even illegal to try and hack your own video game console.

But at the same time the very scale of the protection makes these consoles an attractive target and one big ‘crackme’ for enthusiasts interested in information security and reverse engineering. The more difficult the puzzle, the more interesting it is to solve. Especially if you’ve grown up with a love for video games.

Protection scheme of DualShock 4

Readers who follow my twitter account may know that I’m a long-time fan of reverse engineering video game consoles and everything related to them, including unofficial game devices. In the early days of PlayStation 4, a publicly known vulnerability in the FreeBSD kernel (which PlayStation 4 is based on) let me and many other researchers take a look at the architecture and inner workings of the new game console from Sony. I carried out a lot different research, some of which included looking at how USB authentication works in PlayStation 4 and how it distinguishes licensed devices and blocks unauthorized devices. This subject was of interest because I had previously done similar research on other consoles. PlayStation 4’s authentication scheme turned out to be much simpler than that used in Xbox 360, but no less effective.

PS4 sends 0x100 random bytes to DualShock 4 and in response the gamepad creates an RSASSA-PSS SHA-256 signature and sends it back among the cryptographic constants N and E (public key) needed to verify it. These constants are unique for all manufactured DualShock 4 gamepads. The gamepad also sends a signature needed for verification of N and E. It uses the same RSASSA-PSS SHA-256 algorithm, but the cryptographic constants are equal for all PlayStation 4 consoles and are stored in the kernel.

This means that if you want to authenticate your own USB device, it’s not enough to hack the PlayStation 4 kernel – you need the private key stored inside the gamepad. And even if someone manages to hack a gamepad and obtains the private key, Sony can still blacklist the key with a firmware update. If after eight minutes a game console has not received an authentication response it stops communication with the gamepad and you need to remove it from the USB port and plug it in again to get it to work. That’s how the early counterfeit gamepads worked by simulating a USB port unplug/plug process every eight minutes, and it was very annoying for anyone who bought them.

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