PSP Firmware Hacking

[Burningmace]

Right, so I've played with it a bit and I've hacked my firmware, but I'm confused as to why Sony let this happen in the first place when there's a cheap solution. Let me explain.

The PSP has an EEPROM chip where its firmware is installed, which you can flash with an update file. The hackers first got around this by using a bug in the GTA game save handling code to inject a custom patch into memory and have it put their own data on the EEPROM. The PSP then reboots and voila - you have a custom firmware installed on your PSP. Since then there have been patches to stop this and the hackers have gotten round it.

So, now to my solution. I know nobody in the public wants this to be fixed but I'm just putting my thoughts out there so that they can be heard.

Here's a hypothetical situation:
Sony writes their firmware and compiles it, making sure that there is a 256-byte array full of zeros. This array must be static on the EEPROM - i.e. it must stay in the same place no matter what you do with the rest of the firmware code. Sony builds the PSP with a tamper-proof secondary processor (something like an FPGA) that had an implementation of RSA's hash signing algorithm on it. They write a simple RSA signing application and generate a 1024-bit RSA key pair. They keep the private key secret within the corporation. They hard-code the public key and the address of the 256-byte array into this processor and "seal" the code onto the chip's ROM.
Sony now hashes their firmware with SHA1 and signs the hash using their RSA private key. They write this value over the array of zeros and put the firmware on the device.

Here's how the EEPROM will look when it is compiled (bla bla bla is firmware code):

bla bla bla bla bla bla bla 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 bla bla bla bla bla

Their software takes that data, hashes it with SHA1, then signs the hash and writes that value over those zeros.

The RSA processor loads the raw EEPROM data into its memory and extracts the key from the 256-byte array and stores it. It then replaces the 256-byte array with zeros again and hashes the EEPROM data as before, but this time it doesn't need to sign it. It then verifies the signature against the generated hash using the public key and RSA's signed hash verification function. If the signature is verified, the RSA processor sends an enable signal to the PSP's main processor allowing it to begin running the firmware. If the signature could not be verified, the PSP does not boot.

If anyone can think of a way to bypass this, shout up. And please, if you don't understand asymmetrical encryption like RSA, don't try to put your 2 cents in because I don't feel like explaining the same thing over and over again.

 

[fail at the internets]

Too many things that i didn't understand in that post

 

[Burningmace]

I'm talking about a lot of concepts that only programmers with security/cryptography knowledge will understand. Also I've referenced some electronics.

I'll whack out some definitions:

FPGA - Field Programmable Gate Array. A kind of programmable microchip that can be used like a small processor and/or for complex logic.
EEPROM - Programmable/erasable flash memory.
SHA1 - A hashing algorithm. Data is hashed to a unique identifier, but this identifier cannot be reversed into the original data. If you hash the same data twice, you get the same hash.
RSA - An asymmetrical encryption algorithm. Given a pair of keys - one public, one private - if you sign some data with the private key you can verify (using only the public key) that it was indeed signed with that specific private key and not any other private key. You cannot derive the public key from the private key or vice versa. This means that if I hash a message and sign it, I can send the message with this signed hash and anyone with my public key can verify that I wrote that message and that it has not been tampered with (if it was tampered with the hash would not match the message).

The idea behind this entire process is that ONLY code signed by Sony can be ran on the PSP. If someone modifies it, the hash does not match the signed data and the PSP will not run it. As nobody but Sony has Sony's private key, nobody but Sony can create firmwares! This works on the same principle as code signing, manifest signing and driver signing in Windows.