A lot of the security features on microcontrollers are poorly implemented. Here's an example where someone recovered the encryption key on a consumer device with encrypted firmware: https://garyodernichts.blogspot.com/2024/10/looking-into-nin... . The microcontroller supports firmware encryption with a 128-bit AES key, which gets written to 4 32-bit write-only registers. Because it's split up like this, it's possible to find 1/4 of the key by writing every possible value to one of the registers and checking that the output matches, before repeating with the other 3 registers. This means that the number of possible combinations to check is reduced from (2^128) to (4*(2^32)), which can be brute forced in a few hours.
