So how could ARM hide a backdoor in their CPU? TrustZone in itself seems to not be suitable for that. I still want to have as little trust as possible and in as few actors as possible.So at some point you have to decide to either trust the actors or not use any device (or use them while consdering them to be compromised).
Would this be as easy to implement and hide as malicious code in the IME would be? Or asked another way: If Intel was malicious and wanted a secret backdoor in our computers, would they use the IME or one of the methods you mentioned?ARM don't make CPUs. They just design the specs . I guess they could try to sneak in some backdoor-enabling designs.
Then there's the company that makes the actual CPU, that could also have a backdoor in the hardware, or in the software that comes with it (the drivers, and all the lower-level software that links the hardware to the OS).
Then there's the company that builds the board that will house the CPU, and all the companies that build the components that are added to that board. Each of them could have some backdoor, either at the hardware level or the software level.
Then there's the company that brings it all together in a device. (And all the companies that supply third-party software to them).
Then there's any point in the chain where any of the parts is sent to another place, where someone could intercept the hardware and tweak it. (This is also true to some extent for the software)