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https://blog.invisiblethings.org/papers/2015/x86_harmful.pdfThe author dismisses CPU-level backdoors in favor of Intel ME backdoors mainly on the basis that, since CPUs can't save state, they can't protect themselves against replay-"attacks", and hence Intel would lose any sort of plausible deniability once an "activation sequence" was ever found in the wild.
But I don't really see how ME is protected against replay-"attacks" either. Sure, you might not be able to replay the sequence to the same CPU, but you can definitely replay it to a different CPU - unless every single CPU out there has a different activation sequence, which is possible, but would be vastly less efficient (if you want to hack someone you now have to know / guess some sort of unique production ID of the CPUs he's using?).
I'm really not seeing the big argument in favor of an ME backdoor here. A backdoor directly in the CPU would work just as well.
Arguably better in fact, since the CPU can just look for the activation sequence in the data it reads (if you send an e-mail or website it's very likely the plaintext string gets copied, and therefore read, by the CPU at some point), whereas the ME processor would only be able to watch specific offsets in memory.