First of all, thanks for writing this, since I did find the technical details quite interesting. But I don't think this puts microcode rootkits beyond the reach

First of all, thanks for writing this, since I did find the technical details quite interesting. But I don't think this puts microcode rootkits beyond the reach of a state actor. It puts them beyond the reach of a normal criminal attacker, which is actually my point. Scenario 1: Supply chain interdiction. You don't need to target the CPU only after it has been manufactured and put into whatever you want to hack, you can start way earlier, including at the factory. Scenario 2: Getting the signing keys from hardware manufacturers, including Intel, seems quite feasible for state actors. You don't even need hacking (I assume Intel's keys are kept air-gapped) or relying on secret court orders, plain old spycraft would probably do the trick. That said, my argument can be rephrased to consider the hard-drive controller or other peripheral firmware rootkits instead, if you prefer and care only about scenarios where the rootkit must be delivered over the network to a clean system without attacking the CPU manufacturer.