&gt; A sufficiently paranoid individual can only run code they wrote themselves.<p>I&#x27;d say that is not sufficient because even in this case you trust someone: the manufacturer of the CPU on which the code would run.<p>It might surprise some people but you can examine code of a piece of software to check whether it has a backdoor even if it is closed-source by reading disassembly. Surely it requires some skills and is a bit time-consuming but it&#x27;s doable for an ordinary individual. Reverse engineering software is not as difficult as many think. And as a matter of fact, a large number of people are reading disassembly of widely-used software to find vulnerabilities to sell in black markets. So I think it&#x27;s unlikely for Windows or iOS to have maliciously planted backdoors.<p>On the other hand, it&#x27;s tremendously difficult to reverse engineer hardware especially CPUs for an individual without a large budget. So if I were them I&#x27;d choose CPU as a place to put a backdoor because virtually nobody reverse engineers a modern CPU and thus it&#x27;d be very unlikely to be found.<p>By the way, contemporary CPUs can update itself through microcode updates.