There's two issues: 1) Hardware has severe limitations on what you can actually implement, because implementing complex behavior with small numbers of low-leve

There's two issues: 1) Hardware has severe limitations on what you can actually implement, because implementing complex behavior with small numbers of low-level logic gates is really difficult. 2) You really don't want to get caught when the evidence of the attack is baked into the hardware itself. Under those constraints, a very subtle attack on an entropy pool could be both easier to implement, and much harder to notice, than an obvious attack of just flipping some "root access bits" or backdooring the key generation directly. It's quite difficult to prevent those kinds of attacks from being detected by debuggers, under the constraints of a hardware implant. In particular, remember that the mechanism where the backdoored RNG can observe state might be a deliberate leak, implemented by the programmer of the RNG pool, working with the people doing the hardware implant. In that scenario, they want something subtle so it'll pass code audits. If you haven't already, I'd really suggest trying out some FPGA development. Basically, everything is done with fixed-function state machines, and abstractions and complex behaviours are really difficult to implement directly. I'd say FPGA design in comparison to, say, assembly, is similar to the difference in abstraction power between assembly and Python.