You can probably hide evil in a few gates, in the right place. A little additional logic that perturbs an MMU mapping or TLB load, or fail to register an exception if a bit of hardware has seen a recent pattern of behavior. Heck, with a few thousand gates you could have a full microprocessor embedded on the sly.

I don't know at what point this stuff becomes noticeable; do you subvert the silicon toolchain? Do you buy off the people who make the masks? Are there feedback loops in the silicon production chain where changes would make it back to a designer, who would say "Hmmm, that's funny..."?

In the early 80s it looked possible for a chip designer to stick in Something Bad without much notice. With billion transistor chips, it seems a lot easier.

Is there a VHDL equivalent of the Underhanded C contest? :-)