Some backdoors can be almost impossible to detect with any kind of traditional wafer inspection tools. E.g., it might be possible to transparently cripple somet

Some backdoors can be almost impossible to detect with any kind of traditional wafer inspection tools. E.g., it might be possible to transparently cripple something like Intel's RdRand instruction by changing only the doping on some of the gates inside the hardware RNG circuit. It would look completely identical under the best microscope and looking at the output wouldn't give you a good indication of whether or not you had a broken RNG or a good one. The design of Intel's RNG involved passing the output of the RNG through some whitening and IIRC AES so it might look completely random and pass every statistical test but the hardware entropy source might only realistically have 10 bits per second of real entropy. I still think it's unlikely that someone would go through the effort to insert a super secret backdoor by surreptitiously modifying the masks but there are some interesting techniques that can make finding a backdoor almost impossible if you don't already know about it.