Die photos and analysis of the revolutionary 8008 microprocessor, 45 years old

I think you're underestimating the cost of building a fab by several orders of magnitude.

The raw resources alone - not considering the labor and technology - required to build a fab and maintain it are so staggering that I just can't see that happening anytime soon.

Besides, the fab itself isn't the issue. Rather, the equipment used during the fabrication process itself are the weakest link, so to speak. Further, if we consider the feasibility of insider attacks, the people who maintain the fab are potential targets as well.

So, I think achieving what you propose would require: 1) access to an immense amount of resources, 2) the capability to self-manufacture the tools required to manufacture ICs, and 3) an AI that can operate the fab.

The third point alone is so far off that I'd conclude it's going to be impossible for a long time.

> how much each reprogrammed circuit has in common with any other

Very little. An FPGA is a set of reprogrammable logic blocks (LUTs) of very small size, plus a number of special purpose peripherals. The "layout" process of assigning functions to LUTs is usually done with simulated annealing and random perturbation. The compiler won't necessarily give the same output from the same input, let alone slightly different input.

The fixed-function blocks and any embedded processors (e.g. Nios) are more targetable. But you could also e.g. set up the clock PLL to leak the FPGA configuration slowly via spread-spectrum modulation.

A subverted FPGA could contain a whole different circuit which turns on based on whatever condition desired (timer, radio command, data pattern seen by the FPGA) and takes over the function of the FPGA in whatever way the subverter intended. As the post to which you are replying said, for good effect, the designer of the subverted circuit would need to know the function the FPGA is performing in your device and design a custom circuit for it. This is possible e.g. by examining the first batch of your devices, and inserting a subverted circuit in your supply chain for subsequent batches. Certainly a lot of trouble to go to, but it's possible theoretically, and people have been writing about the possibility.

Now, the FPGA is not necessarily an ideal place to insert your circuit; personally, I would put it in something like one of those flat ribbons which connect components inside so many devices, or a socket - they may have access to all the bus pins of the gadget, and they will be less conspicuous. I don't think 3D printing is the answer to this, since too many things would have to be 3D printed.

Now, I don't know how I would go about protecting from such attacks, or if this is even a real-life concern right now, but I would think that some kind of automated high-resolution X-ray imaging and analysis technology would be a more realistic direction.