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Sadly, there's more in an FPGA than "just" a dumb lookup table for implementing logic functions. There's not much more reason to trust an FPGA than an Intel CPU.

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Sounds interesting. I thought most of FPGA is logic blocks, then there's some MCU that would handle loading the design, maybe some ADC and/or DACs, and then there isn't much more than this. The MCU, theoretically, could try to analyze your design and modify it, but that would require either a targeted adversary that knows the design or a good amount of computational power and fancy algorithms to analyze what's going on.

Maybe you have any links with a good further reading on that topic?

The point is there may be an additional, secret, logic block which allows malicious access to the flops.

There could be, but mustn't it be aware of schematics FPGA is soldered into? A secret block to just manipulate flops isn't enough - it must be able to be controlled by someone.

Well, in theory it must not, because it could detect "oh, this looks exactly like one of popular Ethernet cores, so I'll bug onto those pins and have networking", but this seems like a hard task. Or, well, it could be that every pin is hooked and a secret block awaits a specifically crafted code (somehow like port knocking), but I'm not sure this is a feasible approach.

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