Project X-Ray [1] and Project U-Ray [2] are the best source for configuration details that Xilinx/AMD doesn't publish. These projects focus, respectively, on the 7-series and UltraScale FPGAs.
My hunch would be that knowing the bitstream format allows a competitor to reverse engineer your FPGA somehow. Otherwise I can't imagine what purpose it serves. The most likely reason is that there are some IP cores with complicated licensing schemes and the encryption acts as some form of DRM.
aliens: "One of them figures out something. They don't tell anyone else how it works. Countless others waste countless time and effort trying to figure out the same thing that has already been figured out. The others punish the first for this by giving them all their fishes."
I know this is a bit off topic, but figured it would be a good general thread to speak up in. My company offers bare metal access to U250 and U55C in pretty beefy boxes. We're starting small and looking for customers / interest who are willing to work with us as we define the business and grow. Right now, we have a few ZK customers who are happy with what we are providing. If you're interested, reach out to me directly. username @ gmail.
I'm not a sales guy (I'm on the tech side), but our intro offer cannot be beat, if you can read into those lines. =) Please contact me and tell me what you're looking for and I'll see what we can do for you.
[1]: https://f4pga.readthedocs.io/projects/prjxray/en/latest/
[2]: https://prjuray.readthedocs.io/en/latest/