How can I make a DRM backup before installing Sailfish X? [duplicate]

Note that the procedure is very difficult without access to a windoze computer, I tried repeatedly many times before giving up on it....

And this is the bloody stuff I do for living :)

It is doable on Linux, but I had to use google to find solutions for problems which came up. Unfortunately, I didn't think about adding my findings to the description.

I can't remember how long it took me exactly, I would say it was a very long evening.

For me it took from 15:30 (sun) to 00:20 (mon) to get it done. Lots of trial & error, and plenty of educated guesses. Minimal aosp and sailfish experience before that, but somewhat plenty (N)770 - N9 flashing experience. (Only) On Linux.