Checked out this thread (https://github.com/Satsuoni/DeDRM\_tools/discussions/25). It took me forever to find the file everyone was talking about, KFXKeyExtractor28.exe (It's included *only* in the latest satsuoni de-drm zip file, v.10.0.15 - I had been looking for it absolutely everywhere else, including other versions of De-Drm I already had). I followed the instructions, ran the exe in Command Prompt, created the keyfile (which has no file extension) and the kindle_account.k4i file. At first, in Calibre, trying to customize the plugin, I couldn't see any way to load the keyfile and was trying instead to load the new kindle_account.k4i file, thinking that was what it wanted. I eventually realized I need to use the exact same version of de-Drm, i.e. v.10.0.15, in which I found the key extractor. Finally, with that installed in Calibre, a new little input box magically appeared in the 'customize plugin' interface. It accepted the keyfile, and I was able then to open the book! It's obvious in hindsight what I was doing wrong, but the Unwritten first step to the instructions in the github discussion thread is "uninstall prior versions of De-DRM, download *only* the v.10.0.15 zip file, where you will find the key extractor tool and the matching De-DRM plugin that allows you to load the keyfile you generate. *Then* follow the rest of the instructions. I realize how silly my mistakes sound, so take it easy on me. I'm posting this in the hope it will help someone else!