How can I remove the DRM from a pdf I've bought on google play books?

Question

I've tried for days using DeDRM_Tools as a Calibre plugin, it doesn't work. I've even run Epubor and used it on Wine, the program runs smoothly, but it didn't remove the DRM, unfortunately.
I've just run into this question How to read an ACSM file on Linux?, which seems to have a pretty satisfying answer. I followed all the steps, it's nothing complicated at all. But here come the issues. First of all, ADE runs on wine 5, but glitches and eventually crashes right when I try giving the Adobe ID credentials.
That's fine, I managed to get the encrypted PDF by asking a friend who uses windows to download it for me on ADE.
However, once I get to point 9. and run the Python Script which should extract the adobe key file, it gives me an error. I suspect there's something broken in adobekey.py, a file which comes in the DeDRM_Tool .zip file. In fact, if I go to the DeDRM plugin on Calibre and click "Customize plugin", it opens a window where you have several options, included Adobe Digital Editions.
If I select that, there's a plus button that should generate the key, but it outputs the same error I get with the python script I was talking about before.
I've even tried downloading that same adobekey.py from older versions of DeDRM, but it still doesn't work.
All I really need is that .der file and then everything else should work fine, I'm desperate at this point.
The book has been bought, it's not illegal to remove a DRM for personal use. I just find DRMs extremely immoral and they should be removed, no matter what.
Please, if any of you knows what's going on with that adobekey.py and knows any workaround, or even knows another way I could get that .der file (you could even send it to me on Telegram here), help me out, that would be great.
Thank you.

Answer 1

You cannot simply generate a decryption key out of nothing, and you cannot extract it from the encrypted file itself – if that were possible it would make data encryption outright useless, and this applies to most encryption uses, not only to DRM.

^{(There are minor exceptions, e.g. PDF "lock for editing" obfuscation, which does store the key within the file itself and is indeed useless.)}

In the case of DRM, the PDF file is encrypted using a key that is only stored on the ADE installation which acquired the ebook (i.e. on your friend's computer), so the only way to "extract" the key is to run the plugin on that same computer.

References:

• Is discussion of techniques for removing DRM permitted?

Answer 2

3 Years later:

As far as I know DRMs from Google Play Books can be removed but it requires using outdated and partly deprecated programs (i.e., MS .NET 3.5 SP 1 and ADE 2.0.1) or libraries (i.e., pyCrypto). I got it working on a QEMU VM with Wine.

Basically Adobe Digital Editions v. 2.0.1 has a vulnerability that enables you export decrypted files outside its scope. Take a look at my blog for a step-by-step of how I got it working.