CMYK indexed color to encrypt/decrypt

[pilotandy]

RE: #2188

[pilotandy]

Imagemagic failures. I'm not sure about those.

[pubpub-zz]

I tried it and the error did not occur... maybe a random error during the pytest run
can you try to commit a small change commit and undo it before commit again to rerun the actions

[pubpub-zz]

@pilotandy
I'm on your problem for a couple of hours. The problem is coming from your test file : getting the image scrumble either pypdf or pillow (not identified which one yet).🤔🤔🤔
Your mod was good👍

[pilotandy]

I thought maybe it was something I broke on my end. So I created a clean fork, venv, using python 3.8, I still get failures without any code changes. Anyway, the new PR simply comes from that new fork. Added some comments too.