[Community Puzzle] Secret Message Decoding - Puzzle discussion

Coding Games and Programming Challenges to Code Better

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Created by @csKingBabar,validated by @Jasper_Boticario and @Denny.dude.
If you have any issues, feel free to ping them.

If you have solved the Running Up That Hill puzzle, some generic matrix and modular arithmethic code can be reused for this one. (The Hill puzzle is much harder because of non-prime modulus.)