Please find a bug or security issue in the algorithm and code

Question

Link: https://code.sololearn.com/cGIaS88cyz6k/?ref=app
The cryptographic algorithm uses the idea, " transposing a matrix twice brings the original matrix ". Please can you go through the code and help me findout any bug in the algorithm (or in the code).
Thanks in advance !!!

Answer 1

In the random docs it is stated to use secret lib instead for security purposes. It is because random is not REALLY random. And there instead of randint(0,19) you would use randbelow(19) from secrets.

Answer 2

Herr Rozwel Thanks for your suggestion : )