This Challenge was part of  EG-CERT CTF, and actually I don't know how to describe this challenge, it was weird, it has a very easy solution but no one could solve it

We got a python code that do encryption and we have to figure out how to write the decryption algorithm and get the flag from the cipher-text that was provided to us

at beginning I tried to solve it without any Mathematical solver, but I straggled in the equations, I believe it's solvable without any  mathematical solver, but usually I make mistakes when I over thinking, and I don't like to use solver if I don't know how mathematically to solve the equations

So after a while, me and Coldzer0 decided to stop trying without a solver and use Z3 to get the flag, and it was very simple code

Flag : EGCTF{b1tBYb1t_m491c!}

Script has a Running time = 8 min (on my PC - may it varies on yours) and it consumed 10GB from Ram while running