Rating: 5.0

tl;dr
1. Recover the encryption logic
2. Write the encryption in form of matrix multiplication
3. Solve some matrix equations

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/mpolycipher

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/mpolycipher).