This is a nice challenge based on simple Linear Algebra - particularly system of equations under mod n.

Full write-up: [here](https://github.com/rsa-ctf/write-ups/blob/master/2019/pragyanctf/crypto/decode_this/README.md)

Original writeup (https://github.com/rsa-ctf/write-ups/blob/master/2019/pragyanctf/crypto/decode_this/README.md).