The challenge is basically a fancy map from ascii onto integers mod n, since its a map, and we are given the structure of part of the message, we can deduce some results of the map, which allows us to calculate the parameters of the function(which is a linear function mod n)

Original writeup (https://github.com/Ariana1729/CTF-Writeups/blob/master/2018/ISITDTU%20Quals%202018/Love%20CryptoGraphy/README.md).