tl;dr

The message is converted to decimal, and `0` is replaced with a random string

Then the cipher takes in one character at a time and computes some value incrementally, so a recursive algorithm is used to bruteforce the characters from back to front.

Original writeup (https://github.com/Ariana1729/CTF-Writeups/blob/master/2019/CryptoCTF/Good%20one%21%21/README.md).