This was partly crypto and partly a reversing challenge. We are provided with an encrypted message and the encrypter along with the cipher file. Our goal is to decrypt with the available information. The main challenge was to understand the encrypter and come up with a decrypter which would reverse the effect of the encrypter. A detailed writeup is available in the below link.

Original writeup (https://c12yptonic.github.io/ctf-writeups/ctfs/deconstructf21#code-decode-).