Summary: Filter function of stream cipher had linear annihilator. Use this to set up a linear system of equations; solving this results in four possible seeds, one is acsii only and the flag.

Original writeup (https://cocalc.com/projects/ee6650d2-8007-4522-afbb-9d301658db0b/files/LFSR%20Stream%20Cipher%20Write%20Up.ipynb?session=default).