I'm quite busy these days, I did this in a couple hours only. Also I'm not that fluent in Golang yet. Long story short: this is by no means clean code in my book. When I produce professional code, I do TDD and I factor/name stuff better than this.
Anyway, I had fun doing this so I thought why not post it for you to check if you want. :)
Note: there's some flakiness in the XOR decipher step. Once in every ten runs or so, it will fail. It didn't prevent me from completing the test, and I can't spend any more time right now to investigate. Still wanted to let it be known.