It is a very simple (and common) encoding... you should get it in no time with a basic analysis.
Edit:
With JPK you can solve _a_lot_ of simple challenges, also it _can_ help to analyze the encoded string.
Just try very basic analyse and the first thing that pops up into your mind.
JPK is _not_ needed, but i think it is nice to introduce this program somehow, as it can be quite useful.
The MirmoDecoder is useful for some infomirmo challenge, but i disabled the function because it was easier to disable it than have two versions of JPK.
Edit2:
MirmoDecoder got written by me because it was a perfect example for some custom encoding, and imho, the "mirmo encoding" is kinda straight forward. I hope mirmo will forgive me the day he finds out i wrote a public decoder :x
The geeks shall inherit the properties and methods of object earth.