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I found the solution by brute forcing all possible 3 char string. Is it possible to find the inverse string without brute force?

Yes, you can. (With a lucky guess) I've just re-done the challenge within 5 minutes; using no brute-force at all.

As knoerxi said:

knoerxi wrote:The encoding is "obviously" -- it is standard -- it can returned with a free accessable OnlineTool==

If just ask again in the "Challenges Solved" forum, you just might get a better answer