Edit: never mind

Edit: never mind, this message gave a hint how to find the decrypted message, if you only have the encrypted message, but nothing else. (So how to crack such a block cipher). I leave this hint here, for those interested in those kind of puzzles.

For those who don't know how to solve cipher challenges, I highly recommend Simon Singh's books and website. See http://www.simonsingh.net/The_Black_Chamber/chamberguide.html. A tip for this particular challenge: If you understand Letter Frequencies, you can solve Caesar ciphers. If you can solve that once, you can solve it multiple times. If you can solve it multiple time, you can solve this puzzle...

Good luck!

PS: here is my take on letter distributions, along with some scripts how to generate them: http://www.macfreek.nl/memory/Letter_Distribution