I've now prepped up to puzzle 6. If you would like to write a puzzle for this site, please feel free. I'll credit any that I use. You should send your puzzle (with a solution, including a note on how to crack the code) to mark_cryptopuzzles (at) murky (dot) org.
The rules are simple, it must be a reasonable puzzle (no one time pads!) and it must be based on a cryptographic method already published on this site. It also must be original, please don't crib from a puzzle book.
Note that if the method used doesn't exist as a post on this site, that's fine - the situation will have to be remedied before the puzzle is published! I'm not sure how many I'll get (if any), but fingers crossed.