Matthew gave this solution to Puzzle 13:
(Solution: the first riddle is "What thing is brown and sticky?", and the second is "What do you call a boomerang that does not return when thrown?" -- the answer to both, and the keyword for the transposition, is a STICK. The hint is encyphered using a 3-step Caesar shift, and says "A known plaintext attack would be fruitful, hoping to find common words in the riddles. For instance, one of the pieces contains the word 'you', 'and' is in a different one, and the other has 'does'."