If you have not yet solved puzzle 6, you may like to do so before reading on.
As is often the case, the solution to this puzzle gives information about how the puzzle is solved.
This is an example of ROT thirteen. It is usually typed with numbers, but I avoided this to keep things as text. ROT thirteen is a trivial Caesar Shift. It is quite often used as 'spoiler protection' in usenet newsgroups. For example, if I had just seen a film that I wanted to discuss on a newsgroup, but not everyone there will have seen it, I would apply ROT thirteen to my posts. Each letter is shifted thirteen places, so A becomes N, B becomes O and so on. We should see that applying ROT thirteen again brings us back where we started. A classic example which demonstrates that applying a monoalphabetic substitutions many times over does not increase security! Many pieces of software for usenet have a ROT thirteen function built in.
ROT thirteen is normally written ROT13.