Further to my last, communication ideas have been bounced around here such as character encoding, cyrptic text, language etc. and some may feel there is still grey area there for cheating
What about a BEHAVIORAL based protocol? It would obviously be established entirely in-game. If I want to get a specific ally, early on I request strange things from him. Simplest example would be backwards orders.
(I am England, I have fleet in Nth. I tell him to CLOSELY watch my Lon -> Nth, then I actually move Nth->Lon)
He might just think I'm an idiot, but after a few turns, he might catch on. And because the moves don't really affect other players, they may not even be noticing.

It is entirely public, and as such runs just as much risk of others catching on.
Most importantly, it is based on watching what a player says he will do against what he does... isn't this practically the basis of Diplomacy? I don't know if it would work, but I woulde DEFINATELY not call it any sort of cheating.

@Golgo1: I have often thought about behaviour-based communication in gunboat games.

I completely disagree with your other post, but I've said what I have to say on that subject.

@Figle - a) You could write Para or Pp b) Pot, kettle, blackness squidgyboy c) Yes it's a coded message. It seems nobody's bothered to crack it yet - I wonder whether anyone would in a game and certainly whether more than one person would. I might try it (although I can't see why this is intrinsically different from using a Public Key cryptography system - surely the rule has to be that if press is public then there should be no attempt to make it impenetrable to others. If we're content with any kind of code then Public Key wins and we may as well play with private messaging)

@Parallelopiped: I tried to crack it :-)

Any luck?

No. I just got as far as deciding it's not a cryptogram. Also I noticed the letter frequencies aren't uniform. I'll think about it some more.

Oh right, I'll have a look at it then.
Which was the hypocritical bit? If it was in different posts then I instead refer to it as increasing my understanding.