Reply to post:

Why encrypt at all? There's so many channels of communication they can't monitor them all. All you need is a Flash movie on a page that provides a public chat box and doesn't store the history. Then just create a free chat site using that Flash movie open to the public, with standard chat topics - Marvel, Superman, Batman, Pokemon, Doctor Who - and allow people to create their own rooms. Now all you do is create one with a seemingly non suspicious name and use the "private chat" function. Done.