So I was talking with a friend who's a medieval historian specialising in military tactics.
He was really excited, as he believed he had found the reacon the Vikings were so successful in their raids, with military victories as far as North Africa, the Caspian Sea, and near total subjugation of the British Isles.
He's found evidence that they had a sophisticated system of communication that allowed them to coordinate their planning without fear of interception.
Turns out they sent secret messages in Norse code.