I've just finished reading a spy thriller book in which the spies passed on messages using Google Gmail. To stop the message being read by the Feds they wrote the message then saved it as a draft. They then logged off. Their mate, another spy, then logged onto the same account and read the draft. No sending of emails so not readable or traceable by the Feds.
I suppose anything is possible when the same minded people are working together?.
I don't know why, but when I read this post, I immediately thought about Navajo Indian's and how the American government and forces used them as 'code talkers' or 'wind whisperers' between 1942 and 1945.