The “Which Message” trick -A non-cryptographic secret message Idea -
Your department intercepts and deciphers several contradictory coded messages. Only one of them, however, is a genuine message but you cannot tell which.
Yet the enemy can because the genuine message contains a single word which marks it as being the real message.
Note: All of the words in all of the messages must be different, this makes it hard to work out what the “Marker” word might be.
Why is this a good idea?- Because cryptography (if all the messages contain different words) will not help you discover which message is the real message, only knowing the “Marker” word will tell you that.