Invisible ink was known as "sympathetic stain" during the Revolutionary War. By any name, it was an effective spying tool.

Secret messages can be hidden inside fake conversations generated by AI chatbots. The technique could give people a way to communicate online without arousing the suspicion of oppressive governments.

Vlogger Reuben Levine tells the story of the Colombian army in 2010 sending a message to prisoners of a revolutionary guerilla group by hiding a Morse code message in a concocted pop song. Watch this ...