Steganography

The term "steganography" refers to the broad art of concealing information in a place that is open to the public. While several of the other items on this list, such as invisible ink, would fit under this category, they are merely examples of a broad field of research, much like cryptography. Steganography, like cryptography, has a lot of potential applications in espionage.

Despite the technique's extensive use throughout history, it has become even more pertinent in the age of computers and the Internet, particularly for counterterrorism and cybersecurity. As a simple illustration, hackers frequently encrypt dangerous information to evade detection software by hiding it in the least significant areas of a file, which could be anything from a document to a movie.

Even in 2001, there were some allegations that claimed Al-Qaeda was utilizing pornographic files to transmit information to its operators around the world, however up until this point, no proof of this has ever been discovered.