TIL of Steganography, a field similar to cryptography, but that involves concealing a message rather than encoding it. For example, hiding an image in audio, or even hiding one image in the data of another. A famous example, a POW in Vietnam blinked TORTURE in morse code during a newscast.

https://en.wikipedia.org/wiki/Steganography

https://www.reddit.com/r/todayilearned/comments/165gs8y/til_of_steganography_a_field_similar_to/