Steganography

Steganography:

Steganography is the “art” of relaying hidden digital messages through supposedly innocuous “surface” messages. The hidden message can only be decoded by the sender and the receiver, so sensitive information can be sent through seemingly unimportant data. Examples include hidden pictures within pictures, text with a code or alternate meaning, or something as complicated as slight delay in packet delivery which will result in a decodable message. The word steganography actually of Greek origin and means "concealed writing"

Steganography has been used by underground poltical and religious associations to relay secret messages through networks and websites. Another use for steganography can be “digital watermarking”, or encoded data that indicates from where a file came and who owns the rights to it.  

Image: http://xiao-steganography.en.softonic.com/

Image: http://www.gfi.com/blog/threats-steganography/

Works Referenced:

http://www.garykessler.net/library/steganography.html