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Steganography
Steganography is a techonology for hiding data in media (usually images).
It gives little change to the original media, making the data hidden in it invisible to humans. For instance, 
this is an image, while 
this is a StegFuck program as an image almost exactly the same as the previous one, but there is a program hidden in it that humans can't see.
The most common method of this techonology is LSB (Least Significant Bit), that is by assigning the last binary bits in pixels of an image. This method requires the media to be uncompressed or compressed with lossless compression (like PNG images).