Steganography is the practice of hiding information within a seemingly ordinary object or message. This can be used, for example, to hide secret messages inside of digital images, that can only be decoded by someone who knows what algorithm was used to encode the secret message. What my program does is take an image and a message to encode, and then write specific values to specific pixels following some calculations, that can then be decoded to retrieve the original message. The program itself is very simple, since images are basically just a stream of pixels, which themselves are just bytes. I used the stb_image library for reading/writing to images. An encoded message consists of two parts: a header, and the actual message. The header contains information that allows you to decode the message, including the size of the message, and the distance between each byte of the message. In order to avoid raising suspicions, I implemented a very simple feature that just distances every byte of the pixel evenly throughout the whole image. This way, on a regular image, which tends to have atleast a million pixels, there'll only be a few dozen pixels spread evenly throughout the image that are modified in a barely noticeable manner. This alone makes the message practically undetectable for the vast majority of images and people. I also realised later that encoding the message in an image that only consists of random pixels is even more foolproof, since there is no way of telling if a message is encoded in the image or not, aswell as where the modified pixels are, unless you have the algorithm to decode the message. This was a very fun and quick program to make, and the results were quite surprising. With the most basic of methods (and steganography as a domain goes much further than what I have done), it is possible to hide a few hundred bytes of data inside of an image without it being noticeable at all.

Can you find the secret messages?

Original image of a computer

Image of a computer with a secret message

Original image of a park

Image of a park with a secret message

Image of random noise with a secret message

The top image is a very high resolution image, and the modified pixels are unnoticeable. The second image is lower resolution (400x300 pixels), thus the modified pixels are noticeable if you pay attention. The final image is just random noise, with the secret message added on top, so the secret message is practically unnoticeable even if you look hard.