steganography

Unlike encryption, where it's obvious that a message is being hidden, steganography hides data in plain view, such as inside a picture. To anyone who isn't aware that it contains hidden data, it looks like just a normal picture.

One of the most widely used techniques and perhaps simplest to understand is the least significant bit technique. This technique changes the last few bits in a byte to encode a message, which is especially useful in an image, where the red, green, and blue values of each pixel are represented by eight bits ranging from 0 to 255 in decimal or 00000000 to 11111111 in binary.

Changing the last two bits in a completely red pixel from 11111111 to 11111101 only changes the red value from 255 to 253, which to the naked eye creates a nearly imperceptible change in color but still allows us to encode data inside of the picture.

This diagram shows two 4-pixel images in both color and binary values. Each block of binary represents the value of the corresponding pixel.

The hidden image is revealed by removing all but the two least significant bits of each color component and a subsequent normalization.

Image of a tree with a steganographically hidden image.

The hidden image is shown below.

Image of a cat extracted from the tree image above.

Sources

• How to Hide Secret Data Inside an Image or Audio File in Seconds

• Wikipedia Steganography