steganography

In computing, camouflaging messages in large computer files, especially those carrying audio, video, or graphics, by appropriating a small percentage of their constituent data. For example, a graphics file measuring 500 × 500 pixels, using 32 bits to represent each pixel, contains 8 million bits. A single bit of each pixel (perhaps the 1st, the 15th, or the 32nd) could be used to insert some 5,000 words of text, chopped into individual bits, without making any perceptible difference to the image. The text message itself can be encrypted using Pretty Good Privacy (PGP) for added security.