Applied Cryptography and Policy Debates

Steganography, from the Greek for "covered writing," differs fundamentally from cryptography in its goal: rather than making a message unreadable to an interceptor, as encryption does, steganography aims to hide the very existence of the message so that an observer does not suspect any communication is taking place at all. Historical examples include writing in invisible ink that only appears under heat or chemical treatment, concealing messages within the shaved and regrown hair of a messenger's scalp as described by the ancient Greek historian Herodotus, and, during the twentieth century, microdots that shrank an entire page of text or a photograph down to the size of a printed period. In the digital era, common steganographic techniques hide data within the least significant bits of an image, audio, or video file, altering pixel or sample values in ways imperceptible to human senses while still allowing a recipient who knows the technique to extract the hidden payload. Steganography and encryption are not mutually exclusive and are often combined, since encrypting a message before hiding it protects the content even if the hidden channel itself is discovered, though the reverse is not true: an encrypted message that is not also hidden is still visibly identifiable as an encrypted communication, which steganography specifically seeks to avoid.