The theory and techniques of hiding information by embedding concealed messages within otherwise innocuous or commonplace materials; also called "stego", as derived from "covered + writing". Unused or useless data bits in regular computer files are replaced with invisible bits of covert information. This hidden information can be plain text, cipher code, or even images, and may be inserted into text, audio, graphic, web, or e-mail files. Unlike encryption, which alters obvious signs by translating something readable into something unreadable, secret writing is not detected because it is not overt and not suspicious. Because encryption provides confidentiality but not secrecy, steganography and cryptography are often combined to augment security. Originally used in ancient Greece, where an inscribed tablet was overlaid with another message in wax, modern practice uses specialized "steganalysis" software to detect or distribute data, by injection or substitution, among the electronic noise of typical files. When data is injected, the host medium often increases noticeably in size; and when data is substituted, the host medium often degrades noticeably in quality. See digital watermark, DAISY, trap door, key, warez, virus, quantum, rune.