Revista Informatica Economică, nr. 4 (44)/2007 37 Steganographically Encoded Data Adrian VASILESCU, Bucharest, Romania Steganography is the art of hiding information in ways that prevent its detection. Though steganography is an ancient craft, the onset of computer technology has given it new life. Computer-based steganographic techniques introduce changes to digital covers to embed in- formation foreign to the native covers. Such information may be communicated in the form of text, binary files, or provide additional information about the cover and its owner such as digital watermarks or fingerprints. This paper explains steganography, provides a brief his- tory and describes how steganography is applied in hiding information in images. Keywords: Steganography, information hiding, digital image, digital watermarking teganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the exis- tence of the message itself is not disguised, but the content is obscured. The word "Steganography" is of Greek ori- gin and means "covered, or hidden writing". Its ancient origins can be traced back to 440 BC. Herodotus mentions two examples of Steganography in The Histories of Herodo- tus. Demeratus sent a warning about a forth- coming attack to Greece by writing it on a wooden panel and covering it in wax. Wax tablets were in common use then as re-usable writing surface, sometimes used for short- hand. Another ancient example is that of His- tiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden. The purpose was to instigate a revolt against the Persians. Later, Johannes Trithemius's book Steganographia is a trea- tise on cryptography and steganography dis- guised as a book on black magic. Generally, a steganographic message will appear to be something else: a picture, an article, a shop- ping list, or some other message. This appar- ent message is the covertext. For instance, a message may be hidden by using invisible ink between the visible lines of innocuous documents. The advantage of steganography over cryp- tography alone is that messages do not attract attention to themselves, to messengers, or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse sus- picion and may in itself be incriminating, as in some countries encryption is illegal Steganographic techniques Modern steganographic techniques • Concealing messages within the low- est bits of noisy images or sound files. • Concealing data within encrypted data. The data to be concealed is first en- crypted before being used to overwrite part of a much larger block of encrypted data. This technique works most effectively where the decrypted version of data being overwrit- ten has no special meaning or use: some cryptosystems, especially those designed for filesystems, add random looking padding bytes at the end of a ciphertext so that its size can't be used to know what was the plaintext size. Examples of software that use this tech- nique include FreeOTFE and TrueCrypt. • Chaffing and winnowing • Invisible ink • Null ciphers • Concealed messages in tampered ex- ecutable files, exploiting redundancy in the i386 instruction set. • Embedded pictures in video material (optionally played at slower or faster speed). • A new steganographic technique in- volves injecting imperceptible delays to packets sent over the network from the key- board. Delays in keypresses in some applica- tions (telnet or remote desktop) can mean a delay in packets, and the delays in the pack- ets can be used to encode data. There is no extra processor or network activity, so the steganographic technique is "invisible" to the S