15-02-2013, 10:07 AM
Computer Science 654 Lecture 6: Steganography
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Introduction
Steganography conceals the fact that a message is being sent. It is a method akin to covert channels, spread spectrum communication and invisible inks which adds another step in security. A message in ciphertext may arouse suspicion while an invisible message will not.
Purpose of Paper
This paper introduces steganography by explaining what it is, providing a brief history with illustrations of some methods for implementing steganography, and comparing available software providing steganographic services. Though the forms are many, the focus of the software evaluation in this paper is on the use of images in steganography.
Structure of Paper
Section 2 will define steganography, provide a brief history, and explain various methods of steganography. Section 3 will review several software applications that provide steganographic services and mention the approaches taken. Section 4 will conclude with a brief discussion of the implications of stegaonographic technology. Section 5 will list the resources used in researching this topic and additional readings for those interested in more in-depth understanding of steganography.
Steganography
The word steganography literally means covered writing as derived from Greek. It includes a vast array of methods of secret communications that conceal the very existence of the message. Among these methods are invisible inks, microdots, character arrangement (other than the cryptographic methods of permutation and substitution), digital signatures, covert channels and spread-spectrum communications. Steganography is the art of concealing the existence of information within seemingly innocuous carriers.
History and Steganography
Another ingenious method was to shave the head of a messenger and tattoo a message or image on the messengers head. After allowing his hair to grow, the message would be undetected until the head was shaved again.
Another common form of invisible writing is through the use of Invisible inks.
Such inks were used with much success as recently as WWII.
An innocent letter may contain a very different message written between the lines [Zim48].
Early in WWII steganographic technology consisted almost exclusively of invisible inks [Kahn67]. Common sources for invisible inks are milk, vinegar, fruit juices and urine. All of these darken when heated.
Document Layout
Even the layout of a document can provide information about that document. Brassil et al authored a series of publications dealing with document identification and marking by modulating the position of lines and words [Brassil-Infocom94, Brassil- Infocom94, Brassil-CISS95].
Similar techniques can also be used to provide some other "covert" information just as 0 and 1 are informational bits for a computer. As in one of their examples, word-shifting can be used to help identify an original document [Brassil-CISS95]. Though not applied as discussed in the series by Brassil et al, a similar method can be applied to display an entirely different message.