When sending important messages, one option is to send a clearly disguised message via the methods of cryptography; however, this is not the only means of such communication. Steganography offers additional methods for the transmission of information which hide the existence of the message within the context of some other medium (Mollin 2001). Set aside from cryptographic methods, throughout history steganography has employed numerous techniques, each classified as either technical or linguistic in nature (Kessler 2004). As historical evidence for the use of technical steganography, Dunbar (2002) indicates that as early as 440 BC, King Darius of Susa recorded a message on the shaved head of a slave, and when the hair had grown to cover the message, the slave was sent to the recipient. More important historically than today, disappearing inks made from fruit juices, milk, and other liquids were used to write messages, often between the lines in other documents, which would appear after exposure to heat (Dunbar 2002).

While false-bottomed shoes or suitcases could be used to conceal a given message, a far more impressive method was the German microdot, first discovered in 1941. This technique presented information in the place of a typewritten period within normal text. Photographic reprints of images or text were reduced to the size of a standard period, thus information could be transmitted in otherwise harmless or unrelated texts (Johnson 2003). The methods presented so far only offer insight into the branch of technical steganography, while linguistic steganography offers further methods for the covert transmission of information. Linguistic steganography may be further reduced to two categories, semagrams which hide information through symbols and signs and open codes which hide information in text (Kessler 2004).

Discussing semagrams, visual semagrams refer to messages which are offered through a particular arrangement of physical objects, for example the hands on a clock could be set to indicate a specific time for a meeting. Text semagrams present information which may be highlighted with slight changes in color, font, or position of text within an otherwise unimportant or unrelated message. Aside from these, open code methods offer information through the use of cue words, jargon, or null ciphers which offer rules for reading a hidden message. To use cue words to convey meaning, specific phrases may be given alternate meanings, thus when the phrase is encountered in the context of a message its alternate meaning can be assumed. Similar to this, jargon can be used in a manner such that these words convey meaning only to those with knowledge of certain related facts and otherwise the jargon will appear meaningless (Johnson and Rude 2001; Kessler 2004).

Covered ciphers describe a method for encoding or decoding open code steganographic communications, and the specific nature of the cipher must be known at both ends of the message for successful communication. The first cipher is a grille cipher which is a template to be placed over the carrier message, allowing only particular words of the message to be read. The second is the null cipher which indicates the specific words or letters that should be read in the carrier medium to produce the coded message. Probably the most commonly cited example with a null cipher was sent by the German Embassy during World War Two, and read “Pershing sails from NY June 1.” The message was extracted from each of two messages, each with its own specific cipher (Kessler 2004).

On the modern technological forefront, steganography has new carriers in digital image and audio transmissions, as well as in digital watermarking of these and other forms of media. Johnson and Rude (2001) indicate that given the unused space in an average image file, a 1024 by 768 pixel image file can hold an additional megabyte of information. In addition to unused space, steganographic methods insert hidden data into redundant areas of a file, where replacement of data will not significantly alter the file. An ideal picture contains a complex and unknown landscape with significant black and white halftones, which provides the confidence that any changes brought forth by the addition of hidden data will go unnoticed. Much of the trust in image and audio steganography is placed in the fact that the human senses are unable to perceive small changes to images, audio, and even in text spacing and fonts as mentioned in some previous techniques (Johnson and Rude 2001; Kessler 2004).

Two important differences between steganography and watermarking illustrate key features of the two methods, these being the capacity, security, and the robustness of the medium. Steganography aims at high capacity and security, which has the effect of decreasing the robustness of the medium, that is, small changes made to the file are capable of destroying the file or message. The reverse of this combination is the robustness which is sought out in the watermarking process, the goal of which is to make the removal of the watermark possible only by severely degrading the quality of the medium. While it is not possible in a brief space to address a great deal of the mathematical and computational details of use in this discipline, numerous resources to follow offer insight into the foundations and methods of modern steganography, the modern steganographic software, and visual and textual examples.

As mentioned previously, a classic example is the German Embassy message from World War Two. The original null ciphers are presented below:

PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE. GRAVE SITUATION AFFECTING INTERNATIONAL LAW. STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS. YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT IMMENSELY.

APPARENTLY NEUTRAL'S PROTEST IS THOROUGHLY DISCOUNTED AND IGNORED. ISMAN HARD HIT. BLOCKADE ISSUE AFFECTS PRETEXT FOR EMBARGO ON BYPRODUCTS, EJECTING SUETS AND VEGETABLE OILS.

Two separate null ciphers provided the same message, “Pershing sails from NY June 1,” by reading either the first letter or the second letter in each word (Kessler 2004).

Literature Cited:

Dunbar, B. 2002. A detailed look at Steganographic Techniques and their use in an Open-Systems Environment. For the SANS Institute.

From: http://www.sans.org/rr/papers/index.php?id=677

Kessler, G.C. An Overview of Steganography for the Computer Forensics Examiner. Forensic Science Communication. July 2004.

Johnson, N.F. 2003. History and Steganography.

From: http://niels.xtdnet.nl/papers/practical.pdf

Johnson, N. F. and T. Rude. 2001. Introduction to Steganography: Hidden Information.

From: http://www.crazynights.com/monkeyboy/rude_johnson_gmu2001_stegob.pdf

Mollin, R.A. 2001. An Introduction to Cryptography. Chapman Hall/CRC Press.

Provos, N. and P. Honeyman. Hide and Seek: An Introduction to Steganography. IEEE Security and Privacy. May/June 2003.

From: http://niels.xtdnet.nl/papers/practical.pdf