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Since the Sept. 11 terrorist attacks, the United States has been on high alert. Government officials have urged reporters not to play videotaped messages from Osama bin Laden for fear that they contain coded messages with instructions for further attacks. Coded public statements are not the only means of coded communications being used, however; evidence in the U.S. Embassy bombing case showed that bin Laden operatives may have hidden terrorist plans within pornographic and MP3 audio files distributed on the Internet. In using coded public messages bin Laden is no trendsetter, though. This process of hiding messages in plain sight, called steganography, has been around since ancient Greece. HISTORICAL USE Steganography means “covered writing,” and it has come to mean the art of information hiding. One low-tech, ancient steganographic method involved shaving a messenger’s head, tattooing the text of the message on his scalp and allowing the hair to grow back. Only after shaving the messenger’s head would the message again be visible. During World War II, the Germans took steganography to a new level with the use of microdots and invisible inks. Microdots are tiny pictures compressed into a period or the dot of an “i” in a text message. The concept behind these kinds of steganographic techniques is that only one who knows that there is a concealed message will find it — that is, they would know to magnify the dot on the “i” or shave the messenger’s head to retrieve the message. Digital files are ideally suited for the application of steganographic techniques. Digital steganography works by taking advantage of the unused or unimportant portions of a digital file. The image, sound recording or text file that holds the hidden message is called “the container.” Insignificant elements in the container file are replaced with information comprising the message. The change between the original file and the file with the embedded information is unnoticeable to the ordinary user. Ideally, the inclusion of the hidden information will not degrade the quality of the visual image or audio file that is the container for the message. In some applications of digital steganography, however, the technology is designed such that the quality of the container file will be deliberately ruined if the embedded information is removed or altered. ALGORITHMIC CHANGES Most digital steganographic programs used with image files employ algorithmic changes in the color palette of the container image to conceal the message. Digital steganography programs used with text messages may employ algorithms for line shifting, word shifting and feature-coding. Steganographic programs used with audio files hide information in the MP3 bit stream during the MP3 compression process. Encryption techniques can be used in conjunction with steganography to protect or further conceal the hidden messages in digital files. The benefit of using encryption is that even if an unintended recipient is aware that there is information steganographically hidden in a digital file and is able to extract it, that information must still be decrypted. The recipient would then need to know the particular program used to encrypt the information in order to obtain the message. Steganography has commercial applications that have nothing to do with surreptitious distribution of messages for purposes of espionage or evasion of law enforcement, such as the protection of intellectual property rights. Copy detection and copy protection techniques such as digital watermarking and digital fingerprinting are a form of steganography used for this purpose. Digital watermarking protects a copyrighted work by embedding copyright ownership information in an invisible image or inaudible sound in a digital file. The embedded copyright ownership information is ordinarily invisible in the image, or inaudible in the audio file, so that it cannot be identified and manipulated or removed. This application of steganography is a way to prove ownership as well as to deter unlawful copying and distribution. Digital fingerprinting embeds a customer’s authentic identification information in a digital file. This steganographic function allows a copyright owner to track any illegal distribution of the digital file by tracing the source of an illegally distributed or copied file. HIGH DEGREE OF ROBUSTNESS These applications of steganography require a high degree of robustness — a quality in the technology such that the watermark or fingerprint is difficult or impossible to remove without degrading the quality of the image, audio file or text in the container file, but the watermark or fingerprint nevertheless remains imperceptible to an ordinary user of the file. A robust digital watermark or fingerprint must also be able to survive such processes as data compression, which may alter some of the data in a file. The use of steganography has caused concern because while it protects private communications and copyrighted works, it also prevents the government from detecting terrorism or viruses being sent through innocent files. However, developments in digital steganography have been followed by improvements in digital steganalysis — the art of detecting steganographic content in digital files. Like steganography, steganalysis has positive and negative uses. While steganalytic programs may uncover espionage and viruses, they may also be used to thwart steganographic copyright protection. The development of effective steganalysis techniques that can be used in this fashion is one of the reasons that digital watermarking and digital fingerprinting have been eclipsed by digital rights management systems (DRM), which are a combination of more advanced technologies.

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