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Steganography is the art of hiding information in ways that prevent the detection of hidden messages. Steganography, derived from Greek, literally means “covered writing.” It includes a vast array of secret communications methods that conceal the message’s very existence. These methods include invisible inks, microdots, character arrangement, digital signatures, covert channels, and spread spectrum communications. Steganography and cryptography are cousins in the spycraft family. Cryptography scrambles a message so it cannot be understood. Steganography hides the message so it cannot be seen. A message in ciphertext, for instance, might arouse suspicion on the part of the recipient while an “invisible” message created with steganographic methods will not. In this article we discuss image files and how to hide information in them, and we discuss results obtained from evaluating available steganographic software. For a brief look at how steganography evolved, see the “Steganography: Some History” sidebar. IMAGE FILES To a computer, an image is an array of numbers that represent light intensities at various points (pixels). These pixels make up the image’s raster data. A common image size is 640 ´ 480 pixels and 256 colors (or 8 bits per pixel). Such an image could contain about 300 kilobits of data. Digital images are typically stored in either 24-bit or 8-bit files. A 24-bit image provides the most space for hiding information; however, it can be quite large (with the exception of JPEG images). All color variations for the pixels are derived from three primary colors: red, green, and blue. Each primary color is represented by 1 byte; 24-bit images use 3 bytes per pixel to represent a color value. These 3 bytes can be represented as hexadecimal, decimal, and binary values. In many Web pages, the background color is represented by a six-digit hexadecimal number—actually three pairs representing red, green, and blue. A white background would have the value FFFFFF: 100 percent red (FF), 100 percent green (FF), and 100 percent blue (FF). Its decimal value is 255, 255, 255, and its binary value is 11111111, 11111111, 11111111, which are the three bytes making up white. This definition of a white background is analogous to the color definition of a single pixel in an image. Pixel representation contributes to file size. For example, suppose we have a 24-bit image 1,024 pixels wide by 768 pixels high—a common resolution for highresolution graphics. Such an image has more than two million pixels, each having such a definition, which would produce a file exceeding 2 Mbytes. Because such 24-bit images are still relatively uncommon on the Internet, their size would attract attention during transmission. File compression would thus be beneficial, if not necessary, to transmit such a file. File compression Two kinds of compression are lossless and lossy.1 Both methods save storage space but have different results, interfering with the hidden information, when the information is uncompressed. Lossless compression lets us reconstruct the original message exactly; Computing Practices Steganography is an ancient art of hiding information. Digital technology gives us new ways to apply steganographic techniques, including one of the most intriguing— that of hiding information in digital images.