7. The Bedrock of Cybersecurity: Cryptography 179 There are an infinitude of ways to perform steganography in both classic and computer cryptography, and that is its strength. People can always come up with new and ingenious ways to communicate secretly in plain sight. One big advantage of steganography over cryptography is that it does not reveal the presence of secret communication. When cryptography is used, the assumption is that eavesdroppers could observe the ciphertext messages and identify the sender and the receiver of the messages. This arouses suspicion and invites further scrutiny because the assumption is that the communicating parties have something to hide. If steganography is successful, the onlookers are oblivious to the secret communication and no suspicions are raised. In cyberspace, one of the canonical ways to hide a message in plain sight is to encode the message in an image. As we saw in Chapter 2, the RGB color model is a 24-bit system. Because eight bits are used per color, this means each pixel has up to 255 parts red, green, and blue. Therefore, the RGB format is capable of encoding more than sixteen million colors ranging from RGB[0, 0, 0] (white) to RGB[255, 255, 255] (black). In this scheme there are many slightly different shades of every color, and these differences are indistinguishable to the naked eye—this is the avenue for steganography. Figure 7.16 Two different colors by three bits that are indistinguishable to the human eye. In Figure 7.16 the two purple blocks look the same but are actually different colors: Left: RGB[100, 25, 200]: 011001000001100111001000 Right: RGB[101, 24, 201]: 011001010001100011001001 To a computer these colors are different because three out of the twenty-four bits are different. But to the human eye, they appear to be the same color purple because the RGB contributions are different by only one part each. This makes it possible to hide meaningful bits in an image without actually changing the appearance of the image. This is done by selecting some of the pixels and changing the 1s and 0s as needed to encode the secret message. The recipient can then examine the selected pixels to reconstruct the secret message. To an eavesdropper, nothing seems out of the ordinary—the image looks normal—so the secret message goes by unnoticed, which is the entire point of steganography.
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