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Literally translated, steganography means, "hidden writing." Although the true origin of steganography is unclear and probably predates history, its use has been well documented throughout time. From ancient Greece to World War II to the Cold War, the use of steganographic techniques to facilitate the communication of covert, sensitive information has helped topple governments and promote illicit liaisons.
The goal of steganography is concealment. In contrast to its relative cryptography whose focus is on transformation of the message into indecipherable nonsense, steganography's strength lies in its ability to pass a message from sender to receiver undetected. Because concealment is the objective, some problems lend themselves more readily to steganographic rather than cryptographic techniques.
For example, communication between two prisoners using cryptography is impractical. In this highly controlled setting, an exchange of encrypted data between the two inmates will immediately arouse the guards' suspicions. Once they are alerted to the existence of the message, no further communication can occur. Whether the guards understand the content of the message or not, the ultimate goal of passing information between the sender and the receiver will be thwarted.
On the other hand, by using steganography to conceal the very existence of the message, the prisoners can evade the watchful eyes of their captors. Hidden messages can pass between the sender and the receiver because the guards remain oblivious. This use of a covert communications channel makes the message invisible to everyone but the intended recipient.
To illustrate, assume that prisoner A wants to convey the details of his secret plan to escape. Verbally yelling the information across the corridor to prisoner B is out of the question. Likewise, encoding the information and passing it to his partner would result in separation of the inmates. Steganography comes to their rescue.
If prisoner A wants to convey the following message "Escape today 8 p.m.," he can use Steganography. He can hide the message by using each letter in the message as the first letter of each word in a sentence: "Ellen's sisters Clara and Paula each took over doing all your 8 prayer meetings."
When taken out of context this message appears a little odd; however, if we embedded it in a longer message it would not raise suspicions. As long...