Now that we have covered some of the basic techniques of steganography and how they have been used in the past, we will move forward into the digital age. As I promised in the beginning I am going to keep this as straightforward and nontechnical as possible, but some of these concepts are very technically rooted and will require getting into the weeds, so to speak. I will do my best, but if I get too technical, I apologize in advance.
When it is all said and done, there are only three ways to hide a digital message in a digital cover: injection, substitution, and generation of new files.
Data injection embeds the secret message directly in the host medium. The problem with this kind of embedding is that it usually makes the host file larger, and therefore the alteration is easier to detect.
Normal data is replaced or substituted with the secret data. This usually results in very little size change for the host file. However, depending on the type of host file and the amount of hidden data, the substitution method can degrade the quality of the original host file.
A cover is generated for the sole purpose of concealing a secret message. As illustrated in "The Prisoners' Problem," Bob creates a picture of something innocent that can be passed to Alice; the innocent picture is the cover that provides the mechanism for conveying the message, which in that example was the particular color of a cow.
Another, more modern form of generation of new files is the Spam Mimic program. Spam Mimic embeds a text message within a rather daunting piece of spam, which can be e-mailed to an intended recipient. While this generated spam does not make a whole lot of sense, it makes enough to be believable, and that is enough.
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