Definitions

Steganography

We give some definitions common to the steganography field:

cover medium

This is the medium in which we want to hide data, it can be an innocent looking piece of information for steganography, or some important medium that must be protected for copyright or integrity reasons.

embedded message

This is the hidden message we want to put in the cover. It can be some data for steganography and some copyright informations or added content for digital watermarking.

stegokey

This is represented by some secret information, which is needed in order to extract the embedded message from the stego-medium

stego-medium

This is the final piece of information that the casual observer can see.

We can define this simple formula:

Steganalisys

As the name may suggest, steganalisys is the science of breaking steganography. Its attacks are of two different types: one is aimed at detecting and/or extracting an embedded message from a stego-medium. The second tries to destroy or render unrecoverable that embedded message. Note that these two tasks can be accomplished separately, since we can destroy a message just applying some transformations to the stego-medium depending on the steganographic method used or supposed, even if we don't know what the message is.

The different steganalisys methods derive their names from their counterparts in cryptoanalisys:

• A stego only attack is one where we only have the stego-medium, and we want to detect and/or extract the embedded message.
• A known cover attack is used when we have both the stego-medium and the cover medium, so that we can make comparisons between the two
• A known message attack assumes that we know the message and the stego-medium, and we want to find the method used for embedding the message
• A chosen stego attack is used when we have both the stego-medium and the steganography tool or algorithm
• Lastly, a chosen message attack is one where the steganalyst generates a stego-medium from a message using some particular tool, looking for signatures that will enable him to detect other stego-media.

References:

[JJ98-2]

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