02-02-2013, 12:07 PM
Digital Steganography
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Introduction
Steganography is the art and science of hiding information by embedding messages within other, seemingly
harmless messages. Steganography means “covered writing” in Greek. As the goal of steganography
is to hide the presence of a message and to create a covert channel, it can be seen as the complement
of cryptography, whose goal is to hide the content of a message.
A famous illustration of steganography is Simmons’ “Prisoners’ Problem” [10]: Alice and Bob
are in jail, locked up in separate cells far apart from each other, and wish to devise an escape plan.
They are allowed to communicate by means of sending messages via trusted couriers, provided they
do not deal with escape plans. But the couriers are agents of the warden Eve (who plays the role of
the adversary here) and will leak all communication to her. If Eve detects any sign of conspiracy, she
will thwart the escape plans by transferring both prisoners to high-security cells from which nobody has
ever escaped. Alice and Bob are well aware of these facts, so that before getting locked up, they have
shared a secret codeword that they are now going to exploit for embedding a hidden information into
their seemingly innocent messages. Alice and Bob succeed if they can exchange information allowing
them to coordinate their escape and Eve does not become suspicious.
According to the standard terminology of information hiding [8], a legitimate communication among
the prisoners is called covertext, and a message with embedded hidden information is called stegotext.
The distributions of covertext and stegotext are known to the warden Eve because she knows what
constitutes a legitimate communication among prisoners and which tricks they apply to add a hidden
meaning to innocently looking messages.
Defining Security
The security of a stegosystem is defined in terms of an experiment that measures the capability of
the adversary to detect the presence of an embedded message. In a secure stegosystem, Eve cannot
distinguish whether Alice is sending legitimate covertext or stegotext.
The attack considered here is a chosen-message attack, where the adversary may influence the embedded
message but has otherwise no access to the encoding and decoding functions. It parallels the
notion of a chosen-plaintext attack against a cryptosystem.
Public-key Steganography
What if Alice and Bob did not have the time to agree on a secret key before being imprisoned? They
cannot use any of the stegosystems presented so far because that would require them to share a common
secret key. Fortunately, steganography is also possible without shared secrets, only with public keys,
similar to public-key cryptography. The only requirement is that Bob’s public key becomes known to
Alice in a way that is not detectable by Eve.
Formally, a public-key stegosystem consists of a triple of algorithms for key generation, message
encoding, and message decoding like a (secret-key) stegosystem, but the key generation algorithm now
outputs a stego key pair (spk, ssk). The public key spk is made available to the adversary and is the only
key needed by the encoding algorithm SE. The decoding algorithm SD needs the secret key ssk as an
additional input.