Steganography - the Art of Hiding Information

March 11, 2008

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2.0 Steganography

Steganographic techniques have been used since World War I and World War II, Chemicals were developed and used as secret inks that become visible when brought in contact with other chemicals. A brief history of steganography would give us a valuable background.

2.1 History

Greek historian Herodotus recorded the earliest records of steganography. When Histiaeus had to send a secret message to his son-in-law, he shaved the head of a slave and tattooed a message, he waited till the hair had grown before dispatching him in order to avoid detection. Another Greek history was when Demeratus scraped the wax off tablets and wrote messages on the underlying wood he then covered the wood with wax again to conceal the message. The tablets appear to be blank and unused when inspected.

Invisible ink has always been a popular method of steganography. Ancient Romans wrote between lines using invisible inks made from substances like milk, urine and fruit juices. When it is heated, the invisible ink would darken and become legible.

Gaspari Schotti wrote the earliest book on steganography in 1665 called Steganographica. A major development in the field occurred in 1883 with the publication of Auguste Kerckhoffs cryptographie militaire. Although the work was mostly on cryptography, it provides valuable principle in the design of new steganographic systems [SEL03].

2.2 Steganography In Principle

Bruce Schneier describes steganography as follows: Steganography serves to hide secret messages in other messages, such that the secret’s very existence is concealed [SCH96]. Another basic definition would simply be the act of hidden communication. Whatever definition you find suitable the fundamental principle should be the same. The message is the information to be hidden and may be an image, audio or anything that can be embedded into a bitstream. The cover and the embedded message create a stego-carrier that may require a stegokey. The stegokey is additional secret information such as a password. A possible formula for the process is represented as follows:

Cover medium + embedded message + stegokey = stego-medium

Hiding information in electronic media requires alterations to the media properties, which may introduce some form of degradation. This degradation can sometimes be visible and point to the signatures of the steganographic methods and tools. These signatures may actually broadcast the existence of the embedded message thereby defeating the purpose of steganography.

Steganographic system is considered broken:

• If the attacker can detect the use of steganography.

• If the attacker can read the embedded message.

Traditional cryptography succeeds by locking up messages in a mathematical safe, but steganography offers some stealth and exploit bit randomness. The possible techniques are as follows:

Noise: The simplest technique is to replace the noise in a sound or image file with the message. For example, one spot in a picture may have 220units of pink on a scale of 0 to 255. The average eye would not notice if that one spot was converted to 219 units of pink. It is possible to hide volumes of information below the threshold of perception if done systematically.

Spread information: Spreading the information increases the resilience to destruction, the algorithm distribute the information in such a way that not all the bits are required to reassemble the original data. Data usually falls into patterns, observing the patterns will enable you to exploit decision process of computers.

Randomness: Information can be hidden in place of the random bits. A few algorithms allow the broadcast of information without revealing its identity [WAY02].

2.3 Stegosystem

The steganographic system is referred to as the stegosystem it defines all the relationship with the data and processes involved.

2.4 Private And Public key Steganography

In order to effectively describe the private and public key steganography, it is important to understand the prisoner’s problem. The prisoners’ problem is considered the standard model for covert communication, first proposed by G.J Simmons in 1983. In this problem, two individuals attempt to communicate covertly without alerting a warden who controls the communication channel. One assumption is that the participants are allowed to share some secret information (encryption key) prior to imprisonment. The other assumption makes the problem more difficult; the warden is allowed to modify and read messages sent between prisoners.

2.4.1 Private-Key Steganography

In this scenario we assume that Alice and Bob are allowed to share a secret key prior to imprisonment. This gives them the opportunity to communicate covertly and defeat an active warden (Wendy). In all previous discussion, steganography simply encrypts a message in such a way that the ciphertext appears random while embedding the bits of the message in a known subliminal channel.

In the presence of an active warden, it would not be enough to embed a message in a known place. If Alice can alter the bits in an image then the warden can do the same thereby destroying message sent through the subliminal channel. A cryptographically secure pseudo-random generator, seeded by a secret key can be used to pick a subset of pixels in an image to conceal the data. If Wendy makes changes to the image, it would only scramble a small percentage of the channel bit since she does not know where they are. The scrambling can be corrected with an error-correcting code. Sharing keys before imprisonment gives a lot of freedom to Alice and Bob, and the public key can be used to sign the secret message, which provides additional security by preventing impersonation. Having to exchange keys far in advance of covert communication makes it a bit difficult in real life.

2.4.2 Public-Key Steganography

In this approach the secret key does not have to be agreed between Alice and Bob before imprisonment, but one must know the public key of the other. This is a more practical requirement in the real world.

1. Alice knows Bob’s public key, encrypts the message to obtain the ciphertext C

2. Alice embeds C in a channel known to Bob (also to known to Wendy) and sends

the stego to Bob.

3. Bob has no knowledge a message was hidden in the channel, if he suspects a

Message he would attempt to decrypt with his private key.

The problem with this approach is that Bob will just have to suspect a hidden message in every object he receives. This is not a serious problem if we assume that the steganographic technique is known to all and can be easily extracted. A more practical related problem is a when a large group of recipient is involved with everyone suspecting a hidden content intended for one recipient.

2.5 Steganographic Methods

The task of embedding a secret message could be performed by the combination of various techniques. Usually most steganographic programs follow these steps:

• Finding the Redundant Bits.

• Choosing the Cover Bits.

• Embedding the Data.

2.5.1 Finding the redundant bits: The assumption that the least significant bits are redundant and can be replaced without analysing the cover object is used by most programmes. A more successful technique would be to embed data in all regions of an object that is not informative. In order to determine these regions the image is split into single bit planes and analysed. For every 8´8 block of each bit plane a test is done to determine the threshold, at certain point above the threshold a secret message can be inserted without significantly altering the cover object.

2.5.2 Choosing the cover bits: The number of bits required to embed a secret message is usually not equal to the number of redundant bit; it is then necessary to choose a subset of the redundant bit.

Most programmes embed the message in the first few redundant bits (n) at the beginning of a cover file than at the end of the cover file, this approach exposes the modified object to visual attack. A pseudo-random permutation ensures that the cover bits are chosen with the same probability by providing equal spreading among the redundant cover bits. This technique depends on a secret key applied to the positions of the redundant cover bits; it also reduces visual and statistical attack.

2.5.3 Embedding the data: There are several techniques for embedding secret messages, but the most common methods overwrite the cover bits with the encrypted secret message. Although this technique provides large capacity by embedding one bit of the secret message with one bit of the cover, it can be detected with visual and statistical attack.

Matrix encoding enables more than two secret bits to be encoded in the cover for every change. This is accomplished by encoding the value of the first secret bit as the parity and the value of the first plus the third cover bit and the second secret bit are xored to form the parity for the second and third cover bit. The drawback is that an increased rate reduces the capacity of the cover file [HET02].

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