STEGANOGRAPHIC PROCESS

The following formula provides a very generic description of the pieces of the steganographic process:

cover_medium + hidden_data + stego_key = stego_medium

In this context, the cover_medium is the file in which we will hide the hidden_data, which may also be encrypted using the stego_key. The resultant file is the stego_medium (which will, of course. be the same type of file as the cover_medium). The cover_medium (and, thus, the stego_medium) are typically image or audio files. In this article, I will focus on image files and will, therefore, refer to the cover_image and stego_image.

Before discussing how information is hidden in an image file, it is worth a fast review of how images are stored in the first place. An image file is merely a binary file containing a binary representation of the color or light intensity of each picture element (pixel) comprising the image.

Images typically use either 8-bit or 24-bit color. When using 8-bit color, there is a definition of up to 256 colors forming a palette for this image, each color denoted by an 8-bit value. A 24-bit color scheme, as the term suggests, uses 24 bits per pixel and provides a much better set of colors. In this case, each pix is represented by three bytes, each byte representing the intensity of the three primary colors red, green, and blue (RGB), respectively. The Hypertext Markup Language (HTML) format for indicating colors in a Web page often uses a 24-bit format employing six hexadecimal digits, each pair representing the amount of red, blue, and green, respectively. The color orange, for example, would be displayed with red set to 100% (decimal 255, hex FF), green set to 50% (decimal 127, hex 7F), and no blue (0), so we would use "#FF7F00" in the HTML code.

The size of an image file, then, is directly related to the number of pixels and the granularity of the color definition. A typical 640x480 pix image using a palette of 256 colors would require a file about 307 KB in size (640 • 480 bytes), whereas a 1024x768 pix high-resolution 24-bit color image would result in a 2.36 MB file (1024 • 768 • 3 bytes).

To avoid sending files of this enormous size, a number of compression schemes have been developed over time, notably Bitmap (BMP), Graphic Interchange Format (GIF), and Joint Photographic Experts Group (JPEG) file types. Not all are equally suited to steganography, however

GIF and 8-bit BMP files employ what is known as lossless compression, a scheme that allows the software to exactly reconstruct the original image. JPEG, on the other hand, uses lossy compression, which means that the expanded image is very nearly the same as the original but not an exact duplicate. While both methods allow computers to save storage space, lossless compression is much better suited to applications where the integrity of the original information must be maintained, such as steganography. While JPEG can be used for stego applications, it is more common to embed data in GIF or BMP files.

Steganography-Introduction

Steganography is the art and science of hiding communication; a

steganographic system thus embeds hidden content in unremarkable cover media so as not to arouse an eavesdropper’s suspicion. In the past, people used hidden tattoos or invisible ink to convey steganographic content. Today, computer and network technologies provide easy-to-use communication channels for steganography. Essentially,the information-hiding process in a steganographic system starts by identifying a cover medium’s redundant bits (those that can be modified without destroying that medium’s integrity).

The embedding process creates a stego medium by replacing these redundant bits with data from the hidden message. Modern steganography’s goal is to keep its mere presence undetectable, but steganographic systems—because of their invasive nature—leave behind detectable traces in the cover medium. Even if secret content is not revealed, the existence of it is: modifying the cover medium changes its statistical properties, so eavesdroppers can detect the distortions in the resulting stego medium’s statistical properties. The process of finding these distortions is called statistical steganalysis.

This article discusses existing steganographic systems and presents recent research in detecting them via statistical steganalysis. Other surveys focus on the general usage of information hiding and watermarking or else provide an overview of detection algorithms. Here, we present recent research and discuss the practical application of detection algorithms and the mechanisms for getting around them.

Steganography is a technique used to hide information within images. Using stenography, watermarks and copyrights can be placed on an image to protect the rights of its owner without altering the appearance of the image. Almost like magic, images, executable programs, and text messages can hide in images. The cover image does not appear to be altered. People look at the cover image and never suspect something is hidden. Your information is hidden in plain sight.

Steganography in History

Steganography comes from Greek and means “covered writing”. The ancient Greeks wrote text on wax-covered tablets. To pass a hidden message, a person would scrape off the wax and write the message on the underlying wood. He/She would then once again cover the wood with wax so it appeared unused. Many developments in steganography occurred during world war II. This included the development of invisible inks, microdots, and encoded messages.

While cryptography is preoccupied with the protection of the contents of a message or information, steganography concentrates on concealing the very existence of such messages from detection. The term steganography is adapted from the Greek word steganographia, meaning “covered writing”, and is taken in its modern form to mean the hiding of information inside other information [1].

Naturally these techniques date back throughout history, the main applications being in couriering information during times of war.

The Greek writer Herodotus gave a famous anecdotal account of this around 440 B.C. His tale was of a Demeratus, a Greek in the Persian court who warned Sparta of an invasion by Xerxes, the King of Persia. He did this by removing the wax from a writing tablet, scoring his message in the wood underneath, and then covering it with wax again before sending it to Sparta [1]. With the invention of digital audio and images files this has taken on a whole new meaning; creating new methods for performing “reversible data hiding” as it is often dubbed.

Steganography and the Attacks

Attacks on Steganography:

Two aspects of attacks on steganography are detection and destruction of embedded message. Any object can be manipulated with the intent of destroying some hidden information whether an embedded message exist or not. .

Attacking steganographic algorithm is very similar to attacking cryptographic algorithms and similar techniques apply. If the original unmodified file used as a cover by the stegosystem is available to an attacker or investigator all he has to do is a bit-by-bit comparison with the suspect version in order to establish steganographic content. That is why publicly available files- sound files from CD or images from internet should never be used as a cover. The strength of a steganographic algorithm depends on its ability to successfully withstand attacks.

A few of the possible attacks are as follows:

File Only: The attacker has access to the file and must determine if there is a message hidden inside. This is the weakest form of attack, but it is also the minimum threshold for a successful steganography. File only attack relies on statistical analysis to reveal the presence of a message in a file.

File an Original copy: In some cases the attacker may have a copy of the file with the encoded message and a copy of the original. If the two files are different, there must be some hidden information inside. The attacker can simply replace the message with the original to destroy the hidden information.

Reformat Attack: One possible attack is to change the format of the file. This can work because different file formats store data in different ways.

Compression Attack. One of the simplest forms of attack is to compress the file. Compression algorithms try to remove extraneous information from a file. A good example is the JPEG, where the image format is not accurate but rather an approximate of the original.

Another possible attack is to simply destroy the message or encode a new message if you have access to the algorithm. A message with hidden information maybe detectable, but this only becomes an issue if someone is trying to detect it. Detecting hidden information will save time with message elimination by processing only the messages with hidden information.

Visual Attack

The visual attack is a stego-only-attack that strips away part of the object in way that allows for a human to search for visual anomalies. The most common attack is to display the least significant bit of an object; Digital equipments such as cameras and scanners are not perfect and often leave echoes in the least significant bits. These completely random noises indicate the existence of a hidden message. The average ear can pick up subtle difference in sound. However, this is a very slow and costly attack.

Structural Attack

Steganographic algorithms leave behind a characteristic structure to the data. The format of the data file is often different when information is embedded. The attacker may detect the presence of a message by examining the statistical profile of the bits. These changes to the data file usually fall into easily detectable pattern that gives an indication of a hidden message.

Statistical Attack

Statistical attack is similar to visual attack. The fact that most programs relies on the assumption that least significant bit of a cover file is random and therefore overwritten with a secret message is not necessarily true. The idea of the statistical attack is to compare the frequency distribution of a potential cover file with the theoretically expected distribution of the cover file. If the new data does not have the same statistical profile as the standard data is expected to have, then it probably contains a hidden message.

STEGANALYSIS

Steganalysis is a relatively new branch of research. While steganography (which is somewhat different from watermarking) deals with techniques for hiding information, the goal of steganalysis is to detect and/or estimate potentially hidden information from observed data with little or no knowledge about the steganography algorithm and/or its parameters. It is fair to say that steganalysis is both an art and a science. The art of steganalysis plays a major role in the selection of features or characteristics a typical stego message might exhibit while the science helps in reliably testing the selected features for the presence of hidden information. While it is possible to design a reasonably good

steganalysis technique for a specific steganography algorithm, the long term goal must be to develop a steganalysis framework that can work effectively at least for a class of steganography methods if not for all. Clearly, this poses a number of mathematical challenges and questions some of which are summarized below.

1.Can the current and future steganography algorithms be categorized into distinct classes of mathematical
techniques?

2.What is a good mathematical definition of steganalysis?

3.What a priori knowledge can we assume the steganalyst possesses?

4.What mathematical properties a class of steganography algorithms must satisfy for which good steganalysis techniques can be developed? This will give rise to a new notion of security in steganography that could be quite different from the popular information theoretic definition.1

5.What are the candidate cost or risk functions that a steganalyst must optimize during hidden data detection or extraction procedure?

6.What are the performance trade-offs involved if a steganalysis algorithm is designed only to detect, only to extract, or detect and extract the hidden message?

We attempt to address some of these questions in this paper and develop a formal theory of steganalysis. We note that in our present analysis we assume the steganalyst has reasonable computational resources and time. In a traditional steganography set-up formulated as a prisoner’s problem,2 Alice wishes to send a secret message to Bob by hiding information in a cover message. The stego message (cover+message) passes through Wendy (a warden) who inspects it to

determine if there is anything suspicious about it. Wendy could perform one or several tests to decide if the message from Alice to Bob contains any secret information. If her decision is negative then Wendy forwards the message to Bob—Wendy acts as a passive warden. On the other hand, Wendy can take a conservative approach and modify all the messages from Alice to Bob irrespective of whether any information is hidden by Alice or not. In this case, Wendy is called an active warden. Of course, Wendy will have constraints such as the maximum allowable distortion when modifying the message etc. For example, if the cover messages are digital images, then Wendy cannot modify the stego message to an extent that perceptually significant distortions are induced. While current steganalysis techniques focus on detecting the presence/absence of a secret message in observed message, to our knowledge there seems to have been no attempt in extracting the secret message. In general, extraction of the secret message could be a harder problem than mere detection.

Therefore, based on the ultimate outcome of the effort we classify steganalysis into two categories:

Passive steganalysis: Detect the presence or absence of a secret message in an observed message

Active steganalysis: Extract a (possibly approximate) version of the secret message from a stego message.

Note that active steganalysis could be different from an active warden case. An active warden manipulates the stego message in the hopes of destroying the

secret message (if any) but an active steganalyst attempts to estimate and extract the secret message without destroying it. In this paper, we discuss a mathematical framework for active steganalysis when a certain class of linear steganography algorithms are employed. We also discuss the strengths and limitations of the proposed framework and provide numerical examples. Without loss of generality we consider digital images as cover messages for our experiments. Our primary goal is to estimate the cover message, secret message, and even perhaps the steganography key using only the observed stego messages. During this process we exploit spatial diversity and temporal diversity information.

Cryptography and Steganography

Cryptography is the science of writing in secret code and is an ancient art; the first documented use of cryptography in writing dates back to circa 1900 B.C. when an Egyptian scribe used non-standard hieroglyphs in an inscription. Some experts argue that cryptography appeared spontaneously sometime after writing was invented, with applications ranging from diplomatic missives to war-time battle plans. It is no surprise, then, that new forms of cryptography came soon after the widespread development of computer communications. In data and telecommunications, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.

Within the context of any application-to-application communication, there are some specific security requirements, including:

• Authentication: The process of proving one's identity. (The primary forms of host-to-host authentication on the Internet today are name-based or address-based, both of which are notoriously weak.)
• Privacy/confidentiality: Ensuring that no one can read the message except the intended receiver.
• Integrity: Assuring the receiver that the received message has not been altered in any way from the original.
• Non-repudiation: A mechanism to prove that the sender really sent this message.

Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication. There are, in general, three types of cryptographic schemes typically used to accomplish these goals: secret key (or symmetric) cryptography, public-key (or asymmetric) cryptography, and hash functions, each of which is described below. In all cases, the initial unencrypted data is referred to as plaintext. It is encrypted into ciphertext, which will in turn (usually) be decrypted into usable plaintext.

In many of the descriptions below, two communicating parties will be referred to as Alice and Bob; this is the common nomenclature in the crypto field and literature to make it easier to identify the communicating parties. If there is a third or fourth party to the communication, they will be referred to as Carol and Dave. Mallory is a malicious party, Eve is an eavesdropper, and Trent is a trusted third party.

There are a large number of steganographic methods that most of us are familiar with (especially if you watch a lot of spy movies!), ranging from invisible ink and microdots to secreting a hidden message in the second letter of each word of a large body of text and spread spectrum radio communication. With computers and networks, there are many other ways of hiding information, such as:

• Covert channels (e.g., Loki and some distributed denial-of-service tools use the Internet Control Message Protocol, or ICMP, as the communications channel between the "bad guy" and a compromised system)
• Hidden text within Web pages
• Hiding files in "plain sight" (e.g., what better place to "hide" a file than with an important sounding name in the c:\winnt\system32 directory?)
• Null ciphers (e.g., using the first letter of each word to form a hidden message in an otherwise innocuous text)

Steganography today, however, is significantly more sophisticated than the examples above suggest, allowing a user to hide large amounts of information within image and audio files. These forms of steganography often are used in conjunction with cryptography so that the information is doubly protected.

Steganographic techniques

Modern steganographic techniques:

Modern steganography entered the world in 1985 with the advent of the Personal Computer applied to classical steganography problems. Development following that was slow, but has since taken off, based upon the number of 'stego' programs available.

* Concealing messages within the lowest bits of noisy images or sound files.
* Concealing data within encrypted data. The data to be concealed is first encrypted before being used to overwrite part of a much larger block of encrypted data.
* Chaffing and winnowing
* Invisible ink
* Null ciphers
* Concealed messages in tampered executable files, exploiting redundancy in the i386 instruction set
* Embedded pictures in video material (optionally played at slower or faster speed).
* A new steganographic technique involves injecting imperceptible delays to packets sent over the network from the keyboard. Delays in keypresses in some applications (telnet or remote desktop software) can mean a delay in packets, and the delays in the packets can be used to encode data.
* Content-Aware Steganography hides information in the semantics a human user assigns a datagram; these systems offer security against a non-human adversary/warden.
* BPCS-Steganography - a very large embedding capacity steganography.
* Blog-Steganography. Messages are fractionalyzed and the (encrypted) pieces are added as comments of orphaned web-logs (or pin boards on social network platforms). In this case the selection of blogs is the symmetric key that sender and recipient are using. The carrier of the hidden message is the whole blogosphere.

• Hidden messages in wax tablets: in ancient Greece, people wrote messages on the wood, then covered it with wax so that it looked like an ordinary, unused tablet.
• Hidden messages on messenger's body: also in ancient Greece. Herodotus tells the story of a message tattooed on a slave's shaved head, hidden by the growth of his hair, and exposed by shaving his head again. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks:

1. It is impossible to send a message as quickly as the slave can travel, because it takes months to grow hair.
2. A slave can only be used once for this purpose.
   

• Hidden messages on paper written in secret inks under other messages or on the blank parts of other messages.
• During and after World War II, espionage agents used photographically produced microdots to send information back and forth. Since the dots were typically extremely small—the size of a period produced by a typewriter or even smaller—the stegotext was whatever the dot was hidden within. If a letter or an address, it was some alphabetic characters. If under a postage stamp, it was the presence of the stamp. The problem with the WWII microdots was that they needed to be embedded in the paper, and covered with an adhesive (such as collodion), which could be detected by holding a suspected paper up to a light and viewing it almost edge on. The embedded microdot would reflect light differently than the paper.
• More obscurely, during World War II, a spy for the Japanese in New York City, Velvalee Dickinson, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed how many of this or that doll to ship. The stegotext in this case was the doll orders; the 'plaintext' being concealed was itself a codetext giving information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.
• Counter-propaganda: During the Pueblo Incident, US crew members of the USS Pueblo (AGER-2) research ship held as prisoners by North Korea communicated in sign language during staged photo ops to inform the United States that they had not defected, but had instead been captured by North Korea and were still loyal to the U.S. In other photos presented to the US, the crew members gave "the finger" to the unsuspecting North Koreans, in an attempt to discredit the pictures that showed them smiling and comfortable.
• The one-time pad is a theoretically unbreakable cipher that produces ciphertexts indistinguishable from random texts: only those who have the private key can distinguish these ciphertexts from any other perfectly random texts. Thus, any perfectly random data can be used as a covertext for a theoretically unbreakable steganography. A modern example of OTP: in most cryptosystems, private symmetric session keys are supposed to be perfectly random (that is, generated by a good Random Number Generator), even very weak ones . This means that users of weak cryptography (in countries where strong cryptography is forbidden) can safely hide OTP messages in their session keys.

Information hiding

In computer science, the principle of information hiding is the hiding of design decisions in a computer program that are most likely to change, thus protecting other parts of the program from change if the design decision is changed. The protection involves providing a stable interface which shields the remainder of the program from the implementation. In modern programming languages, the principle of information hiding manifests itself in a number of ways, including encapsulation polymorphism.

The term is also used frequently as a synonym for steganography, the science of hiding a secret message often by mixing it into the noise of another. One of the academic conferences devoted to the subject is held annually.

Information hiding serves as an effective criterion for dividing any piece of equipment, software or hardware, into modules of functionality. For instance a car is a complex piece of equipment. In order to make the design, manufacturing, and maintenance of a car reasonable, the complex piece of equipment is divided into modules with particular interfaces hiding design decisions. By designing a car in this fashion, a car manufacturer can also offer various options while still having a vehicle which is economical to manufacture.

For instance, a car manufacturer may have a luxury version of the car as well as a standard version. The luxury version comes with a more powerful engine than the standard version. The engineers designing the two different car engines, one for the luxury version and one for the standard version, provide the same interface for both engines. Both engines fit into the engine bay of the car which is the same between both versions. Both engines fit the same transmission, the same engine mounts, and the same controls. The differences in the engines are that the more powerful luxury version has a larger displacement with a fuel injection system that is programmed to provide the fuel air mixture that the larger displacement engine requires.

In addition to the more powerful engine, the luxury version may also offer other options such as a better radio with CD player, more comfortable seats, a better suspension system with wider tires, and different paint colors. With all of these changes, most of the car is the same between the standard version and the luxury version. The radio with CD player is a module which replaces the standard radio, also a module, in the luxury model. The more comfortable seats are installed into the same seat mounts as the standard types of seats. Whether the seats are leather or plastic, lumbar support or not, doesn't matter.

The engineers design the car by dividing the task up into pieces of work which are assigned to teams. Each team then designs their component to a particular standard or interface which allows the sub-team flexibility in the design of the component while at the same time ensuring that all of the components will fit together.

As can be seen by this example, information hiding provides flexibility. This flexibility allows a programmer to modify functionality of a computer program during normal evolution as the computer program is changed to better fit the needs of users. When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes.

Cars provide another example of this in how they interface with drivers. They present a standard interface -pedals, wheel, shifter, signals, gauges, etc. on which people are trained and licensed. Thus, people only have to learn to drive a car; they don't need to learn a completely different way of driving every time they drive a new model.

Steganography - the Art of Hiding Information

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.

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..

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.

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.

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 . 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.

Digital watermarking

Digital watermarking is the process of embedding auxiliary information into a digital signal -the cover signal. Depending on the context, the notion digital watermark either refers to the information that is embedded into the cover signal or to the difference between the marked signal and the cover signal. Digital watermarking is related to physical paper watermarking, in this case the auxiliary information being an unobtrusive ingrained logo. Watermarking is also closely related to steganography, the art of secret communication.

A digital watermark is called robust with respect to a class of transformations T if the embedded information can reliably be detected from the marked signal even if degraded by any transformation in T. Typical image degradations are JPEG compression, rotation, cropping, additive noise and quantization. For video content temporal modifications and MPEG compression are often added to this list. A watermark is called imperceptible if the cover signal and marked signal are indistinguishable with respect to an appropriate perceptual metric. In general it is easy to create robust watermarks or imperceptible watermarks, but the creation of robust and imperceptible watermarks has proven to be quite challenging . Robust imperceptible watermarks have been proposed as tool for the protection of digital content, for example as an embedded 'no-copy-allowed' flag in professional video content .

• Robustness
  • A watermark is called fragile if it fails to be detected after the slightest modification. Fragile watermarks are commonly used for tamper detection.
  • A watermark is called semi-fragile if it resist benign transformations but fails detection after malignant transformations. Semi-fragile watermarks are commonly used to detect malignant transformations.
  • A watermark is called robust if it resists a designated class of transformations. Robust watermarks are commonly used in copyright applications (to carry ownership or forensic information) and copy protection applications (to carry copy and access control information).
• Perceptibility
  • A watermark is called imperceptible if the original cover signal and the marked signal are (close to) perceptually indistinguishable.
  • A watermark is called perceptible if its presence in the marked signal is noticeable, but non-intrusive.
  • Modification to an original work that are clearly noticeable are commonly not referred to as watermarks, but referred to as generalized barcodes.
• Embedding method
  • A watermarking method is referred to as spread-spectrum if the marked signal is obtained by an additive modification. Spread-spectrum watermarks are known to be modestly robust, but also to have a low information capacity due to host interference.
  • A watermarking method is referred to be of quantization type if the marked signal is obtained by quantization. Quantization watermarks suffer from low robustness, but have a high information capacity due to rejection of host interference.
  • A watermarking method is referred to as amplitude modulation if the marked signal is embedded by additive modification method which it similar to spread spectrum method but this method is especially embedded in spatial domain.

Steganography In Principle

Steganography is one of the oldest arts that people have always wanted to have since they started communicating with each other, but sadly the least researched. Most people study steganography either as academic discipline or out of curiosity and I belong to the latter camp. Although steganography is used in military and commercial circuits the level of application and understanding is very low.

The term steganography as well cryptography was derived from the Greek language. The prefix crypto comes from the Greek word kryptos, which means hidden or secret. The suffix graphy was derived from graphia, which means writing. Cryptography is essentially the art of secret writing and the goal is to maintain the secrecy of the message even if it is visible. Steganography is also a form of writing -concealed writing. The Greek word steganos means unseen or hidden. Steganography is a form of hidden communication, it should not be seen as a replacement for cryptography but rather as a complement to it. Steganography, although closely related to cryptography, is different. The goal of cryptography is to conceal the content of a message, while the goal of steganography is to conceal the existence of a message. However, these two techniques can be combined effectively by first encrypting the secret message before embedding in a cover data. Concealing the transmission of encrypted messages enhances their overall security since outsiders are unaware of the communication.

Encrypted data can attract the attention of hackers and investigators through its mere existence, however if concealed, no attempt would be made to break the code or to obtain the secret key. Steganographic methods primarily use image or audio files to hide encrypted data, such techniques conceal information in the least significant bits of the carrier medium, which serves as a hiding place. It is important that the carrier medium does not lose its appearance after the embedding process.

Another technique similar to steganography is watermarking, the goal of watermarking is to mark an image or sound file to the owner by making elusive modifications to the file. These modifications should not be noticeable but rather, very robust; nobody should be able to remove an existing mark or mark an already marked file as belonging to him. This technique is of great interest to the entertainment industry because it gives an efficient way to determine if a file was illegally downloaded from the web or rightfully purchased.

A good steganography system should fulfil the same requirements posed by the Kerckhoff principle in cryptography that security of a system should not rely on the on its method of operation being unknown to the enemy, but rather on the choice of a secret key.

In recent years there has been an exciting convergence of information protection technologies and the main emphasis is information hiding as oppose to encryption. The two big policy issues of copyright protection and state surveillance motivated this development. The more information that is placed on the Internet or public media, the more the owner of the information need to protect themselves from theft and abuse. The entertainment industry is particularly very nervous due to the ease at which exact copies of digital music and video can be made. The way forward is to embrace advance technology to protect investment rather than oppose it. Part of the solution may be a change in the sale process of music and video; one mechanism is copyright marking hiding notices and serial numbers in a way that would be difficult for pirates to remove. Systems and techniques that can uncover hidden information will be useful in computer forensics and digital traffic analysis. Understanding the limitations of current techniques can help develop more robust techniques. The principal focus is hiding information or at least stopping other people from hiding information.

Steganography-Uses

Steganography is the secret transmission of a message. It is distinct from encryption, because the goal of encryption is to make a message difficult to read while the goal of steganography is to make a message altogether invisible. A steganographic message may also be an encrypted as an extra barrier to interception, but need not be. Steganography has the advantage that even a talented code-cracker cannot decipher a message without knowing it is there.

Like most other forms of cryptography and secret writing, steganography has thrived in the digital era. Most digital documents contain useless or insignificant areas of data, or involve enough redundancy that some of their information can be altered without obvious effect. For instance, one might conceal a message bitstream inside a digital audio file by replacing the least-significant bit of every waveform sample -or every n th waveform sample with a message bit; the only effect on the file, if played back as audio, would be a slight decrease in the sound quality -probably imperceptible. Although steganographic messages can be hidden in any kind of digital files, image files, because they contain so much data to begin with, are usually used for digital steganography. Today a number of commercial or shareware programs exist for encoding text into steganographic images , and are used by millions of people worldwide who wish to evade surveillance, especially by governments. This includes people who have reason to fear punishment for expressing their political ideas, as well as terrorists.

Steganography is also used for the less dramatic purpose of watermarking , which is the hiding of information indicating ownership or origin inside a digital file. Physical watermarking, the practice after which digital watermarking is named, is the impression of a subtle pattern on paper using water. A watermark is only visible when the paper is held up to a light. Watermarking can be used for digital authentication -i.e., to prove that certain party was indeed the source of a file or to check whether a digital file was obtained in violation of copyright.