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19-02-2011, 03:19 PM

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Steganography is the art of hiding the fact that communication is taking place, by hiding information inother information. Many different carrier file formats can be used, but digital images are the mostpopular because of their frequency on the Internet. For hiding secret information in images, there exists alarge variety of steganographic techniques some are more complex than others and all of them haverespective strong and weak points. Different applications have different requirements of thesteganography technique used. For example, some applications may require absolute invisibility of thesecret information, while others require a larger secret message to be hidden. This paper intends to givean overview of image steganography, its uses and techniques. It also attempts to identify the requirementsof a good steganographic algorithm and briefly reflects on which steganographic techniques are moresuitable for which applications .
1. Introduction
Since the rise of the Internet one of the most important factors of information technology and communicationhas been the security of information. Cryptography was created as a technique for securing the secrecy ofcommunication and many different methods have been developed to encrypt and decrypt data in order to keepthe message secret. Unfortunately it is sometimes not enough to keep the contents of a message secret, it mayalso be necessary to keep the existence of the message secret. The technique used to implement this, is calledsteganography.Steganography is the art and science of invisible communication. This is accomplished through hidinginformation in other information, thus hiding the existence of the communicated information. The wordsteganography is derived from the Greek words “stegos” meaning “cover” and “grafia” meaning “writing” [1]defining it as “covered writing”. In image steganography the information is hidden exclusively in images.The idea and practice of hiding information has a long history. In Histories the Greek historian Herodotus writesof a nobleman, Histaeus, who needed to communicate with his son-in-law in Greece. He shaved the head of oneof his most trusted slaves and tattooed the message onto the slave’s scalp. When the slave’s hair grew back theslave was dispatched with the hidden message [2]. In the Second World War the Microdot technique wasdeveloped by the Germans. Information, especially photographs, was reduced in size until it was the size of atyped period. Extremely difficult to detect, a normal cover message was sent over an insecure channel with oneof the periods on the paper containing hidden information [3]. Today steganography is mostly used oncomputers with digital data being the carriers and networks being the high speed delivery channels.Steganography differs from cryptography in the sense that where cryptography focuses on keeping the contentsof a message secret, steganography focuses on keeping the existence of a message secret [4]. Steganography andcryptography are both ways to protect information from unwanted parties but neither technology alone is perfectand can be compromised. Once the presence of hidden information is revealed or even suspected, the purpose ofsteganography is partly defeated [4]. The strength of steganography can thus be amplified by combining it withcryptography.Two other technologies that are closely related to steganography are watermarking and fingerprinting [5]. Thesetechnologies are mainly concerned with the protection of intellectual property, thus the algorithms have differentrequirements than steganography. These requirements of a good steganographic algorithm will be discussedbelow. In watermarking all of the instances of an object are “marked” in the same way. The kind of informationhidden in objects when using watermarking is usually a signature to signify origin or ownership for the purposeof copyright protection [6]. With fingerprinting on the other hand, different, unique marks are embedded indistinct copies of the carrier object that are supplied to different customers. This enables the intellectual propertyowner to identify customers who break their licensing agreement by supplying the property to third parties [5].In watermarking and fingerprinting the fact that information is hidden inside the files may be public knowledge– sometimes it may even be visible – while in steganography the imperceptibility of the information is crucial[4]. A successful attack on a steganographic system consists of an adversary observing that there is informationhidden inside a file, while a successful attack on a watermarking or fingerprinting system would not be to detectthe mark, but to remove it [5].Research in steganography has mainly been driven by a lack of strength in cryptographic systems. Manygovernments have created laws to either limit the strength of a cryptographic system or to prohibit it altogether[7], forcing people to study other methods of secure information transfer. Businesses have also started to realisethe potential of steganography in communicating trade secrets or new product information. Avoidingcommunication through well-known channels greatly reduces the risk of information being leaked in transit [8].Hiding information in a photograph of the company picnic is less suspicious than communicating an encryptedfile.This paper intends to offer a state of the art overview of the different algorithms used for image steganography toillustrate the security potential of steganography for business and personal use. After the overview it brieflyreflects on the suitability of various image steganography techniques for various applications. This reflection isbased on a set of criteria that we have identified for image steganography. The remainder of the paper isstructured as follows: Section 2 gives the reader an overview of steganography in general and differentiatesbetween different kinds of steganography. In section 3 the most popular algorithms for image steganography arediscussed and compared in section 4. In Section 5 a conclusion is reached
.2. Overview of Steganography
To provide an overview of steganography, terms and concepts should first be explained. An overview of thedifferent kinds of steganography is given at a later stage.
2.1 Steganography concepts
Although steganography is an ancient subject, the modern formulation of it is often given in terms of theprisoner’s problem proposed by Simmons [9], where two inmates wish to communicate in secret to hatch anescape plan. All of their communication passes through a warden who will throw them in solitary confinementshould she suspect any covert communication [10].The warden, who is free to examine all communication exchanged between the inmates, can either be passive oractive. A passive warden simply examines the communication to try and determine if it potentially containssecret information. If she suspects a communication to contain hidden information, a passive warden takes noteof the detected covert communication, reports this to some outside party and lets the message through withoutblocking it. An active warden, on the other hand, will try to alter the communication with the suspected hiddeninformation deliberately, in order to remove the information [5].
2.2 Different kinds of steganography
Almost all digital file formats can be used for steganography, but the formats that are more suitable are thosewith a high degree of redundancy. Redundancy can be defined as the bits of an object that provide accuracy fargreater than necessary for the object’s use and display [11]. The redundant bits of an object are those bits thatcan be altered without the alteration being detected easily [5]. Image and audio files especially comply with thisrequirement, while research has also uncovered other file formats that can be used for information hiding.Figure 1 shows the four main categories of file formats that can be used for steganography.
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19-01-2012, 10:07 AM

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