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Steganography- An Enhanced Approach

 

 

Abstract – The word “Steganography” is of Greek origin and means “covered, or hidden writing. The advantage of steganography over cryptography alone is that messages do not attract attention to themselves, to messengers, or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating, as in some countries encryption is illegal.

Cryptographic techniques “scramble” messages so if intercepted, the messages cannot be understood. Steganography, in an essence, “camouflages” a message to hide its existence and make it seem “invisible” thus concealing the fact that a message is being sent altogether. An encrypted message may draw suspicion while an invisible message will not. Steganography is the art of concealing the existence of information within seemingly innocuous carriers. Steganography can be viewed as akin to cryptography. Both have been used throughout recorded history as means to protect information. At times these two technologies seem to converge while the objectives of the two differ.

This paper provides brief history and basics of steganography, and describes the design of the tool Stego Viewer to hide the stego images or clips even on the computers of source and destination to make stegnography even more secure.In general we are using two different images at the time of encoding the stego image and separate these two files at the time of watching the image there are chances to leave traces of the images which is to be hide from general viewer. Our design of of stego viewer is one step ahead in protecting the traces of hidden image or clip which might be identified by any inside or outside intruders and hence make the sense of steganography more secure.

(Note: This Paper was presented in ICSIP Please Download the paper for proper formatting, images and symbols)

I. INTRODUCTION

Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the content is obscured. Nevertheless, this paper will treat steganography as a separate field.[1]

Steganography hides the covert message but not the fact that two parties are communicating with each other. The steganography process generally involves placing a hidden message in some transport medium, called the carrier. The secret message is embedded in the carrier to form the steganography medium. The use of a steganography key may be employed for encryption of the hidden message and/or for randomization in the steganography scheme. In summary:

steganography_medium=hidden_message + carrier + steganography_key.

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. [3]

III. CLASSIFICATION OF STEGANOGRAPHY TECHNIQUES

Figure 1. Classification of Steganography Techniques (Adapted from Bauer 2002).

Figure 1 shows a common taxonomy of steganographic techniques

· Technical steganography uses scientific methods to hide a message, such as the use of invisible ink or microdots and other size-reduction methods.

· Linguistic steganography hides the message in the carrier in some nonobvious ways and is further categorized as semagrams or open codes.

· Semagrams hide information by the use of symbols or signs. A visual semagram uses innocent-looking or everyday physical objects to convey a message, such as doodles or the positioning of items on a desk or Website. A text semagram hides a message by modifying the appearance of the carrier text, such as subtle changes in font size or type, adding extra spaces, or different flourishes in letters or handwritten text.[2]

· Open codes hide a message in a legitimate carrier message in ways that are not obvious to an unsuspecting observer. The carrier message is sometimes called the overt communication whereas the hidden message is the covert communication. This category is subdivided into jargon codes and covered ciphers.

· Jargon code, as the name suggests, uses language that is understood by a group of people but is meaningless to others. Jargon codes include warchalking (symbols used to indicate the presence and type of wireless network signal [Warchalking 2003]), underground terminology, or an innocent conversation that conveys special meaning because of facts known only to the speakers. A subset of jargon codes is cue codes, where certain prearranged phrases convey meaning.

· Covered or concealment ciphers hide a message openly in the carrier medium so that it can be recovered by anyone who knows the secret for how it was concealed. A grille cipher employs a template that is used to cover the carrier message. The words that appear in the openings of the template are the hidden message. A null cipher hides the message according to some prearranged set of rules, such as “read every fifth word” or “look at the third character in every word.”

As an increasing amount of data is stored on computers and transmitted over networks, it is not surprising that steganography has entered the digital age. On computers and networks, steganography applications allow for someone to hide any type of binary file in any other binary file, although image and audio files are today’s most common carriers.

DIGITAL CARRIER METHODS

There are many ways in which messages can be hidden in digital media. Digital forensics examiners are familiar with data that remains in file slack or unallocated space as the remnants of previous files, and programs can be written to access slack and unallocated space directly. Small amounts of data can also be hidden in the unused portion of file headers (Curran and Bailey 2003).

Information can also be hidden on a hard drive in a secret partition. A hidden partition will not be seen under normal circumstances, although disk configuration and other tools might allow complete access to the hidden partition (Johnson et al. 2001). This theory has been implemented in a steganographic ext2fs file system for Linux. A hidden file system is particularly interesting because it protects the user from being inextricably tied to certain information on their hard drive. This form of plausible deniability allows a user to claim to not be in possession of certain information or to claim that certain events never occurred. Under this system users can hide the number of files on the drive, guarantee the secrecy of the files’ contents, and not disrupt nonhidden files by the removal of the steganography file driver (Anderson et al. 1998; Artz 2001; McDonald and Kuhn 2000).

Another digital carrier can be the network protocols. Covert Transmission Control Protocol by Craig Rowland, for example, forms covert communications channels using the Identification field in Internet Protocol packets or the sequence number field in Transmission Control Protocol segments (Johnson et al. 2001; Rowland 1996).

There are several characteristics of sound that can be altered in ways that are indiscernible to human senses, and these slight alterations, such as tiny shifts in phase angle, speech cadence, and frequency, can transport hidden information (Curran and Bailey 2003).

Nevertheless, image and audio files remain the easiest and most common carrier media on the Internet because of the plethora of potential carrier files already in existence, the ability to create an infinite number of new carrier files, and the easy access to steganography software that will operate on these carriers. For that reason, the manuscript focus will return to image and audio files.

The most common steganography method in audio and image files employs some type of least significant bit substitution or overwriting. The least significant bit term comes from the numeric significance of the bits in a byte. The high-order or most significant bit is the one with the highest arithmetic value (i.e., 27=128), whereas the low-order or least significant bit is the one with the lowest arithmetic value (i.e., 20=1).

As a simple example of least significant bit substitution, imagine “hiding” the character ’G’ across the following eight bytes of a carrier file (the least significant bits are underlined):

10010101 00001101 11001001 10010110

00001111 11001011 10011111 00010000

A ’G’ is represented in the American Standard Code for Information Interchange (ASCII) as the binary string 01000111. These eight bits can be “written” to the least significant bit of each of the eight carrier bytes as follows:

10010100 00001101 11001000 10010110

00001110 11001011 10011111 00010001

In the sample above, only half of the least significant bits were actually changed (shown above in italics). This makes some sense when one set of zeros and ones are being substituted with another set of zeros and ones.

Least significant bit substitution can be used to overwrite legitimate RGB color encodings or palette pointers in GIF and BMP files, coefficients in JPEG files, and pulse code modulation levels in audio files. By overwriting the least significant bit, the numeric value of the byte changes very little and is least likely to be detected by the human eye or ear.

Least significant bit substitution is a simple, albeit common, technique for steganography. Its use, however, is not necessarily as simplistic as the method sounds. Only the most naive steganography software would merely overwrite every least significant bit with hidden data. Almost all use some sort of means to randomize the actual bits in the carrier file that are modified. This is one of the factors that make steganography detection so difficult.

One other way to hide information in a paletted image is to alter the order of the colors in the palette or use least significant bit encoding on the palette colors rather than on the image data. These methods are potentially weak, however. Many graphics software tools order the palette colors by frequency, luminance, or other parameter, and a randomly ordered palette stands out under statistical analysis (Fridrich and Du 2000).

Newer, more complex steganography methods continue to emerge. Spread-spectrum steganography methods are analogous to spread-spectrum radio transmissions (developed in World War II and commonly used in data communications systems today) where the “energy” of the signal is spread across a wide-frequency spectrum rather than focused on a single frequency, in an effort to make detection and jamming of the signal harder. Spread-spectrum steganography has the same function-avoid detection. These methods take advantage of the fact that little distortions to image and sound files are least detectable in the high-energy portions of the carrier (i.e., high intensity in sound files or bright colors in image files). Even when viewed side by side, it is easier to fool human senses when small changes are made to loud sounds and/or bright colors (Wayner 2002).

DETECTING STEGANOGRAPHY

The Prisoner’s Problem (Simmons 1983) is often used to describe steganography, although it was originally introduced to describe a cryptography scenario.

The problem involves two prisoners, Alice and Bob, who are locked in separate prison cells and wish to communicate some secret plan to each other. Alice and Bob are allowed to exchange messages with each other, but William, the warden, can read all of the messages. Alice and Bob know that William will terminate the communications if he discovers the secret channel (Chandramouli 2002; Fridrich et al. 2003B).[4]

William can act in either a passive or active mode. In the passive warden model, William examines each message and determines whether to forward the message or not based on his ability to detect a hidden message. In the active warden model, William can modify messages if he wishes. A conservative or malicious warden might actually modify all messages in an attempt to disrupt any covert channel so that Alice and Bob would need to use a very robust steganography method (Chandramouli 2002; Fridrich et al. 2003B).

Steganalysis, the detection of steganography by a third party, is a relatively young research discipline with few articles appearing before the late-1990s. The art and science of steganalysis is intended to detect or estimate hidden information based on observing some data transfer and making no assumptions about the steganography algorithm (Chandramouli 2002).

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