Steganography: a Brief Overview
Steganography refers to the practice of embedding or hiding information into an inconspicuous medium in such a way that it is difficult or impossible to be seen without knowing the correct extraction technique . Steganography comes from the Greek \(\sigma\tau\epsilon\gamma\alpha\nu\omega\), and literally means "covered writing" . Unlike cryptography where the aim is to make a message unreadable, the aim in steganography is to make a message undetectable . Steganography is therefore often more useful than cryptography, or especially useful if used in conjunction, as it allows for discrete messages to be sent in plain sight: whereas a cryptographically encoded message may attract attention. Although steganography is often associated with embedding messages or hidden images within a cover image, there is no requirement for a visual medium in steganography, merely a hidden message.
An ancient form of steganography existed in Greece, where one would engrave a message on a timber tablet, then cover the surface with wax. The resulting tablet would appear blank, but if the wax were scraped off the message would be revealed . Other popular methods through history were various schemes involving invisible ink . The field has come a long way since then, especially since the development of the digital age.
In modern steganography, there are three separate types, each with a separate purpose: information hiding, watermarking and fingerprinting . Information hiding is the classical application for steganography described above, where a hidden message is to be passed to a recipient . Watermarking it commonly used by publishers and broadcasters to encrypt copyright marks into digital media, including images, film, recordings and books . Finally, in fingerprinting a specific identity is associated with a copy of a given media. Fingerprinting steganography can be used when distributing sensitive information, such as intellectual property, to later determine which copy was leaked, and therefore identify the sharer. From herein, the information hiding type of steganography is considered.
There are three different criteria by which the performance of steganography can be measured : security, also known as robustness , describes the ability for the message to survive a deliberate attempt to corrupt any hidden information within the message; capacity describes the ratio of the amount of information that can be contained against the required size of the hiding medium; and imperceptibility, also known as undetectability , is the property that the encoded message appears to be a regular message. Often a trade-off is required, whereby increasing the performance in one of these three criteria reduces the performance in the other two .
The most common method for steganographically embedding a message is to do so within an image . One such method is called least-significant bit (LSB) based steganography. In this method, data is embedded in the LSB of given pixels. Doing so has a minimal effect on the image, and allows a message to be embedded. This method allows for a relatively high capacity, and can give a good level of imperceptibility for appropriate cover images, but has a low security, and is susceptible to attacks.
An alternative method for steganographically embedding a message within an image is the transform domain technique . In this method, the data is stored in an alternative domain, which may be the Laplace domain, the discrete cosine transform domain, the wavelet transform domain, or others . As opposed to LSB techniques, transform domain techniques are far more secure and robust, and able to survive some lossy compression or other distortions, although at the expense of having a far lower capacity .
An alternate form of steganography, known as linguistic steganography, seeks to conceal a message as innocuous text . Although linguistic steganography is one of the oldest forms of steganography , it is still a studied field in the modern, digital age . Linguistic steganography therefore includes both old methods, such as null ciphers, and modern techniques for large-scale automated linguistic steganography .
Natural Language Watermarking and Tamperproofing
In Natural language watermarking and tamperproofing, Atallah et al.  propose a method for steganographically embedding watermarks or fingerprints into plain text. Modern natural language steganography methods attempt to apply steganographic methods to text, without relying on modifying specific formatting parameters, such as LaTeX or HTML. In this work, authors continue from previous work , and use a similar base concept. The concept depends on fundamental redundancies in language structure and semantics. These two separate areas of redundancy are exploited within this text to embed the hidden message.
Firstly, a sentence can be restructured, and maintain the exact same meaning . For example, "Ashley submitted a perfect assignment" and "A perfect assignment was submitted by Ashley." In these sentenced, the order of the subject and object have been reversed, however the meaning has not been changed. Thus if the meaning of a sentence can be extracted, and can be reconstructed in a number of different, but mathematically defined ways, the specific reconstruction method can be used to encode extra information. This is known at the syntactic marking scheme.
Secondly, the semantics of parts of a sentence, or sentences within a paragraph can be changed, but result in the same overall meaning . For example, information can be grafted from one sentence to another. Grafting "The perfect assignment received an HD grade" into "Ashley submitted a perfect assignment" might form "Ashley submitted a perfect assignment, which received an HD grade." Another option is to prune redundant data, that might be shared and repeated between nearby sentences; or to substitute data with equivalent data.
Unlike other linguistic steganography methods, these methods allow a hidden message, such as a fingerprint or watermark, to be embedded within a text, without modifying the overall meaning of the text. In addition, the use of the two methods in conjunction allows the document to become self-error-checking, thereby any tampering becomes evident.
The algorithm would first step through and modify the semantic meaning using grafting, pruning and substitution, encoding the hidden message within. The message was encoded using the sentence text-meaning representation (TMR) tree . TMR provides a method by which a sentence can be encoded and represented by its pure meaning, as opposed to its syntactic structure. A typical sentence can encode up to a byte in the TMR, however in this text only four bits are encoded per sentence, allowing for the modified sentences to be marked.
After this first pass, the syntactic pass was also made in order to allow for tampering detection. The syntax encoding method used was the syntax tree. By modifying the syntax of a sentence without modifying the TMR, both the sentence meaning and the watermark encoding are maintained. The syntax tree was modified in such a way to encode the a signature of the document itself. In this way, tampering becomes immediately evident based on a syntactic change in the document.
An example stego-image is seen in figure 2. This was produced using the manytools.org steganography tool . The raw LaTeX code used do generate §Natural Language Watermarking and Tamperproofing was entered into the Hide message field, and the image in figure 1 was uploaded to the Host-image area.
Figure 1: Original image
Figure 2: Stego-image
Figure 3: Original image shoulder zoom
Figure 4: Stego-image shoulder zoom
A number of distortions to the host image are noted in the stego-image in figure 2. Most obviously is that the transparent background has been destroyed and is in fact black in the stego-image. The shoulder close ups in figure 4 also reveal more subtle distortions, directly due to the data encoding. The data revealed upon decoding by uploading the image to the same tool is given below.
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