What is Steganography ?
Steganography is the general practice of “hiding” data within a cover medium, sometimes called a “host file.” This file can be any kind of file, really, although usually it’s a graphics file. While there are different methods of steganography, what they all share is an abstract view of a stegosystem.
Stegosystem, Stegokey & Stegomedium
A stegosystem is composed of a message embedded in the cover medium along with a stegokey. Taken together, these three items create a stegomedium. The recipient of this medium will use the stegokey and the stegomedium to derive the (embedded) message.
Below are two non-malicious examples of steganography.
Medical records can have a patient ID inserted within them to make sure that the information applies to a particular patient, even if other identifiers were lost or modified.
Steganography can be used in the workplace to keep communications private that concern intellectual property: For example, details of a secret project could be communicated among project members without leaving a “paper” trail.
Steganography is useful in any circumstance when we wish to incorporate extra information in a particular digital object such that a casual observer wouldn’t know that information is being exchanged.
There are three major methods of steganography: technical, linguistic, and digital.
Technical steganography
It uses an outside medium, such as chemicals or liquids, to reveal the hidden message. One example of this kind of steganography is the time-honored technique of writing a message using lemon juice and milk, and then exposing the paper to low heat to reveal the message.
Linguistic steganography
It consists of two types: semagrams and open codes. Semagrams either can be visual, such as arranging objects to communicate the message, or can use variations in font, spelling, and punctuation to embed the message in a plain text file.
Open codes modify plain text files to send the message. One technique uses argot codes, where information is communicated via a language that only the participants can understand. Another is covered ciphers, where the message is embedded within plain text, but only the receiver knows how to extract the message. The null cipher embeds the text of the message in the host file according to some pattern, such as the first word of each sentence, the first letter of every third word, vertically, horizontally, and so forth. Grille ciphers require a special form or template to be placed over the text that reveals the letters or characters of the message. No grille, no message.
Digital steganography
It hides messages in a digital medium, which is to say graphics files, audio, video, and so forth. Following techniques are used in digital steganography.
- Injection
- Least significant bit (LSB)
- Transform-domain techniques
- Spread-spectrum encoding
- Perceptual masking
- File generation
- Statistical methods
- Distortion technique
We can categorize these techniques as belonging to one of two strategies: insertion, where we include that message with the host file, and substitution, where we replace bits of the host file with bits from the message.
Steganography Techniques in Graphics Files
Messages can be hidden in graphics files by using LSB insertion, masking and filtering, and algorithmic transformation. The LSB technique is used for other media as well as graphics. In this technique, the LSB of, say, a 24-bit quantity would be replaced with bits from the secret message. In the case of graphic files that utilize 24- or 32-bit color, the resulting changes would not be discernible to the viewer. In contrast to techniques like open space steganography, LSB encoding, algorithmic techniques, masking, and filtering all represent a substitution approach.
Steganography Techniques in Audio Files
A suspect can hide messages in audio files using variations on the LSB technique or by manipulating various frequencies. Techniques include phase coding, spread spectrum, and echo data hiding. Phase coding separates the sound into discrete segments and then introduces a reference phase that contains the information. Spread spectrum techniques encode the information across the entire frequency spectrum using direct-sequence spread spectrum (DSSS). Echo data hiding means introducing an echo into the original medium, and properties of the echo are used to represent information.
Steganography Techniques in Video Files
Messages can be hidden in video files using any technique that applies to audio or graphic files (a video is an audio stream overlaid on a series of images). One such technique is modifying the coefficients used in the discrete cosine transform (DCT) computation, a technique used to compress JPEG files, so as to hide parts of the messages in areas of the file that will survive expansion and recompression.
