Modern Steganography

Abstract

This paper introduces steganography, the art and science of hidden writing. The purpose of steganography is to hide the existence of a secret message from a third party. The most popular modern application of steganography involves concealing messages within least significant bits of image or sound files. Another application of modern steganography is a steganographic file system. Steganography is also used by some modern printers, where tiny yellow dots that contain encoded printer serial numbers, as well as other information, are added to each printed page.

I chose modern steganography as a topic for my course paper primarily because I have had an interest in the topic before, and would like to use this paper as an opportunity to expand my knowledge of steganographic techniques, especially in their modern applications.

Introduction

Steganography is the art and science of hidden writing. The purpose of steganography is to hide the existence of a secret communication from a third party. Usually, a steganographic message will appear to be something else: a picture, an audio file, or some other message (the covertext). The advantage of steganography over cryptography alone is that steganographic message does not attract attention to communicating parties. An apparent encrypted message, no matter how unbreakable it is, can arouse suspicion and may in itself be incriminating. If suspicion is raised, then the goal of steganography is defeated. Since there is always a possibility that secret communication will be uncovered, steganography is often used together with cryptography. A steganographic message (the plaintext) is first encrypted by some modern encryption algorithms, and then a covertext is modified in some way to contain the encrypted message (the ciphertext), resulting in stegotext.

Digital Steganography

A typical digital steganographic encoder is presented on Figure 1. The message is the data that the sender wishes to remain secret. It can be text, image, audio, video, or any other data that can be represented by a stream of bits. The cover media is the carrier medium in which the message is embedded and serves to hide the presence of the message. The message embedding technique is strongly dependent on the structure of the cover media, and in this discussion it is restricted to being digital image. The image with the secretly embedded message produced by the encoder is the stegoimage. The purpose of the stegoimage is to resemble the coverimage under casual inspection and analysis. In addition, the encoder usually employs a stegokey which ensures that only recipients who know the corresponding decoding key will be able to extract the message from a stegoimage.

Figure 1. Steganographic Encoding

Recovering the message from a stegoimage requires the stegoimage itself and a corresponding decoding key if a stegokey was used during the encoding process. The original coverimage may or may not be required. In most applications it is desirable that the cover image is not needed to extract the message. A typical digital steganographic decoder is shown on Figure 2.

Figure 2. Steganographic Decoding

Most modern computer graphics standards use a combination of tree component colors - red, green, and blue to represent a picture element (pixel). The RGB color cube is shown on Figure 3. The absence of all colors yields black, shown as the intersection of the zero point of the three-color axes. The mixture of 100% red, 100% blue, and the complete absence of green forms magenta; cyan is 100% green and 100% blue without any red; 100% green and 100% red with no blue form yellow. White is 100% mixture of all three colors.

Figure 3. The RGB Color Cube

Most digital image encoding standards today support 24-bit true color, where each pixel is encoded in 24 bits, comprising the three RGB bytes as described above.

The most common steganography method in digital 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). 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.

As a simple example of least significant bit substitution, imagine "hiding" the character 'H' across the following eight bytes of a coverimage (the least significant bits are underlined):

10010100 10101100 11001001 10010110

01001011 10001010 10011111 10000100

A 'H' is represented in the American Standard Code for Information Interchange (ASCII) as the binary string 01001000. These eight bits can be "written" to the least significant bit of each of the eight carrier bytes as follows:

10010100 10101101 11001000 10010110

01001011 10001010 10011110 10000100

In the sample above, less than a half of the least significant bits were actually changed (shown above in italic).

Least significant bit substitution is a simple and common technique for steganography. However, its use is not necessarily as simplistic as it may sound. Only the most naive steganographic software would simply overwrite every least significant bit with hidden data. Almost all steganographic tools try to randomize the actual bits in the coverimage that are modified. This is one of the factors that makes steganography hard to detect.

Modern Applications

Steganography can be used for digital watermarking, a technique which allows an entity to add hidden copyright notices or other identification messages to digital images, audio, video, and other electronic documents. In this case a message identifier is hidden in a file being watermarked so that its source can be tracked or verified.

Another application of modern steganography is a steganographic file system. This kind of file system was first proposed by Ross Anderson, Roger Needham, and Adi Shamir in 1999. In a steganographic file system, files are not simply stored, but encrypted and the entire partition is randomized. When files are