08-17-2017, 12:23 AM
LEAST SIGNIFICANT BIT EMBEDDINGS: IMPLEMENTATION AND DETECTION
INTRODUCTION
Steganography is a method of hiding digital information so that it will escape detection that has been used by al-Qaeda, drug cartels, and others. Properly-executed steganography allows for large quantities of information to be hidden inside a file, while making no perceivable changes to that file's contents. Steganography can be applied to many types of data, including audio, video, and images and can hide any kind of digital information. Steganography provides a significant challenge to security as it hides the act of communication; if illicit communication is not discovered it cannot be prevented or decoded. Unlike encryption, which secures the content of a message, steganography hides the message s existence. The term steganography derives from the Greek for covered writing [9], which is a good representation of the central idea of steganography. The end goal of steganography is to hide data in a digital object so that it cannot be detected through observation (or even complex analysis). Least Significant Bit Embeddings (LSB) are a general steganographic technique that may be employed to embed data into a variety of digital media, but one of the most studied applications is using LSB embedding to hide one image inside another.
RELATED WORK
A significant amount of work in steganography has been published since the late 1990s. Vision of the Unseen provides a survey of the current work in steganography and other topics in digital forensics [6]. Steganography has been evolving over the last decade, as designers of embedding algorithms have developed more complex embedding strategies and others try to detect and decode them [8]. In LSB embedding with images, work has been done to naively detect embedding in images with a variety of embedding strategies [1, 2, 5, 9]. Steganographic tools for embedding and decoding have been made freely available online as published and proprietary techniques for both embedding are implemented [4, 7].
THE COVER
Steganography requires two pieces of data: the cover, and the data to be hidden. The cover is the medium into which we will embed the data. Choosing an appropriate cover is an important decision, as it is a large part of what determines the effectiveness of the steganographic technique. Steganography s reliance on hiding the data behind the cover prevents it from being considered secure, unlike encryption. Once embedding is suspected it may be trivial to retrieve the embedded data. The cover will also be the container for the given message. If the cover itself raises suspicion, it could result in detection. A Brazilian drug trafficker had messages hidden with steganographic algorithms hidden on his computer inside images of a cartoon character [6]. Perhaps if the cover image were more innocuous the messages would have escaped detection. While any image can serve as a cover, the images which make the best covers have several properties that make it possible for more of the image's data to be replaced without creating any visually detectable distortion. The most important characteristic of a potential cover is that the image should have a large variety of colors. Images with few colors will make the embedding easier to detect. If, for example, the image is a single color there will be two colors after embedding. These colors will be very similar, but not the same. In Fig. 1, there are two grays that would be produced by a least significant bit embedding. The left side has the gray value 125, and the right 124.
STEGANOGRAPHY
Steganography is a technique for transmitting information without detection. It has been used in drug cartels and other organizations to avoid detection of their illicit communication [6]. Steganography relies on the fact that it is difficult to detect in order to remain secure. It uses parts of an image that do not strongly influence the colors shown to embed data. Where embedding is most practical varies with different image formats, but one technique that works well across formats is least significant bit embedding. Other algorithms, such as Jsteg, exploit the design of a specific image file format to embed without detection. The general principle of steganography is that perturbing a particular value in an image using a value from the data will create a small difference in the original image. The image created by this process is a stego object. The stego object contains data from the cover and information about the data that was used to perturb the cover image. The stego object can then be decoded by the intended recipient(s) and the hidden message retrieved. Because the values in the original image are only changed slightly, an observer will struggle to visually detect that an embedding has taken place. Through this series of minor perturbations based on the message s contents the data is hidden in the cover image. A third party will have to analyze the image in order to determine if an embedding has taken place. The development of different analyses has led to an arms race between those developing steganographic algorithms and those trying to detect embeddings. From this point forward, it is assumed that the cover and data to hide are both images.