Nov 28, 2014
CHAPTER 1 INTRODUCTIONWith advancements in digital communication technology and the growth of computer power and storage, the difficulties in ensuring individuals privacy become increasingly challenging. The degrees to which individuals appreciate privacy differ from one person to another. Various methods have been investigated and developed to protect personal privacy. Encryption is probably the most obvious one, and then comes steganography.
Steganography is an old art which has been in practice since time unknown. Steganography, from the Greek, means covered or secret writing and is thus the art of hiding messages inside innocuous cover carriers, e.g. images, audio, video, text, or any other digitally represented code or transmission, in such a manner that the existence of the embedded messages is undetectable. The hidden message may be plaintext, ciphertext, or anything that can be represented as a bit stream. Encryption lends itself to noise and is generally observed while steganography is not observable.
Steganography and cryptography, though closely related, they are not the same. The former has the intent to hide the existence of the message whereas the later scrambles a message to absolute illegibility.
The goal of steganography is to avoid drawing suspicion to the transmission of a hidden message. It hide messages inside other harmless messages in a way that does not allow any enemy to even detect that there is a second secret message present. If suspicion is raised, then this goal is defeated. Discovering and rendering useless such covert messages is another art form known as steganalysis.
This approach of information hiding technique has recently become important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but
imperceptible marks, which may contain a hiding copyright notice or serial number or even help to prevent unauthorized copying directly.
Military communications system make increasing use of traffic security technique which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections.
1.1 SteganographySteganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity.
Figure: The different embodiment disciplines of Information Hiding. The arrow indicates an extension and bold face indicates the focus of this study. Intuitively, this work makes use of some nomenclature commonly used by steganography and watermarking communities. The term cover image is used throughout this thesis to describe the image designated to carry the embedded bits. An image with embedded data, payload, is described as stego-image while steganalysis or attacks refer to different image processing and statistical analysis approaches that aim to break steganography algorithms. People use to confuse steganography with cryptography, which is wrong.
Steganography and cryptography, though closely related, they are altogether different. The former hides the existence of the message, while the latter scrambles a message so that it cannot be understood (Sellars, 1999). But the two techniques must not be perceived as mutually exclusive and if used together can prove more powerful. As we have said of steganography, the embedded data is not necessarily encrypted; hidden message may be plaintext, ciphertext, or anything that can be represented as a bit stream. Embedding encrypted message could be more secure and effective.
Figure 1: General scheme of steganography
1.2 Steganography vs. CryptographyBasically, the purpose of cryptography and steganography is to provide secret communication. However, steganography is not the same as cryptography. Cryptography hides the contents of a secret message from a malicious people, whereas steganography even conceals the existence of the message. Steganography must not be confused with cryptography, where we transform the message so as to make it meaning obscure to a malicious people who intercept it. Therefore, the definition of breaking the system is different . In cryptography, the system is broken when the attacker can read the secret message. Breaking a steganographic system need the attacker to detect that steganography has been used and he is able to read the embedded message. In cryptography, the structure of a message is scrambled to make it meaningless and unintelligible unless the decryption key is available. It makes no attempt to disguise or hide the encoded message. Basically, cryptography offers the ability of transmitting information between persons in a way that prevents a third party from reading it. Cryptography can also provide authentication for verifying the identity of someone or something.
In contrast, steganography does not alter the structure of the secret message, but hides it inside a cover-image so it cannot be seen. A message in ciphertext, for instance, might arouse suspicion on the part of the recipient while an invisible message created with steganographic methods will not. In other word, steganography prevents an unintended recipient from suspecting that the data exists. In addition, the security of classical steganography system relies on secrecy of the data encoding system. Once the encoding system is known, the steganography system is defeated. It is possible to combine the techniques by encrypting message using cryptography and then hiding the encrypted message using steganography. The resulting stego-image can be transmitted without revealing that secret information is being exchanged. Furthermore, even if an attacker were to defeat the steganographic technique and detect the message from the stego-object, he would still require the cryptographic decoding key to decipher the encrypted message. Table below shows a comparision between the three techniques. Criterion/ MethodCarrier Steganography Watermarking Cryptography
any digital media
mostly image/audio files
usually text based, with some extensions to image files plain text changes the structure necessary blind
no changes to the structure Key Detection blind optional usually
informative, i.e., original cover or watermark is needed for recovery Authentication full retrieval of data usually achieved by cross correlation Copyright preserving watermarkedfile robustness image processing sometimes It is removed/ replaced usually becomes an attribute of the cover image. The cover is more important than the message. cover choice is restricted modern era full retrieval of data
Objective Result Concern Type of attacks Visibility Fails when Relation to cover
secrete communication stego-file delectability/ capacity steganalysis never it is detected not necessarily related to the cover. The message is more important than the cover.
data protection cipher-text robustness cryptanalysis Always de-ciphered N/A
free to choose any suitable cover very ancient except its digital version
N/A modern era
Figure 2: Different steganography fields Our work is Data Hiding (protection against detection). We have used the cover object as digital image and stego object(secret data) as the text file.
CHAPTER 2 DIGITAL IMAGE STEGANOGRAPHYSteganography can also be classified a on the basis of carrier media. The most commonly used media are text, image, audio and video. So here Digital Images are used as the carrier media.
DIGITAL IMAGESA digital image is defined for the purposes of this document as
a raster based, 2-dimensional, rectangular array of static data elements called pixels, intended for display on a computer monitor or for transformation into another format, such as a printed page. To a computer, an image is an array of numbers that represent light intensities at various points, or pixels. These pixels make up the image's raster data. Digital images are typically stored in 32-, 24- or 8-bit per pixel files. In 8-bit color images, (such as GIF files), each pixel is represented as a single byte. A typical 32 bit picture of width=n pixels and height = m pixels can be represented by an m x n matrix of pixels.
Figure 3: Matrix and bits representation of an image file. The three 8 bit parts - red-R, blue-B and green-G - constitute 24 bits which means that a pixel should have 24 bits. 32 bit refers to the image having an "alpha channel". An alpha channel is like an extra color, although instead of displaying it as a color, it is rendered translucently (see-through) with the background.
IMAGE FORMATSThere are several image formats in use nowadays. Since raw image files are quite large, some suitable compression technique is applied to reduce the size. Based on the kind of compression employed a given image format can be classified as lossy or lossless. Lossy compression is used mostly with JPEG files and may not maintain the original image's integrity despite providing high
compression. Obviously it would infect any data embedded in the image. Lossless compression does maintain the original image data exactly but does not offer such high compression rates as lossy compression. PNG, BMP, TIFF and GIF etc are example lossless formats. Some commonly used formats are JPEG, BMP, TIFF, GIF and PNG; the last two types of images are also called palette images. We discuss here all these formats briefly: 1. TIFFTagged Im age File Format (TIFF),