Steganography with Digital ImagesByKarl SchrammCS-265Fall 2002IntroductionA Generic Steganographic SystemLSB ModificationJPEG AlgorithmsPatchworkSteganalysisConclusionBibliographySteganography with Digital ImagesByKarl Schramm CS-265 Fall 2002IntroductionThough the fields of steganography and cryptography are associated with one another, there is a distinction to be made. Cryptography is the art of jumbling a message so that a would-be eavesdropper cannot interpret the message. Steganography, on the other hand, is the art of hiding a message so that a would-be eavesdropper is unaware of the message’s presence.While steganography has been around for centuries, the Digital Revolution has sparked a renewed interest in the field. For instance, the mass media industry has shown increasing interest in steganography to fight piracy. It is even rumored that the terrorist organization, Al-Qaeda, has employed steganography to transmit orders to its operatives over the internet. All digital file formats can be used to hide secret messages. This paper, however, focuses specifically on the techniques employed in hiding information in digital image files.A Generic Steganographic SystemAs with any other science, steganography has its own set of terminology. The term cover is used to describe the original message in which we will hide our secret message. Once we embed our secret message into the cover, the new message is known as the stego data. The stego data is analogous the cipher text of cryptography.A generic steganographic system, or stegosystem, works thusly. A secret message is embedded into the cover data using some sort of embedding algorithm. The cover datamay be a single file, but that is not necessarily the case. The embedding algorithm then outputs the stego data. There is, however, a minor detail that needs to be added to the system. Recall Kerckhoff’s principle, which states that the security of a system should not be based on the obscurity of the algorithm, but on the strength of its key. Therefore the embedding algorithm should require a key as an input. Additionally it is advisable to encrypt the secret message prior to embedding it.Though embedding algorithms may take many forms, there are some requirements that all embedding algorithms should meet. Firstly, the distortion of the cover data as a result of the embedding algorithm should be as imperceptible as possible. Secondly, no part of the secret message should be contained in the header of the stego data file. The message must become part of the cover data and should be immune to manipulation attacks such as re-sampling or filtering. Ideally, it would also be a good idea to include error correcting codes into the message so that if the stego data is damaged, the message can still be recovered. Finally, it is imperative that the original cover data never fall into the hands of an eavesdropper or be used twice. Since the embedding process is additive, the secret message can be recovered if an eavesdropper has different stego files which utilize the same cover data.We will now explore some of the more popular techniques for embedding messages into cover.LSB ModificationLSB modification is perhaps the most popular method to embed a message into cover data. As its name suggests, this method works by modifying the least significant bit of one of the RGB values of the pixel data. The secret message data is then scattered pseudo-randomly across the image. This technique is analogous to the spread spectrum communication technique of frequency hopping.Figure 1 An overview of a generic steganographic system.Embedding AlgorithmKeyStego DataCover DataSecret MessageThis method is quite effective against human detection because it is difficult for the human eye to discern an LSB modified pixel. Also, any modifications that are made could easily be attributed to “noise” that may already be contained in the image. However, computer generated images, such as those generated by vector drawing applications like Adobe Illustrator or Macromedia Flash, do not contain much noise and would therefore make a poor choice as cover data. While 24-bit true-color RGB data formats are best suited for LSB modification, itis possible to use this method with 8-bit color-index data formats. This can be tricky, however, because the palette is much smaller and pixel luminescence variation may be much greater and more easily detected. Therefore, it is wise to attempt LSB modificationwith a grayscale or monochromatic cover image. There are, however, problems with LSB modification. For one, this method will only work with raw image data. Lossy image compression formats, such as JPEG, do notstore images in an RGB format and are therefore not as forgiving to simple bit manipulation. Another problem with LSB manipulation is that if the stego data is compressed with a lossy compression algorithm, the secret message may be destroyed.JPEG AlgorithmsWhile JPEG files are not as tolerant to bit manipulation as uncompressed image files, it is still possible to use them as cover data. The key is to know where to hide the information. The JPEG algorithm works by dividing an image into several 8 x 8 pixel matrices.Discrete cosine transform (DCT) coefficients are then calculated for each matrix. The coefficients are then multiplied by a quantization matrix. The results are then rounded offFigure 2 A comparison of an LSB altered color tone.Blue 220 Blue 221to the nearest integer. The rounded numbers are then further compressed and the results are saved. It is in these DCT values where we can hide our data. A typical approach involvesslightly altering a set of the largest DCT coefficients. These larger values contain the most “energy” and would therefore produce the least amount of distortion in the image. Another approach is to choose DCT coefficients that fall into a particular range so as to avoid perception.The popular JPEG steganography algorithms, F5 and JSteg, both use DCT modification to embed data. And while both algorithms generally escape human detection, they are both detectable through statistical analysis.PatchworkPatchwork, an early steganographic algorithm, works by hiding a single bit in pairs of pixels that are in different parts of the image. Here is a sketch of the Patchwork algorithm:- Create a pseudo-random bit stream to choose pairs of pixels from the cover data.- For each pair, let d be the difference
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