Steganography - The art of hiding information
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03-09-2009, 04:35 PM


Steganography ,from the Greek, means covered or secret writing, and is a long-practiced form of hiding information. Although related to cryptography, they are not the same. Steganography's intent is to hide the existence of the message, while cryptography scrambles a message so that it cannot be understood.

Steganography includes a vast array of techniques for hiding messages in a variety of media. Among these methods are invisible inks, microdots, digital signatures, covert channels and spread-spectrum communications. Today, thanks to modern technology, steganography is used on text, images, sound, signals, and more.


The advantage of steganography is that it can be used to secretly transmit messages without the fact of the transmission being discovered. Often, using encryption might identify the sender or receiver as somebody with something to hide. For example, that picture of your cat could conceal the plans for your company's latest technical innovation.

In fact, it is common practice to encrypt the hidden message before placing it in the cover message. However, it should be noted that the hidden message does not need to be encrypted to qualify as steganography. The message itself can be in plain English and still be a hidden message. However, most steganographers like the extra layer of protection that encryption provides. If your hidden message is found, and then at least make it as protected as possible
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19-03-2010, 10:36 AM

please read topicideashow-to-steganography-full-report--8121 for more about Steganography seminar and presentation informations
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07-05-2011, 11:44 AM


.doc   SEMINAR_ REPORT.DOC (Size: 125 KB / Downloads: 93)
Introduction
Steganography, from the Greek, means covered or secret writing, and is a long-practiced form of hiding information. Although related to cryptography, they are not the same. Steganography's intent is to hide the existence of the message, while cryptography scrambles a message so that it cannot be understood.
More precisely,
“the goal of steganography is to 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.''
Steganography includes a vast array of techniques for hiding messages in a variety of media. Among these methods are invisible inks, microdots, digital signatures, covert channels and spread-spectrum communications. Today, thanks to modern technology, steganography is used on text, images, sound, signals, and more.
The advantage of steganography is that it can be used to secretly transmit messages without the fact of the transmission being discovered. Often, using encryption might identify the sender or receiver as somebody with something to hide. For example, that picture of your cat could conceal the plans for your company's latest technical innovation.
However, steganography has a number of disadvantages as well. Unlike encryption, it generally requires a lot of overhead to hide a relatively few bits of information. However, there are ways around this. Also, once a steganographic system is discovered, it is rendered useless. This problem, too, can be overcome if the hidden data depends on some sort of key for its insertion and extraction.
In fact, it is common practice to encrypt the hidden message before placing it in the cover message. However, it should be noted that the hidden message does not need to be encrypted to qualify as steganography. The message itself can be in plain English and still be a hidden message. However, most steganographers like the extra layer of protection that encryption provides. If your hidden message is found, and then at least make it as protected as possible.
This seminar and presentation aims to outline a general introduction to steganography - what it is, and where it comes from. Methods for hiding data in three varied media (text, image, and audio) will be described, and some guidelines for users of steganography will be provided where necessary. In addition, we will take a brief look at steganalysis, the science of detecting steganography, and destroying it.
Introduction to Terms used
In the field of steganography, some terminology has developed.
The adjectives cover, embedded and stego were defined at the Information Hiding Workshop held in Cambridge, England. The term ``cover'' is used to describe the original, innocent message, data, audio, still, video and so on. When referring to audio signal steganography, the cover signal is sometimes called the ``host'' signal.
The information to be hidden in the cover data is known as the ``embedded'' data. The ``stego'' data is the data containing both the cover signal and the ``embedded'' information. Logically, the processing of putting the hidden or embedded data, into the cover data, is sometimes known as embedding. Occasionally, especially when referring to image steganography, the cover image is known as the container.
Steganography under Various Media
In the following three sections we will try to show how steganography can and is being used through the media of text, images, and audio.
Often, although it is not necessary, the hidden messages will be encrypted. This meets a requirement posed by the ``Kerckhoff principle'' in cryptography. This principle states that the security of the system has to be based on the assumption that the enemy has full knowledge of the design and implementation details of the steganographic system. The only missing information for the enemy is a short, easily exchangeable random number sequence, the secret key. Without this secret key, the enemy should not have the chance to even suspect that on an observed communication channel, hidden communication is taking place. Most of the software that we will discuss later meets this principle.
When embedding data, it is important to remember the following restrictions and features:
• The cover data should not be significantly degraded by the embedded data, and the embedded data should be as imperceptible as possible. (This does not mean the embedded data needs to be invisible; it is possible for the data to be hidden while it remains in plain sight.)
• The embedded data should be directly encoded into the media, rather than into a header or wrapper, to maintain data consistency across formats.
• The embedded data should be as immune as possible to modifications from intelligent attacks or anticipated manipulations such as filtering and resampling.
• Some distortion or degradation of the embedded data can be expected when the cover data is modified. To minimize this, error correcting codes should be used.
• The embedded data should be self-clocking or arbitrarily re-entrant. This ensures that the embedded data can still be extracted when only portions of the cover data are available. For example, if only a part of image is available, the embedded data should still be recoverable.
Steganography in Text
The illegal distribution of documents through modern electronic means, such as electronic mail, means such as this allow infringers to make identical copies of documents without paying royalties or revenues to the original author. To counteract this possible wide-scale piracy, a method of marking printable documents with a unique codeword that is indiscernible to readers, but can be used to identify the intended recipient of a document just by examination of a recovered document.
The techniques they propose are intended to be used in conjunction with standard security measures. For example, documents should still be encrypted prior to transmission across a network. Primarily, their techniques are intended for use after a document has been decrypted, once it is readable to all.
An added advantage of their system is that it is not prone to distortion by methods such as photocopying, and can thus be used to trace paper copies back to their source.
An additional application of text steganography suggested by Bender, et al. is annotation, that is, checking that a document has not been tampered with. Hidden data in text could even by used by mail servers to check whether documents should be posted or not.
The marking techniques described are to be applied to either an image representation of a document or to a document format file, such as PostScript or Textiles. The idea is that a codeword (such as a binary number, for example) is embedded in the document by altering particular textual features. By applying each bit of the codeword to a particular document feature, we can encode the codeword. It is the type of feature that identifies a particular encoding method. Three features are described in the following subsections:
Line-Shift Coding
In this method, text lines are vertically shifted to encode the document uniquely. Encoding and decoding can generally be applied either to the format file of a document, or the bitmap of a page image.
By moving every second line of document either 1/300 of an inch up or down, it was found that line-shift coding worked particularly well, and documents could still be completely decoded, even after the tenth photocopy.
However, this method is probably the most visible text coding technique to the reader. Also, line-shift encoding can be defeated by manual or automatic measurement of the number of pixels between text baselines. Random or uniform respacing of the lines can damage any attempts to decode the codeword.
However, if a document is marked with line-shift coding, it is particularly difficult to remove the encoding if the document is in paper format. Each page will need to be rescanned, altered, and reprinted. This is complicated even further if the printed document is a photocopy, as it will then suffer from effects such as blurring, and salt-and-pepper noise.
Word-Shift Coding
In word-shift coding, codewords are coded into a document by shifting the horizontal locations of words within text lines, while maintaining a natural spacing appearance. This encoding can also be applied to either the format file or the page image bitmap. The method, of course, is only applicable to documents with variable spacing between adjacent words, such as in documents that have been text-justified. As a result of this variable spacing, it is necessary to have the original image, or to at least know the spacing between words in the unencoded document.
The following is a simple example of how word-shifting might work. For each text-line, the largest and smallest spaces between words are found. To code a line, the largest spacing is reduced by a certain amount, and the smallest is extended by the same amount. This maintains the line length, and produces little visible change to the text.
Word-shift coding should be less visible to the reader than line-shift coding, since the spacing between adjacent words on a line is often shifted to support text justification.
However, word-shifting can also be detected and defeated, in either of two ways.
If one knows the algorithm used by the formatter for text justification, actual spaces between words could then be measured and compared to the formatter's expected spacing. The differences in spacing would reveal encoded data.
A second method is to take two or more distinctly encoded, uncorrupted documents and perform page by page pixel-wise difference operations on the page images. One could then quickly pick up word shifts and the size of the word displacement.
By respacing the shifted words back to the original spacing produced under the formatter, or merely applying random horizontal shifts to all words in the document not found at column edges, an attacker could eliminate the encoding. However, it is felt that these methods would be time-consuming and painstaking.
Feature Coding
A third method of coding data into text is known as feature coding. This is applied either to the bitmap image of a document, or to a format file. In feature coding, certain text features are altered, or not altered, depending on the codeword. For example, one could encode bits into text by extending or shortening the upward, vertical endlines of letters such as b, d, h, etc. Generally, before encoding, feature randomization takes place. That is, character endline lengths would be randomly lengthened or shortened, then altered again to encode the specific data. This removes the possibility of visual decoding, as the original endline lengths would not be known. Of course, to decode, one requires the original image, or at least a specification of the change in pixels at a feature.
Due to the frequently high number of features in documents that can be altered, feature coding supports a high amount of data encoding. Also, feature encoding is largely indiscernible to the reader. Finally, feature encoding can be applied directly to image files, which leaves out the need for a format file.
When trying to attack a feature-coded document, it is interesting that a purely random adjustment of endline lengths is not a particularly strong attack on this coding method. Feature coding can be defeated by adjusting each endline length to a fixed value. This can be done manually, but would be painstaking. Although this process can be automated, it can be made more challenging by varying the particular feature to be encoded. To even further complicate the issue, word shifting might be used in conjunction with feature coding, for example. Efforts such as this can place enough impediments in the attacker's way to make his job difficult and time-consuming.
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20-07-2012, 10:06 AM


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