08-03-2012, 02:51 PM
steganography
[attachment=18112]
Introduction
1.1 What is Steganography?
Steganography comes from the Greek and literally means, "Covered or secret writing".
Although related to cryptography, they are not the same. Steganography’s intent is to hide theexistence of the message, while cryptography scrambles a message so that it cannot be understood.
Steganography is one of various data hiding techniques, which aims at transmitting amessage on a channel where some other kind of information is already being transmitted.This distinguishes Steganography from covert channel techniques, which instead of tryingto transmit data between two entities that were unconnected before.
The goal of Steganography is to hide messages inside other “harmless” messages ina way that does not allow any “enemy” to even detect that there is a second secret messagepresent. The only missing information for the “enemy” is the short easily exchangeablerandom number sequence, the secret key, without the secret key, the “enemy” should nothave the slightest chance of even becoming suspicious that on an observed communicationchannel, hidden communication might take place.
1.2 Introduction to Terms used:
In the field of steganography, some terminology has developed.
The adjectivesCover,Embedded Andstego were defined at the Information Hiding
Workshop held in Cambridge, England. The term ``cover'' is used to describe the origina
innocent message, data, audio, still, video and so on. When referring to audio signal
Steganography, the cover signal is sometimes called the ``host'' signal.
Steganography in Text
The illegal distribution of documents through modern electronic means, such aselectronic mail, means such as this allow infringers to make identical copies of documentswithout 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 isIndiscernible to readers, but can be used to identify the intended recipient of a document just byExamination of a recovered document
The techniques they propose are intended to be used in conjunction with standardsecurity measures. For example, documents should still be encrypted prior to transmission acrossa 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 annotationthat is, checking that a document has not been tampered with. Hidden data in text could even byused by mail servers to check whether documents should be posted or not.
2.1 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 thebitmap of a page image.
By moving every second line of document either 1/300 of an inch up or down, it wasfound that line-shift coding worked particularly well, and documents could still be completelydecoded, 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 ofpixels between text baselines. Random or uniform respacing of the lines can damage anyattempts to decode the codeword.
However, if a document is marked with line-shift coding, it is particularly difficult toremove 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, asit will then suffer from effects such as blurring, and salt-and-pepper noise.
2.2 Word-Shift Coding:
In word-shift coding, codewords are coded into a document by shifting the horizontallocations of words within text lines, while maintaining a natural spacing appearance. Thisencoding can also be applied to either the format file or the page image bitmap. The method, ofcourse, is only applicable to documents with variable spacing between adjacent words, such as indocuments that have been text-justified.
[attachment=18112]
Introduction
1.1 What is Steganography?
Steganography comes from the Greek and literally means, "Covered or secret writing".
Although related to cryptography, they are not the same. Steganography’s intent is to hide theexistence of the message, while cryptography scrambles a message so that it cannot be understood.
Steganography is one of various data hiding techniques, which aims at transmitting amessage on a channel where some other kind of information is already being transmitted.This distinguishes Steganography from covert channel techniques, which instead of tryingto transmit data between two entities that were unconnected before.
The goal of Steganography is to hide messages inside other “harmless” messages ina way that does not allow any “enemy” to even detect that there is a second secret messagepresent. The only missing information for the “enemy” is the short easily exchangeablerandom number sequence, the secret key, without the secret key, the “enemy” should nothave the slightest chance of even becoming suspicious that on an observed communicationchannel, hidden communication might take place.
1.2 Introduction to Terms used:
In the field of steganography, some terminology has developed.
The adjectivesCover,Embedded Andstego were defined at the Information Hiding
Workshop held in Cambridge, England. The term ``cover'' is used to describe the origina
innocent message, data, audio, still, video and so on. When referring to audio signal
Steganography, the cover signal is sometimes called the ``host'' signal.
Steganography in Text
The illegal distribution of documents through modern electronic means, such aselectronic mail, means such as this allow infringers to make identical copies of documentswithout 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 isIndiscernible to readers, but can be used to identify the intended recipient of a document just byExamination of a recovered document
The techniques they propose are intended to be used in conjunction with standardsecurity measures. For example, documents should still be encrypted prior to transmission acrossa 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 annotationthat is, checking that a document has not been tampered with. Hidden data in text could even byused by mail servers to check whether documents should be posted or not.
2.1 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 thebitmap of a page image.
By moving every second line of document either 1/300 of an inch up or down, it wasfound that line-shift coding worked particularly well, and documents could still be completelydecoded, 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 ofpixels between text baselines. Random or uniform respacing of the lines can damage anyattempts to decode the codeword.
However, if a document is marked with line-shift coding, it is particularly difficult toremove 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, asit will then suffer from effects such as blurring, and salt-and-pepper noise.
2.2 Word-Shift Coding:
In word-shift coding, codewords are coded into a document by shifting the horizontallocations of words within text lines, while maintaining a natural spacing appearance. Thisencoding can also be applied to either the format file or the page image bitmap. The method, ofcourse, is only applicable to documents with variable spacing between adjacent words, such as indocuments that have been text-justified.