Ch02-Computer Security
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This document discusses the topic of steganography, which is hiding secret messages within other harmless messages. It outlines different techniques for hiding messages in text, images, and audio files. For text, it describes line shift coding, word shift coding, and feature coding methods. For images, it explains least significant bit insertion and exploiting the limitations of the human visual system. For audio, it mentions low-bit encoding and other techniques like phase coding and spread spectrum. It also discusses steganalysis, which aims to detect and destroy hidden messages within files.
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Ch02-Computer Security
- 1. 1 241-427-SV-2-2553-COE-PSU 1 241-427 Computer Security Chapter II: Steganography Dr. Sangsuree Vasupongayya 241-427-SV-2-2553-COE-PSU 2 Outline History What is steganography Steganography Text Image Audio Steganalysis 241-427-SV-2-2553-COE-PSU 3 Steganography The art and science of communicating in a way which hides the existence of the communication Goal to hide messages inside other harmless messages in away that does not allow any enemy to even detect that there is a second secret message present E.g., Watermarking Fingerprinting
- 2. 2 241-427-SV-2-2553-COE-PSU 4 Watermarking and Fingerprinting Watermarking: hiding a trademark in a digital object Fingerprinting: hiding characteristics such as a serial number within a digital object to distinguish it from a similar digital object Used to fight piracy Watermarking is for detection Fingerprinting is for prosecution 241-427-SV-2-2553-COE-PSU 5 Steganography in Text Modern application is for detecting illegal document distribution Also detects document tampering Words with document images or document format files (postscript, TEX, etc.) Alters specific features Lines spacing coding Word spacing coding Feature-specific coding 241-427-SV-2-2553-COE-PSU 6 Line-Shift Coding Subtle shifting of line spacing transmits information Easy to implement with modern laser printers Shift lines 1-2 dots/pixels up or down to convey a bit of information Easy to detect More successful with handwritten documents on unlined paper
- 3. 3 241-427-SV-2-2553-COE-PSU 7 Word-Shift Coding Harnesses variable spacing between words to convey information Fixed spacing systems cannot do this Variable spacing in word processors can do this Easily detected/ defeated if spacing algorithm is known Encoding needs to preserve overall line length and content Encoded lines should not be longer / shorter Encoding should not introduce new line breaks 241-427-SV-2-2553-COE-PSU 8 Feature Coding Features of individual characters are modified Modifying height of endlines on letters: b, d, h, etc. Modifying length of drop on letters: c, e, s, etc. Syntax encoding (how we punctuate) bread, butter, and milk bread, butter and milk Semantic encoding (the words we use) Alternating synonyms (big / large, etc.) Synonym substitution 241-427-SV-2-2553-COE-PSU 9 Steganography in Images Exploiting the limited powers of the human visual system (HVS) Humans are not very good at distinguishing subtle changes in Color Brightness Busy, gray-scale images are preferred
- 4. 4 241-427-SV-2-2553-COE-PSU 10 Image Compression Two main types lossless Lossy Lossless Preferred as hidden data remains unaffected GIF, BMP Lossy Could have negative impact Depends on the encoding algorithm JPEG 241-427-SV-2-2553-COE-PSU 11 LSB Insertion Common and simple Fragile when lossy compression is used Or when format is converted We steal bits from the image in some way From the pixels themselves From the color table if one is in use Different stealing: mechanisms have different impacts on the resultant image 241-427-SV-2-2553-COE-PSU 12 Steganography in Audio The human audio system (HAS) is more sensitive than the HVS The opportunity occurs in volume Large sounds tend to drown out small sounds In general, it is easy for HAS to detect added noise Except where the noise is added to the backgraound, smaller, sounds Low-Bit Encoding similar to image LBE Steal the low order bit of each audio byte In general, not a good technique Introduces audible noise Channel noise and other factors can destroy the hidden signal Phase coding, Spread spectrum, Echo data hiding
- 5. 5 241-427-SV-2-2553-COE-PSU 13 Steganalysis Steganography Avoidance of suspicion to hidden messages in other data Steganalysis Aims to discover and render useless such covert messages Methods: detection or destruction of the hidden messages Examples Watermark detector: degrading image until no watermark can be found; random signals are added so that the watermark cannot be detected 241-427-SV-2-2553-COE-PSU 14 References Steganography Past, Present and Future http://www.sans.org/reading_room/whitepapers/st enganography/steganography-past-present- future_552 http://www.lia.deis.unibo.it/Courses/RetiDiCalcolat ori/Progetti98/Fortini/history.html J. Brassil, S. Low, N. Maxemchuk, and L. O' Garman. Electronic marking and identification techniques to discourage document copying. In IEEE Infocom 94, pages 1278-1287, 1994. W.Bender, D. Gruhl, N. Morimoto, and A. Lu. Techniques for data hiding. In IBM Systems Journal, Vol. 35, No. 3-4, pages 313-336, February 1996.