Information Theory and Communications CSM25 Secure Information Hiding Dr Hans Georg Schaathun University of Surrey Spring 2007 Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 1 / 44
Learning Outcomes become familiar with fundamental concepts in communications Entropy and Redundancy Error-control coding Compression be able to link communications fundamentals to steganography Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 2 / 44
Communications essentials Communications and Redundancy Outline Communications essentials 1 Communications and Redundancy Digital Communications Shannon Entropy Security Prediction Compression 2 Recollection Huffmann Coding Huffmann Steganography Grammars 3 Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 3 / 44
� Communications essentials Communications and Redundancy The communications problem Alice Bob m ˆ m Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
� � Communications essentials Communications and Redundancy The communications problem Alice Bob Noisy m ˆ m channel Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
� � Communications essentials Communications and Redundancy The communications problem Alice Bob Noisy m ˆ m channel Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
� � Communications essentials Communications and Redundancy The communications problem Alice Bob Noisy m ˆ m channel Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
� � � Communications essentials Communications and Redundancy The communications problem Alice Bob Noisy c � r m ˆ Enc. Dec. m channel Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
� � � Communications essentials Communications and Redundancy The communications problem Alice Bob Noisy c � r m ˆ Enc. Dec. m channel Bob’s problem Estimate m , given (partly) random output ˆ m from the channel How much (un)certainty does Bob have about m ? Information theory and Shannon entropy. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 4 / 44
Communications essentials Communications and Redundancy Redundancy of English Fact The English language is more than 50% redundant. Message destroyed on the channel Redundancy allows Bob to determine the original m . Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 5 / 44
Communications essentials Communications and Redundancy Redundancy of English Fact The English language is more than 50% redundant. t** p*oce*s o**hid**g *ata**nsid* o*her**ata. For ex*****, a **xt f*le c**ld*** hid*** "in**de"****im*ge or***s**nd *ile* By look****at t*e im*g***or list***** to th**s**nd,*yo* w*u*d n*t *no**that***ere is *x*ra info******* *r*sent. from http://www.cdt.org/crypto/glossary.shtml Message destroyed on the channel Redundancy allows Bob to determine the original m . Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 5 / 44
Communications essentials Communications and Redundancy Redundancy of English Fact The English language is more than 50% redundant. t** p*oce*s o**hid**g *ata**nsid* o*her**ata. For ex*****, a **xt f*le c**ld*** hid*** "in**de"****im*ge or***s**nd *ile* By look****at t*e im*g***or list***** to th**s**nd,*yo* w*u*d n*t *no**that***ere is *x*ra info******* *r*sent. from http://www.cdt.org/crypto/glossary.shtml Message destroyed on the channel Redundancy allows Bob to determine the original m . Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 5 / 44
Communications essentials Communications and Redundancy Redundancy of English Fact The English language is more than 50% redundant. t*e p*oce*s o* hid**g *ata*insid* o*her*data. For ex*m***, a t*xt f*le c**ld*b* hidd** "ind*de" a**im*ge or*a*s*und *ile* By look**g*at t*e im*g*,*or list**in* to th* s**nd,*yo* w*uld n*t *no**that *here is *x*ra info*****on *r*sent. from http://www.cdt.org/crypto/glossary.shtml Message destroyed on the channel Redundancy allows Bob to determine the original m . Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 5 / 44
Communications essentials Communications and Redundancy Redundancy of English Fact The English language is more than 50% redundant. the process of hiding data inside other data. For example, a text file could be hidden "inside" an image or a sound file. By looking at the image, or listening to the sound, you would not know that there is extra information present. from http://www.cdt.org/crypto/glossary.shtml Message destroyed on the channel Redundancy allows Bob to determine the original m . Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 5 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy Benefits of redundancy Cross-word puzzles Understand foreigners with imperfect pronounciation. How much would you understand of a lecture without redundancy? Hear in a noisy environment. Read bad hand writing How could I mark exam scripts without redundancy? Cryptanalysis? Steganalysis? Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 6 / 44
Communications essentials Communications and Redundancy What if there were no redundancy? No use for steganography! Any text would be meaningful, in particular, ciphertext would be meaningful Simple encryption would give a stegogramme indistinguishable from cover-text. Dr Hans Georg Schaathun Information Theory and Communications Spring 2007 7 / 44
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