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Steganography Presentation

Dec 15, 2014



Presentation by Zachary Burt,

CMSC 24000. Information Theory and Coding (PSYC28800/38800). Professor Abraham Bookstein.

  • 1. Steganography a presentation by Zachary Burt information theory - fall quarter

2. My Dilemma: The BeginningMy friend Mark is very interested in the e- book market. E-books are sold over the internet for anywhere from $2 to $500E-books usually are distributed as PDFs, sometimes password-protected, and sometimes in proprietary formats that require a special reader and password 3. My Dilemma: WTF, PDFPasswords can be cracked using brute-force methods with cheap software on the internetRaster images from e-books delivered through proprietary readers can be captured via screenshots and bundled as unlocked PDFs 4. My Dilemma: Yarr.Unlocked PDFs can be pirated with increasing ease due to the advent of P2P technologies such as BitTorrentAlthough a long-term solution may be to nd alternative, piracy-friendly economic models, this is a problem! 5. My Dilemma: A Idea Springs ForthWhat you need to do is code the information so that you have a unique way of identifying a signature, while at the same time not signicantly shift the information any way (as to avoid arousing suspicion)! The text itself (content) must be structured in a way that you can infer extra information: a signature The signature will be the id of the purchaser 6. Surprise!Apparently Im not the rstperson to consider this problem Steganography is the art and science of writing hidden messages so that none but sender and recipient realize there is a hidden message 7. F.Y.I.; BTW Cryptography (not the same thing)obscures the meaning of a messagewithout concealing the message This is Bill Nye the Science Guy. I was going for a itself Did you know that?...Now you know! vibe 8. Terminology Steganography usually employs both a covertext and a messageThe message is producedA covertext is modied to contain itThis results in stegotext 9. Cryptography Quick Tangent Message can be plaintext and then converted into ciphertext for added security before it becomes stegotextThis requires an encryption algorithm 10. Humanity and HVS Most steganography methods take advantage of human psychology and the human visual system.Think Change blindness 11. CovertextA covertext can be anything if youre clever enough about it. Well look at ways to be clever with a few different types of media.text (.doc, .txt, .html, newspapers)images (pictures, periods)sounds (.mp3, radio transmissions)human being 12. TextLine shifting (as little as .003 in.)Word shifting (spaces between words)Change features of characters (b, d, T, i, etc.)Ordering (xml)Word choice (esp. spam messages!) Words map to a dictionary nth character signicantProblem: easy to normalize text 13. Images LSB encoding: least signicant bit. 3 bits available for 24-bit images, 1 bit available for 8 bit images (R -> 255, G -> 255, B -> 255)You can do this without the HVS detecting, but it is very vulnerable to attacks as simple as changing formatting from GIF to JPEG 14. ImagesLSB Encoding ExampleHost pixel: 10110001Secret pixel: 00111111 New Image pixel: 10110011 Transform 10110011 into 00110000 Uses only 4 bits, fairly low loss for host and secret 15. Images LSB Encoding OutcomeChanging the number of bits used has an effect on quality of both the original and secret imageThe sweet spot may be around 4 bits 16. Images Youd Never Expect ItMicrodot techniques take an image and reduce it to the size of a grammatical unit such as a period. Any arbitrary covertext can be used as long as it contains periods.J. Edgar Hoover described their use as the enemys masterpiece of espionage 17. ImagesOther TechniquesEmbed a digital watermarkDirect Cosine TransformationsThis extends the data of the original image as opposed to hiding information inside the dataScatter black pixels, disguised as noise, in even or odd blocks 18. Sound Fun TechniquesBinary data can be encoded as noise, but recognized with a proper decoding keyEncoding data in mp3 les requires you to store data in the parity bit during the compression process decompress and read all parity bits 19. Human Being 1. Shave the head of a human being, preferably a slave 2. Tattoo a message on his head3. Wait for the hair to grow back 20. Defeating Steg Steganalysis Color histogram, eliminate spikesBitmap images and near-duplicate colors color table, LSB creates dupes, arouses supicion 21. Defeating Steg The BattleAnticipate with inverse transformationsError correcting codes, redundancy Normalize the imageChange the formatD+W+W 22. Defeating Steg StirMark StirMark applies geometricdistortions, a random lowfrequency deviation based aroundthe center of the image, and atransfer function to introduceerror into all the sample values The change in the image is nearlyimpossible to detect but anywatermark is likely destroyed 23. The Bottom Line Steganography is useful but has its drawbacksNormalization, confusionBest when combined with cryptography 24. My Dilemma:Proposed ResolutionsSubtle changes in the spacing of the image might be possible to detect using a diff program, or by comparing the hashes of two instances of a copyrighted e-bookThey could be defeated by scanning the text, normalizing it, and binding it as a plain PDFChange kerning to interfere with OCR 25. The MediaAl Qaeda rumors: eBay, pornographyPedophiles using stego to hide their images 26. Remember the picture on the front page?I didnt think so.(Maybe you did; after all, this was a presentation about steganography and it may have appeared pretty conspicuous...) 27. Catty TitleIf you remove all but the last 2 bits of every color component in the rst image, you get an almost completely black image.You werent expecting this, were you.When you make it 85 times brighter, though, kittens start to purr. 28. Conclusions: The Future been StegoIt probably would have of more appropriate to have a space-age stegosaurus for the picture, but whateverCriticism: it only works when nobody expects itNew techniques being researched DNASometimes the best place to hide something may be in plain sight