Steganography Presentation


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Presentation by Zachary Burt,

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

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Steganography Presentation

  1. 1. Steganography a presentation by Zachary Burt information theory - fall quarter
  2. 2. My Dilemma: The Beginning • My friend Mark is very interested in the e- book market. E-books are sold over the internet for anywhere from $2 to $500 • E-books usually are distributed as PDFs, sometimes password-protected, and sometimes in proprietary formats that require a special reader and password
  3. 3. My Dilemma: WTF, PDF • Passwords can be cracked using brute-force methods with cheap software on the internet • Raster images from e-books delivered through proprietary readers can be captured via screenshots and bundled as unlocked PDFs
  4. 4. My Dilemma: Yarr. • Unlocked PDFs can be pirated with increasing ease due to the advent of P2P technologies such as BitTorrent • Although a long-term solution may be to find alternative, piracy-friendly economic models, this is a problem!
  5. 5. My Dilemma: A Idea Springs Forth • What 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 significantly 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. 6. Surprise! • Apparently I’m not the first person 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. 7. F.Y.I.; BTW • Cryptography (not the same thing) obscures the meaning of a message without 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. 8. Terminology • Steganography usually employs both a covertext and a message • The message is produced • A covertext is modified to contain it • This results in stegotext
  9. 9. Cryptography Quick Tangent • Message can be plaintext and then converted into ciphertext for added security before it becomes stegotext • This requires an encryption algorithm
  10. 10. Humanity and HVS • Most steganography methods take advantage of human psychology and the human visual system. • Think “Change blindness”
  11. 11. Covertext • A covertext can be anything if you’re clever enough about it. We’ll 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. 12. Text • Line 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 significant • Problem: easy to normalize text
  13. 13. Images • LSB encoding: least significant 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. 14. Images LSB Encoding Example • Host pixel: 10110001 • Secret pixel: 00111111 • New Image pixel: 10110011 • Transform 10110011 into 00110000 • Uses only 4 bits, fairly low loss for host and secret
  15. 15. Images LSB Encoding Outcome • Changing the number of bits used has an effect on quality of both the original and secret image • The sweet spot may be around 4 bits
  16. 16. Images You’d Never Expect It • Microdot 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 enemy’s masterpiece of espionage”
  17. 17. Images Other Techniques • Embed a digital watermark • Direct Cosine Transformations • This extends the data of the original image as opposed to hiding information inside the data • Scatter black pixels, disguised as noise, in even or odd blocks
  18. 18. Sound Fun Techniques • Binary data can be encoded as noise, but recognized with a proper decoding key • Encoding data in mp3 files requires you to store data in the parity bit during the compression process • decompress and read all parity bits
  19. 19. Human Being • 1. Shave the head of a human being, preferably a slave • 2. Tattoo a message on his head • 3. Wait for the hair to grow back
  20. 20. Defeating Steg Steganalysis • Color histogram, eliminate spikes • Bitmap images and near-duplicate colors • color table, LSB creates dupes, arouses supicion
  21. 21. Defeating Steg The Battle • Anticipate with inverse transformations • Error correcting codes, redundancy • Normalize the image • Change the format • D+W+W’
  22. 22. Defeating Steg StirMark •  StirMark applies geometric distortions, a random low frequency deviation based around the center of the image, and a transfer function to introduce error into all the sample values •  The change in the image is nearly impossible to detect but any watermark is likely destroyed
  23. 23. The Bottom Line • Steganography is useful but has its drawbacks • Normalization, confusion • Best when combined with cryptography
  24. 24. My Dilemma: Proposed Resolutions • Subtle 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-book • They could be defeated by scanning the text, normalizing it, and binding it as a plain PDF • Change kerning to interfere with OCR
  25. 25. The Media • Al Qaeda rumors: eBay, pornography • Pedophiles using stego to hide their images
  26. 26. Remember the picture on the front page? • I didn’t think so. • (Maybe you did; after all, this was a presentation about steganography and it may have appeared pretty conspicuous...)
  27. 27. Catty Title • If you remove all but the last 2 bits of every color component in the first image, you get an almost completely black image. You weren’t expecting this, were you. • When you make it 85 times brighter, though, kittens start to purr.
  28. 28. Conclusions: The Future been Stego • It probably would have of more appropriate to have a space-age stegosaurus for the picture, but whatever • Criticism: “it only works when nobody expects it” • New techniques being researched • DNA • Sometimes the best place to hide something may be in plain sight