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Steganography a presentation by Zachary Burt information theory - fall quarter

Steganography Presentation

Dec 15, 2014




Presentation by Zachary Burt,

CMSC 24000. Information Theory and Coding (PSYC28800/38800). Professor Abraham Bookstein.
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Page 1: Steganography Presentation

Steganography a presentation by Zachary Burt

information theory - fall quarter

Page 2: Steganography Presentation

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

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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

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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!

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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

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• 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

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• Cryptography (not the same thing) obscures the meaning of a message without concealing the message itself

This is Bill Nye the Science Guy. I was going for a “Did you know that?...Now you know!” vibe

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• Steganography usually employs both a covertext and a message

• The message is produced

• A covertext is modified to contain it

• This results in stegotext

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Cryptography�Quick Tangent

• Message can be plaintext and then converted into ciphertext for added security before it becomes stegotext

• This requires an encryption algorithm

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Humanity and HVS

• Most steganography methods take advantage of human psychology and the human visual system.

• Think “Change blindness”

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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

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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

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• 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

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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

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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

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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”

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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

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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

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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

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Defeating Steg�Steganalysis

• Color histogram, eliminate spikes

• Bitmap images and near-duplicate colors

• color table, LSB creates dupes, arouses supicion

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Defeating Steg�The Battle

• Anticipate with inverse transformations

• Error correcting codes, redundancy

• Normalize the image

• Change the format

• D+W+W’

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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

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The Bottom Line

• Steganography is useful but has its drawbacks

• Normalization, confusion

• Best when combined with cryptography

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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

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The Media

• Al Qaeda rumors: eBay, pornography

• Pedophiles using stego to hide their images

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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...)

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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.

• When you make it 85 times brighter, though, kittens start to purr.

You weren’t expecting this, were you.

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Conclusions:�The Future of Stego • It probably would have been 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


• Sometimes the best place to hide something may be in plain sight