Steganography Implementation & Detection Robert Krenn rkrenn@xidc.nl January 21, 2004
Overview ● What is steganography? ● Implementations ● Detection ● Defeating steganography ● Conclusion ● Questions
What is steganography? Stega- covered, from the Greek “stegos” or roof -nography writing, from the Greek “graphia”
What is steganography? ● The art of hiding information inside information Since everyone can read, encoding text in neutral sentences is doubtfully effective
What is steganography? ● The art of hiding information inside information Since everyone can read, encoding text in neutral sentences is doubtfully effective Since Everyone Can Read, Encoding Text In Neutral Sentences Is Doubtfully Effective ⇒ Secret inside
What is steganography? What is it used for? ● Hiding the fact that you are sending messages ● Hiding several messages inside data ● Digital watermarking Kerckhoffs' principle ● Secure with knowlegde of the system ● Message can only be read with secret key
What is steganography? Hiding several messages ● Deniable cryptography ● Knowledge of encrypted data ● No proof of how much information is stored ● Rubberhose (http://www.rubberhose.org)
What is steganography? Digital watermarking ● Not really hiding information ● Important: not modificable ● Uses the hiding-property of steganography ● Recent: MPAA hides watermarks in movies
Implementation Hiding information ● Text / Webpages ● Images ● Audio ● Video
Implementation Text / Webpages ● Use of a codebook ● Layout of texts ● Every Nth character ● Use of whitespaces and newlines ● Can be difficult to detect and decode
Implementation Text / Webpages In the midway of this our mortal life, I found me in a gloomy wood, astray Gone from the path direct: and e'en to tell It were no easy task, how savage wild That forest, how robust and rough its growth, Which to remember only, my dismay Renews, in bitterness not far from death. Yet to discourse of what there good befell, All else will I relate discover'd there. How first I enter'd it I scarce can say
Implementation Text / Webpages In the midway of this our mortal life, I found me in a gloomy wood, astray Gone from the path direct: and e'en to tell It were no easy task, how savage wild That forest, how robust and rough its growth, Which to remember only, my dismay Renews, in bitterness not far from death. Yet to discourse of what there good befell, All else will I relate discover'd there. How first I enter'd it I scarce can say 06081913030629170827 ⇒ meet at dawn
Implementation Images ● Cover image ● Identify redundant data ● Replace (a subset of) redundant bits with data
Implementation Images ● JPEG uses discrete cosine transformations (DCT) ● Transform pixel blocks to DCT coefficients ● Get least significant bit of each DCT coefficient ● Replace LSB with secret message bit ● Insert modified DCT into output image
Implementation Images Original image Data embedded inside
Implementation Images ● JPEG stegencryption ● Operates in transformation space ● No visual changes ● GIF and BMP stegencryption ● Operates in low bit planes ● Subject to visual attacks
Implementation Images Original image Data embedded inside
Implementation Audio ● Use frequencies inaudible to humans ● Embed data using the LSB ● Encoding musical tones
Implementation Video ● Visible signals (gestures, movements) ● Same techniques as with images ● Same techniques as with audio
Detection Text ● Look for (disturbings of) patterns ● Odd use of language ● Line heights and whitespaces
Detection Images ● Examine color palette ● Size of the image ● Differences: − Format − Last modified date
Detection Images ● Statistical analysis ● Analyze frequency of DCT coefficients ● Entropy of redundant data − Hidden content ⇒ higher entropy
Detection Images ● Apply filters to steganograms ● Visible representation of statistical data
Detection Filtered Original image Embedded data
Detection Filtered Original image Embedded data
Detection Audio ● Statistical analysis ● Analyze patterns in background noise ● Distortions ● Measure inaudible frequencies
Detection Video ● Look for distinguishing movements ● Same techniques as with images ● Same techniques as with audio
Defeating steganography Text ● Change spacing, interpunction ● Modify line heights, layout ● Add or remove words
Defeating steganography Images ● Compress with lossy compression ● Convert to different format ● Resize the image ● Modify colors
Defeating steganography Audio ● Compress with lossy compression ● Change bitrate ● Alter / limit frequency spectrum
Defeating steganography Video ● Same techniques as with images ● Same techniques as with audio
Conclusion ● Hide data practically everywhere ● Detection of steganography − Possible, but can be difficult − Hard when valid to Kerckhoffs' principle ● Defeating steganography is not hard
Steganography ● Questions?
Steganography
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