robert krenn [email protected] january 21, 2004 steganography implementation & detection
TRANSCRIPT
Overview
● What is steganography?
● Implementations● Detection● Defeating steganography● Conclusion● Questions
What is steganography?
Stega-covered, from the Greek “stegos” or roof
-nographywriting, 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, astrayGone from the path direct: and e'en to tellIt were no easy task, how savage wildThat forest, how robust and rough its growth,Which to remember only, my dismayRenews, 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, astrayGone from the path direct: and e'en to tellIt were no easy task, how savage wildThat forest, how robust and rough its growth,Which to remember only, my dismayRenews, 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