Robert Krenn

rkrenn@xidc.nl

January 21, 2004

SteganographyImplementation & Detection

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