Robert Krenn January 21, 2004 Steganography Implementation & Detection

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 (

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 ⇒ 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