Steganography

1
Steganography Steganalysis

• Multimedia Security Systems
• Spring 2006
• Hi Tae Shin

2
Overview

• Introduction
• Steganography Background
• Information Hiding in JPEG files
• Steganalysis
• Statistical Analysis
• Advanced Steganography

3
Steganography Background

• Classical Steganography
• Security relies on secrecy of encoding system
• Tattoo on a shaved head (440 B.C),
• Last word in every sentence
• Least-significant bits in an image
• Modern Steganography
• Detectable only if secret information is known
• Invasive nature leaves detectable traces in
  mediums characteristic

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Steganography Background

• Information Hiding Process
• Identify redundant bits in cover medium
• Select a subset of bits to be replaced with data
  from hidden message
• Insert into cover medium to create stego medium
• Modifying redundant bits changes statistical
  properties of cover medium

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Information Hiding in JPEG files

• JPEG image format uses a discrete cosine
  transform (DCT).
• Transforms 8x8-pixel block into 64 DCT
  coefficients
• Modifying one coefficient affects all 64 image
  pixels
• Least-significant bits of quantized DCT
  coefficients are used as redundant data
• No known visual attacks against JPEG
  steganography

6
Information Hiding in JPEG files
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Steganalysis

• The goal of steganalysis is to identify suspected
  information streams, determine whether or not
  they have hidden messages encoded into them, and,
  if possible, recover the hidden information.
• Visual Attack
• Enhancement of intensity of LSB image
• Statistical Attack
• Chi-square test

8
Statistic Analysis

• Statistical tests can reveal that an image has
  been modified by steganography by determining
  that an images statistical properties deviate
  from a norm.
• Westfeld and Pfitzman observe that embedding of
  encrypted data changes the histogram of color
  frequencies.
• In encrypted data, 0 and 1 bit are equally likely
• If color 2 is more frequent than color 3,
  inserting encrypted data will change 2 more often
  to 3, than vice versa.
• Same is true for JPEG. Take frequency of the DCT
  coefficients instead.

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Statistical Analysis

• Given uniformly distributed message bits, if n2i
  n2i1, then pixels with color (DCT coefficient)
  with 2i are changed more frequently to color 2i1
  than pixels with color 2i1 are changed to color
  2i.
• Thus, n2i n2i1 n2i - n2i1

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Statistical Analysis

• Least Significant Bit
• PoV (pairs of value)
• , (-4,-3), (-2,-1), (0,1) (2,3),
• number of a particular PoV remain constant even
  after embedding

JPEG DCT Coefficient Cover Image
-3
-2
-1
-4
2
0
1
3
4
0
0
0
1
1
0
1
1
1
1
1
0
0
0
0
1
2 7
0
2 7
1
-3
-2
-1
-4
2
0
1
3
4
JPEG DCT Coefficient Stego Image
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Statistical Analysis

• ?2 test determine whether an image shows
  distortion from embedding hidden data.
• The probability p that the two distributions are
  equal.
• When two distributions are equal (p 1), the
  entropy in the LSBs is maximal, indicating with
  certainty that a message has been hidden

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Statistical Analysis

• The probability of embedding calculated for
  different areas of an image.

13
Statistical Analysis

• Limitation
• Targeted Analysis Jsteg
• Inability to detect steganographic images created
  by other tools
• Simple chi-square test does not detect advanced
  steganography
• Slight modification to the original method
• Advanced Steganography
• Redundant Bit Selection
• keyed PRNG to select the redundant bits
• Error Correcting Codes
• use the unmarked redundant bits and embed error
  correcting codes
• flipp neighboring PoV that does not fall into
  selected redundant bit set

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Advanced Steganography

• Extended chi-square test

• Simple chi-square test

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References

• Niels Provos and Peter Honeyman. "Hide and Seek
  An introduction to Steganography."
  http//niels.xtdnet.nl/papers/practical.pdf
  (MAY/JUNE 2003).
• Neil F. Johnson and Sushil Jajodia. "Steganalysis
  of Images Created Using Current Steganography
  Software." http//www.jjtc.com/ihws98/jjgmu.html
  (1995-2003).
• Ross J. Anderson, Fabien A.P. Petitcolas. "On The
  Limits of Steganography." http//www.petitcolas.ne
  t/fabien/publications/jsac98-limsteg.pdf (May
  1998).
• Hany Farid. "Detecting Steganographic Messages in
  Digital Images." http//www.cs.dartmouth.edu/fari
  d/publications/tr01.pdf
• Saraju P. Mohanty. "Digital Watermarking A
  Tutorial Review." http//www.cs.unt.edu/smohanty/
  research/Reports/MohantyWatermarkingSurvey1999.pdf
  (1999)

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• Thank You !