steganography paul gretes gangster pay ho dennis pattersonno sniper dna test a lepers gut
TRANSCRIPT
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Steganography
Paul Gretes
Gangster pay ho
Dennis Pattersonno sniper DNA testA lepers gut
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Topics
Driving Interest and Examples Prisoner’s Problem Public/Private Key Steganography Attacks Basic Theory Watermarking/Fingerprinting
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phgloji
m
ei
ss
un
afyn e
i
r sn
r
Apparently neutral’s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by-products, ejecting suets and vegetable oils.
Intercepted message:
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phgloji
m
ei
ss
un
afyn e
i
r sn
r
(Perishing sails from NY June I)
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Steganography
Comes from the greek words and – literally, “covered writing”
Steganography conceals the fact that a message even exists.
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Driving Interest behind Steganography
Military and Intelligence agencies
– Battlefield Communications Criminals
– Or terrorists (Bin Laden suspected use) Law Enforcement and counter intelligence agencies
– Interested in weaknesses
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Earlier Examples
Prisoners hid messages in letters home using the dots and dashes on i, j, t, & f to spell out a hidden text in Morse Code
Herodotus tells us Histiaeus shaved the head of his most trusted slave and tattooed it with a message which disappeared after the hair had regrown.
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Example
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Steganography diagram
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Definitions…
Embedded data – message you want to send Cover object – text, image, audio, or other object to hide
data in Stego-key – used to control hiding process Stego-object – resulting object when data is hidden in
cover object Robustness –>
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Steganography in pictures
How?– Least Significant Bit Insertion
– Masking and Filtering
• More robust than LSB Insertion
– Algorithms and Transformations
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LSB Insertion
3 pixels of a picture might be (without compression) (00100111 11101001 11001000)
(00100111 11001000 11101001)
(11001000 00100111 11101001)
Binary value for A is 10000011 New data for 3 pixels would be (00100111 11101000 11001000)
(00100110 11001000 11101000)
(11001000 00100111 11101001)
Change won’t be visually noticeable!
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Steganography in Audio
Echo hiding
– We cannot perceive short echoes (millisecond short)
– Introduce two types of short echo with different delays to encode zeros and ones
Example program: MP3Stego
– Information hidden during compression process
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Aphex Twin
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Steganography in Programs
Example program: Hydan– Exploits redundancy in the i386 instruction set by
defining sets of functionally equivalent instructions
• Can add 50 or subtract –50
• XOR DX,DX MOV BX, 4MOV AX,3 versus MOV AX, 3MOV BX,4 XOR DX, DXMUL BX MUL BX
– Same code, new order---when paired with original, can give meaning
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Marks should not degrade the perceived quality of the work
Detecting the presence and/or value of a mark should require knowledge of a secret
Multiple marks should not interfere with each other Mark should survive all attacks that do not degrade work’s
perceived quality– Resampling, dithering, compression, and combinations
of these
Qualities of a Robust Marking System
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Channels
Covert Channels– Usually non-digital– Flower pot example
Subliminal channel– Exploits existing randomness– Regular communication through stego-object
“Supraliminal” channel– Low bandwidth – maybe establish session key– Information not hidden but cannot be modified
• Example: a novel
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Prisoners’ Problem Two individuals attempt to communicate covertly without
alerting a “warden” who controls the communications channel
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Prisoners’ Problem, continued
Passive Warden – monitors traffic and signal to some process outside the system if unauthorized message traffic is detected; (just spies on channel)
Active Warden – tries to remove all possible covert messages from cover texts that pass through their hands; (can slightly modify data being sent)– Much harder to deal with
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Pure Steganography
In the best case, prisoners would not have to communicate prior to imprisonment (to trade encryption keys)
Is it possible?
– Very difficult to engineer
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Public/Private Key Steganography
Intertwined with Prisoner’s Problem Private-Key Steganography assumes that Alice and
Bob are allowed to share a secret key prior to imprisonment, or even trade public keys
Public-Key Steganography – shared key isn’t necessary – one only needs to know the other’s public key (may have to check all objects for messages)
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Role of Randomness
Average information rate given by entropy Example: entropy of monochrome images is generally
around 4 – 6 bits/pixel
– Use this difference to hide information
– All the gain provided by compression is used for hiding
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Watermarking/Fingerprinting Cousin to steganography Not limited to images (but is main focus) Fingerprint – hidden serial number Watermarking – hidden copyright message
– Visible• Company logo (image)
– Invisible• More applications• Fragile – mark destroyed if image manipulated (ex: in
Court)• Robust – mark is resistant to image manipulation
(verify ownership)
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Types of Marking
Private Marking – require original for comparison– What if original is tampered with?
Public Marking (or Blind Marking) – requires neither secret original or embedded mark– More challenging– More useful
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Attacks Robustness attack
– Try to diminish or remove mark Presentation attack
– Modify content such that detector cannot find mark
Interpretation attack– Devise a situation which prevents assertion of
ownerships Many attacks are combinations of above
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Original image
Rotated
Stretched
Cropped
Blurred(would more likely be a combination of manipulations)
Robustness Attacks
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Robustness Attacks
Program: StirMark– Slightly stretches, shifts, bends, rotates by
an unnoticeable, random amount. Then, adds a low frequency deviation to each pixel. Also adds smoothly distributed error.
Attack on Echo Hiding– Try to detect echo and remove it
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Presentation Attack
Mosaic Attack– Chop image into
smaller images
– Prevents web crawlers from finding whole image and checking for watermark
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Interpretation Attack
Owner of document d encodes watermark w, publishes the marked version d + w and has no other proof of ownership.
Attacker registers his watermark as w’ can claim that original unmarked version of it was d + w – w’.
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Goal: discover covert messages Extended 2 tests Stuff way to complicated for this presentation
Statistical Steganalysis
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Conclusions/Summary
Complicated Many methods of implementation Implementation depends on situation
– Many situations
– Many assumptions Must ensure robustness (in most cases)
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Works used Most information:
– Fabien A. P. Petitcolas, Ross J. Anderson, and Markus G. Kuhn. Information hiding - a survey. Proceedings of the IEEE, 87(7), pp. 1062-1078, July 1999.
– Neil F. Johnson, Sushil Jajodia. Exploring Steganography: Seeing the Unseen, IEEE Computer, February 1998. pp. 26-34
– Niels Provos. Defending Against Statistical Steganalysis. In Proceedings of the 10th USENIX Security Symposium, pages 323-335, August 2001.
– R. Anderson, "Stretching the limits of steganography," in Information Hiding, Springer Lecture Notes in Computer Science vol. 1174, pp. 39--48, 1996.
– R. Anderson and F. Petitcolas. On the limits of steganography, ieee journal on selceted areas in communications 16, pp. 474-481, may 1998., 1998.
– ‘Resolving Rightful Ownerships with Invisible Watermarking Techniques: Limitations, Attacks, and Implications.’ Craver, N. Memon, B.-L. Yeo, M. M. Yeung, IEEE Journal of Selected Areas in Communications, vol. 16 no. 4 pp. 573–586, May 1998, Special issue on copyright & privacy protection.
– S. Craver, "On Public-Key Steganography in the Presence of an Active Warden." in Information Hiding II, Springer Lecture Notes in Computer Science v 1525 (April 1996), pp 355—368.
Other information: various websites