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EPICS Spring 2003 Slide 1
Digital Watermarking:An Introduction
Edward J. Delp
Purdue University School of Electrical and Computer Engineering
Purdue Multimedia TestbedVideo and Image Processing Laboratory (VIPER)
West Lafayette, Indiana
email: [email protected] http://www.ece.purdue.edu/~ace
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EPICS Spring 2003 Slide 2
Outline
• Provide an introduction to watermarking and data hiding and overview its use
• Describe how security techniques may/will impact multimedia systems
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EPICS Spring 2003 Slide 3
Multimedia Security
• “Everything” is digital these days - a copy of a digital media element is identical to the original
• How can an owner protect their content?• Are images still “fossilized light”?• What does all of this mean in terms of law?• Does any security system really work or does it just make
us feel good!
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EPICS Spring 2003 Slide 4
What Do We Want From a Security System?
• Access Control • Copy Control
• Auditing (fingerprinting)– Who did what and when?
Playback Control
Record Control
Generation Control⇒
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EPICS Spring 2003 Slide 5
What Do Users Want?
• Time-shifting• Space-shifting• Single copy (back ups?)
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EPICS Spring 2003 Slide 6
Digital Communication System
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EPICS Spring 2003 Slide 7
Typical Cryptography System:Trusted Users
Source UserInsecure Channel
Attacker Is this a pirate
device?
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EPICS Spring 2003 Slide 8
Cryptography System:User Not Trusted
Source UserInsecure Channel
Authentication
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EPICS Spring 2003 Slide 9
Media Elements
• Audio• Video• Documents (including HTML documents)• Images• Graphics• Graphic or Scene Models • Programs (executable code)
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EPICS Spring 2003 Slide 10
Multimedia Security - Tools Set
• Encryption
• Authentication
• Hashing
• Time-stamping
• Watermarking
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EPICS Spring 2003 Slide 11
Multimedia Security Applications
• Privacy• Forgery Detection ⇒ watermarking• Copyright Protection ⇒ watermarking• Proof of Purchase (non-deniable)• Proof of Delivery (non-deniable)• Intruder Detection
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EPICS Spring 2003 Slide 12
What is Watermarking?
• The use of perceptually invisible authentication techniques
– “controlled” distortion is introduced in a multimedia element
• Visible watermarks also exists
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EPICS Spring 2003 Slide 13
• Scenario– an owner places digital images on a network
server and wants to “protect” the images• Goals
– verify the owner of a digital image– detect forgeries of an original image– identify illegal copies of the image– prevent unauthorized distribution
Watermarking Scenario
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EPICS Spring 2003 Slide 14
Where are Watermarks Used?• Watermarks have been used or proposed in:
– digital cameras– DVD video– audio (SDMI)– broadcast video (in US - ATSC)
• visible watermarks now used– “binding” mechanism – key distribution systems– preventing forgery of bank notes
Usually as secondary security ⇒ conversion to “analog”
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EPICS Spring 2003 Slide 15
Steganography
Steganography - (covered writing) techniques used to hide information within other information to conceal the very existence of the messageUsed much longer than cryptography
Different than crytography in that an illegal user may intercept the message
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EPICS Spring 2003 Slide 16
Why is Watermarking Important?
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EPICS Spring 2003 Slide 17
Why is Watermarking Important?
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EPICS Spring 2003 Slide 18
Why Watermarking is Important?
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EPICS Spring 2003 Slide 19
Why is Watermarking Important?
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EPICS Spring 2003 Slide 20
Why is Watermarking Important?
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EPICS Spring 2003 Slide 21
Why is Watermarking Important?
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EPICS Spring 2003 Slide 22
Why is Watermarking Important?
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EPICS Spring 2003 Slide 23
Why is Watermarking Important?
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EPICS Spring 2003 Slide 24
Watermarking
• The use of perceptually invisible authentication techniques is one form of watermarking
– distortion is introduced in the data• Other forms include visible watermarks
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EPICS Spring 2003 Slide 25
A Review of Watermarking Techniques
• Spatial watermarking • Sub-band (wavelet) watermarking• DCT coefficient modulation• Visible watermarks
Watermarking Algorithm
Watermark
WatermarkedImage
OriginalImage
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EPICS Spring 2003 Slide 26
Components of aWatermarking Technique
• The watermark, W– each owner has a unique watermark
• The marking algorithm– incorporates the watermark into the image
• Verification algorithm– an authentication procedure (determines the
integrity / ownership of the image)
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EPICS Spring 2003 Slide 27
Watermark Detection
• The tradeoff of detectability vs. visibility (host signal interference)
• Do you need the original image for detection?– If not ⇒ blind detection
• What about the “key?”– private or public?
• These are very important when with video
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EPICS Spring 2003 Slide 28
Main Principles
• Transparency - the watermark is not visible in the image under typical viewing conditions
• Robustness to attacks - the watermark can still be detected after the image has undergone linear and/or nonlinear operations (this may not be a good property -fragile watermarks)
• Capacity - the technique is capable of allowing multiple watermarks to be inserted into the image with each watermark being independently verifiable
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EPICS Spring 2003 Slide 29
Fragile Watermarks
• Changes to image easily detected and localized
• Used for authentication, rather than copy detection
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EPICS Spring 2003 Slide 30
Fragile Watermarks
• Are we asking too much from a robust watermark?– it is a very interesting signal processing/signal
detection problem but will lead to a solution or an arms race
• Fragile watermarks do not have to worry about this!
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EPICS Spring 2003 Slide 31
Attacks
• Compression• Filtering• Printing and rescanning• Geometric attacks - cropping, resampling, rotation• Collusion - spatial and temporal• Conversion to analog
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EPICS Spring 2003 Slide 32
Current Research Issues
• Theoretical Issues– capacity and performance bounds– models of the watermarking/detection process
• Robust Watermarks– linear vs. nonlinear– scaling and other geometric attacks – watermarking analog representations of content– new detection schemes– what should be embedded (watermark structure)
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EPICS Spring 2003 Slide 33
Fixed-length DCT Watermark
• W is a sequence of random numbers– bipolar binary sequence, or N(0,1)
• XD and YD are DCT of X and Y• a = scaling factor:
Y i X i aWD D( ) ( )( )= +1
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EPICS Spring 2003 Slide 34
DCT Watermark
• W* is the extracted version of the watermark• Verification:
• T = user-defined threshold• If S > T, image is authentic
S W WW W
W W( , )*
*
* *=
⋅
⋅
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EPICS Spring 2003 Slide 35
Original Image
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EPICS Spring 2003 Slide 36
Fixed-length DCT Watermark a = 0.1
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EPICS Spring 2003 Slide 37
Fixed-length DCT Watermark a = 0.5
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EPICS Spring 2003 Slide 38
Fixed-length DCT Watermark a = 1.0
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EPICS Spring 2003 Slide 39
Fixed-length DCT Watermark a = 5.0
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EPICS Spring 2003 Slide 40
Levels of Transparency
1. Spatial Watermarks– watermark embedded in lower order bit planes – mark imperceptible, or indistinguishable from noise
2. Transform Domain Watermarks– watermark sequence added to transform coefficients– transforms based on human visual system– DCT, Wavelet
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EPICS Spring 2003 Slide 41
Levels of Transparency
3. Image Adaptive (IA) Watermarks– transform based
• IA-W : 9-7 biorthogonal wavelet• IA-DCT : Discrete Cosine Transform
– different transform coefficients can tolerate different amounts of change, before changes are noticed
– amounts determined by formal visual models– amplitude of watermark adjusted according to these
amounts– most robust of the three levels
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EPICS Spring 2003 Slide 42
Visual Models
• Used extensively in image compression• Determine how much a transform coefficient can
imperceptibly change
• “Just noticeable difference” (JND) value determines:– quantization step size– watermark amplitude
• Models developed for DCT, Wavelet
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EPICS Spring 2003 Slide 43
Image Adaptive Watermarks
• JND scales the watermark before insertion into image
• Xu,v = original transform coefficient• Ju,v = JND value for particular coefficient• Wu,v = watermark element• Yu,v = marked coefficient
>+
=otherwiseX
JXWJXY
vu
vuvuvuvuvuvu ,
,
,
,,,,,,
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EPICS Spring 2003 Slide 44
Image Adaptive Watermarks (DCT)
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EPICS Spring 2003 Slide 45
Image Adaptive Watermarks (DCT)
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EPICS Spring 2003 Slide 46
Image Adaptive Watermarks (DCT)
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EPICS Spring 2003 Slide 47
IBM ATTACK• Description
– X ~ original, Y1 ~ image marked with W1
– Create counterfeit original, XF
– Y1 now appears to be a marked version of XF
11 WXY +=
21 WYX F −=
21 WXY F +=⇒
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EPICS Spring 2003 Slide 48
IBM Attack
• What is the original, X or XF ?
• Attacker can claim XF is true original
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EPICS Spring 2003 Slide 49
Thwarting the IBM Attack
• Time stamp original image, X
• Let time stamp certificate (S) of X be part of W– W now depends on X, owner name and creation date– only one legitimate W per original image– attacker needs correct time stamp of XF before
generating XF
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EPICS Spring 2003 Slide 50
Steganography
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EPICS Spring 2003 Slide 51
Introduction
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EPICS Spring 2003 Slide 52
Digital Color Images• Visual data is organized as array of pixel values in the
spatial domain• Many different image formats
• In personal computing:– RGB (Red, Green, Blue) color space is common– Each color component assigned an integer value
between 0 to 255, or 8 bits– (0,0,0) corresponds to darkest black; (255,255,255)
corresponds to brightest white
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EPICS Spring 2003 Slide 53
24-Bit Color Images
• In images with 24 bits/pixel, there is a Red, Green, and Blue value for each pixel location
1 1 1 1 0 0 0 0 Red
Bits MSB LSB
1 1 0 1 1 0 0 0 Green
1 1 0 0 1 1 1 0 BlueRGB values of
pixel
(240 216 206)
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EPICS Spring 2003 Slide 54
8-Bit Color Images
• In images with 8 bits/pixel, pixel values represent indices in a color lookup table or palette
– Each entry in palette has Red, Green, Blue values– Size (# of entries) of palette == maximum number of
colors used in the image– Size can be up to 256 entries for 8 bits/pixel– Pixel values do not correspond to RGB values
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EPICS Spring 2003 Slide 55
8-Bit Color Images
RGB values of pixel
23 240 32 24 85 2 33 25 96 97 35 27
94 96 2 4
Array of Pixel Values
Index R G B 0 0 0 0 1 32 84 2
239 160 2 233 240 240 216 206 241 239 200 102
254 255 255 255 255 128 127 124
Color Lookup Table
Pixel value == index in color table
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EPICS Spring 2003 Slide 56
Digital Image Steganography– Objective: covertly embed data inside images– Steganography: “Covered Writing” (Greek)
Cover Image
Stego Image
Stego Key
Secret Message
Steganographic Encoder (S-Tools)
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EPICS Spring 2003 Slide 57
Digital Image Steganography
• Cover Image = Original image• Message = Data (files) to be hidden• Stego Image = Altered image containing hidden data• Stego Key = Secret needed to embed or recover message
• Cover Image often innocuous to avoid suspicion• Stego Image appears identical to cover image under
causal observation• Maximum size of message (capacity) depends on
steganography technique and cover image
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EPICS Spring 2003 Slide 58
Digital Image Steganography
• Secret Message can be recovered by decoder– If stego image has not been altered, extracted message
will be identical to the secret message
Cover Image
Stego Image
Stego Key
Extracted Message
Steganographic Decoder (S-Tools)
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EPICS Spring 2003 Slide 59
Digital Image Steganography• Example cover and stego images produced by S-Tools 4.0
– Message: This is a test message demonstrating the S-Tools 4.0 steganography software.
– Stego key: STEGO
Cover Image (8 bits/pixel)
Stego Image (8 bits/pixel)
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EPICS Spring 2003 Slide 60
Steganalysis• Examination of suspect images:
– Visual inspection (color artifacts, graininess, excess noise)
– Stochastic or statistical analysis– Histogram analysis
• Extracting an embedded message without knowledge of stego key
• Deducing the stego key
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EPICS Spring 2003 Slide 61
Overview of S-Tools Software
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EPICS Spring 2003 Slide 62
S-Tools 4.0 Overview• Created in 1996 by Andrew Brown
– Not compatible with previous versions of S-Tools– Software handles both encoding and decoding
• Capacity is 3 bits/pixel with ~128 bits overhead
• Different methods of embedding for– 24-bit images ( BMP )– 8-bit images ( BMP, GIF )– Does not support JPEG, TIFF, PNG images– Can also hide data in audio ( WAV ) files
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EPICS Spring 2003 Slide 63
S-Tools 4.0 Encoding Process
Message Processing
Cover Image
Stego Key
Message (Files)
Message Embedding
Image size
Stego Image
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EPICS Spring 2003 Slide 64
Message Processing
• Goal: Encrypt (scramble) the message
Passphrase
Message (Files)
Overhead Compression (Optional)
Z-Lib
Encryption Permute Bits Encrypted Message
M M*
K
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EPICS Spring 2003 Slide 65
Message Encryption Details
• Cipher Feedback Mode (CFB)• User can select block cipher (defaults to IDEA)
Key Generator
MD5 (128-bit)
Block Cipher Feedback Buffer Block
Cipher Key
64-bit buffer Initialization
IDEA, DES, 3DES, MDC Passphrase
Message + Overhead X
K
K1
K2
To Bit Permutation Y
Binary XOR
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EPICS Spring 2003 Slide 66
Bit Permutation Details
• Message bits are shuffled randomly– The decoder can reverse shuffling process with
knowledge of the passphrase– Complicates message recovery for an attacker
– Permutation performed on the entire message, not 64-bit blocks
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EPICS Spring 2003 Slide 67
Message Embedding
• Goal: Embed the encrypted message into cover image by altering pixel values
– The altered image becomes the stego image
• Embedding process differs for 24-bit images and 8-bit images
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EPICS Spring 2003 Slide 68
Message Embedding in 24-Bit Images
• One encrypted message bit is embedded in the LSB of the Red, Green, and Blue values of each pixel
1 1 1 1 0 0 0 x Red
Bits MSB LSB
1 1 0 1 1 0 0 x Green
1 1 0 0 1 1 1 x Blue
These bits are changed to that of the encrypted message
RGB values of pixel
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EPICS Spring 2003 Slide 69
Embedding in 8-Bit Images• Problem: Pixel values represent index in color table, not
intensity values– In general, changing color indices renders image
useless
• Strategy: Reduce number of colors in the image, then embed message in reduced color image
– User also has choice to convert 8-bit image to 24-bits and using 24-bit embedding
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EPICS Spring 2003 Slide 70
Embedding in 8-Bit Images
• Color reduction reduces colors in image• Dithering can improve visual quality• 8-bit embedding inserts encrypted message
Color Reduction
Cover Image
Dithering (Optional)
Median-Cut Floyd-Steinberg
8-bit Embedding
Stego Image
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EPICS Spring 2003 Slide 71
Color Reduction
• Median Cut Algorithm described inP. S. Heckbert, “Color image quantization for frame buffer display”, Computer Graphics, vol. 16, no. 3, pp. 297-303, 1982.
– Used in S-Tools to reduce the number of colors in the 8-bit image from 256 to 32
– Color reduction can be noticeable, usually reducing visual quality of image
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EPICS Spring 2003 Slide 72
Color Reduction Examples
256 color “Fruit” 32 color “Fruit”
256 color “Barbara” 32 color “Barbara”
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EPICS Spring 2003 Slide 73
Dithering
• Dithering process improves visual quality by reducing “contouring” effects
R. Floyd and L. Steinberg, “An adaptive algorithm for spatial grayscale,” Proceedings of the Society for Information Display, vol. 17, no. 2, 1976, pp. 75-77.
– Floyd-Steinberg dithering does not change color table or increase the colors used
• User can disable dithering if desired (default is enabled)
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EPICS Spring 2003 Slide 74
Dithering Examples
256 color “Fruit” 32 color “Fruit”
256 color “Barbara” 32 color “Barbara”
32 color “Fruit” (dithered)
32 color “Barbara” (dithered)
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EPICS Spring 2003 Slide 75
Message Embedding in 8-Bit Images
• Message is embedded in least 3-LSBs of each pixel– A 256 entry color table is constructed for stego image,
but every 8 color entries is similar (differing LSB)• Thus, the lowest 3-LSBs can be changed to any combination and
image will appear similar• Palette entries may be shuffled after embedding to avoid suspicion
c c c c c x x x Color Index
Bits MSB LSB
Selects 1 of the 32 colors used in the
reduced color image
Set to match 3 bits of the encrypted
message
Palette
RGB values of pixel
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EPICS Spring 2003 Slide 76
Summary
• Steps: Message Processing, Message Embedding– Message Processing encrypts messages– Message Embedding inserts the encrypted message
into cover image• Message embedding differs for 8-bit and 24-bit cover
images– 24-bit: Straightforward LSB embedding– 8-bit: Color reduction, dithering, 3-LSB embedding
• Decoding is straightforward
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EPICS Spring 2003 Slide 77
Attacking S-Tools
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EPICS Spring 2003 Slide 78
Cover Image Issues• Cover image can strongly influence steganalysis• Poor cover images can produce stego images that can be
suspect by visual inspection
• Poor cover images include:– Images with large monochromatic regions– Images with smooth transitions in color– Images with too few colors– 8-bit images with too many colors– Computer generated images– Decompressed JPEG images [Fridrich]
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EPICS Spring 2003 Slide 79
Original Fruit (24 bpp)
LSB Original Fruit
Stego Image (24 bpp)
LSB Stego Image
Is LSB Independent of Image?
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EPICS Spring 2003 Slide 80
Statistical Steganalysis vs. LSB
• Attacks exploiting Pair of Values– Chi-squared comparison between suspect image and
image with pair-wise color values [Westfeld]– Raw Quick Pairs technique [Fridrich]
• Examining smoothness of groups of pixels (RS) [Fridrich]
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EPICS Spring 2003 Slide 81
Other Attacks
• Palette Analysis [Johnson]– S-Tools creates “clusters” of similar colors in palette
for 8-bit images because of color reduction– Attack examines palette for color clustering
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EPICS Spring 2003 Slide 82
Conclusions
• S-Tools steganography uses least-significant bit (LSB) embedding for embedding message
• For many cover images, LSB is not independent of the image content
• Many methods for inspecting suspect images• Determining message without stego key or the stego key
itself may be difficult
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EPICS Spring 2003 Slide 83
References
• S-Tools 4.0 software by Andrew Brown ([email protected], 1996)– Z-LIB compression component by Jean-loup Gailly and Mark Adler– CRYPTLIB encryption component by Peter Gutmann, Eric Young and Colin
Plumb
• P. S. Heckbert, “Color image quantization for frame buffer display”, Computer Graphics, vol. 16, no. 3, pp. 297-303, 1982.
• R. Floyd and L. Steinberg, “An adaptive algorithm for spatial grayscale,” Proceedings of the Society for Information Display, vol. 17, no. 2, 1976, pp. 75-77.
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EPICS Spring 2003 Slide 84
Steganalysis References• J. Fridrich, M. Goljan, “Practical steganalysis of digital images—State of the art,”
Proc. SPIE Security and Watermarking of Multimedia Contents IV, vol. 4675, San Jose, CA, January 2002.
• J. Fridrich, M. Goljan, and R. Du, “Steganalysis based on JPEG compatibility,” Proc. SPIE Multimedia Systems and Applications IV, Denver, CO, August 20-24, 2001.
• J. Fridrich, R. Du, and L. Meng, “Steganalysis of LSB encoding in color images,” Proc. IEEE International Conference on Multimedia and Expo, New York, NY, July 30-August 2, 2000.
• N. Johnson and S. Jajodia, “Steganalysis of images created using current steganography software,” Lecture Notes in Computer Science, vol. 1525, Springer-Verlag, Berlin, 1998, pp. 273-289.
• A. Westfeld and A. Pfitzmann, “Attacks on steganographic systems: Breaking the steganographic utilities EzStego, Jsteg, Steganos, and S-Tools—and some lessons learned,” Proc. Workshop on Information Hiding, Dresden, Germany, September 29-October 1, 1999.
• R. Chandramouli, “A mathematical approach to steganalysis,” Proc. SPIE Security and Watermarking of Multimedia Contents IV, vol. 4675, San Jose, CA, January 2002, pp.14-25.
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EPICS Spring 2003 Slide 85
“Anti-Watermarking” Techniques
• UnSign software– http://altern.org/watermark/
• StirMark– http://www.cl.cam.ac.uk/~fapp2/watermarking/image
_watermarking/stirmark/
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EPICS Spring 2003 Slide 86
Research at Purdue
• Fragile and semi-fragile watermarks for forensic imaging
– are fragile watermarks better than hashing?• Extending concept of robust image adaptive watermarks
to video (with Chris Podilchuk)– is there a temporal masking model that works?
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EPICS Spring 2003 Slide 87
VW2D Watermarked Image
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EPICS Spring 2003 Slide 88
VW2D Difference Image
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EPICS Spring 2003 Slide 89
Results - Girls
← Original “Girls”
Altered “Girls” →
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EPICS Spring 2003 Slide 90
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EPICS Spring 2003 Slide 91
Video Watermarking Issues
• A video sequence cannot simply be treated as an ordered collection of images:
– visibility issues in the use of “still” image watermarks– visibility issues in stop frames– human perception of motion is not accounted for in
visual models for still images – embedding the same watermark in all the frames of a
video sequence is not secure, an attacker can correlate across the entire sequence to estimate the watermark (temporal collusion)
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EPICS Spring 2003 Slide 92
Video Watermarking Issues
– embedding completely different watermarks in successive frames of a video sequence is not secure
– successive video frames are highly correlated, an attacker can exploit this to estimate and remove a watermark
– the techniques for compressing video do not necessarily encode each frame of the sequence identically
– the synchronization of the audio with the video sequence may be a consideration for watermark protection
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EPICS Spring 2003 Slide 93
Video Watermarking
• Use still image approaches– may have problem in MPEG with B and P frames
• Hash parts of the compressed video stream• Techniques could be used to prevent multiple viewing,
copying, and editing (e. g. inserts)• Can the watermark survive the conversion back to an
analog signal?
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EPICS Spring 2003 Slide 94
Video Watermarking
• Cannot trivially extend image watermarking techniques– additional attacks are possible– computationally very expensive
• Unique attacks on video watermarks– frame shuffling / insertion– inter-frame collusion
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EPICS Spring 2003 Slide 95
Watermarking of Compressed Video
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EPICS Spring 2003 Slide 96
Digital Video Compression
• Uncompressed digital video:
Video Dimensions Bits/pixel Frames/sec Bits/sec CIF 352 x 288 12 24 29,196,288 CCIR601 720 x 480 16 30 165,888,000 HDTV 1920 x 1080 20 60 2,488,320,000
• Requires lots of network bandwidth or storage space
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EPICS Spring 2003 Slide 97
Digital Video Compression
• Motion Compensated Block DCT Coding– MPEG-1, MPEG-2, MPEG-4, H.261, H.263
• Remove spatial and temporal redundancy– Discrete Cosine Transform– Block-based motion compensation
• Quantization• Entropy coding
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EPICS Spring 2003 Slide 98
Spatial Image Coding (DCT)• Very similar to JPEG
• After DCT and quantization, many coefficients are zero
• Non-zero coefficients are coded:– Location (zig-zag)– Quantization index/value
• Zero coefficients not coded
µ
?
0 1 2 3 4 5 6
7
0 1 2 3 4 5 6 7
DC
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EPICS Spring 2003 Slide 99
Temporal Prediction• Successive frames of video typically similar
– Differences often in areas of motion
• Strategy: Predict contents of frame by indicating displacement of blocks from previously decoded frame(s)
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EPICS Spring 2003 Slide 100
Temporal Prediction
• Encoding (Motion Estimation):– Computationally expensive search– For each block in predicted frame, must find most
suitable match in reference frame
• Decoding (Motion Compensation):– Relatively simple block copying
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EPICS Spring 2003 Slide 101
Temporal Prediction
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Beginning of stream (Decoded First)
End of stream(Decoded Later)
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EPICS Spring 2003 Slide 102
Compressed Video Data
• I-Pictures: “JPEG”, no motion vectors Header Encoded DCT Coefficients of Picture
• P/B-Pictures: Motion vector + DCT of residual
Header Motion Vector Data DCT Coefficients of PEF
• Other data in compressed video stream:– Synchronization and multiplexing– Audio
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EPICS Spring 2003 Slide 103
Watermarking Compressed Video• “Classical” approach:
– Decompress the original video– Insert watermark– Compress the watermarked video
• Disadvantages:– Computationally expensive– Compression can damage watermark– Must insert watermark with excess strength– Watermark embedder does not know compression
parameters– Re-compression can degrade video further
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EPICS Spring 2003 Slide 104
Watermarking Compressed Video
• Compressed-domain approach– Partially decode the compressed video– Insert watermark by altering syntactic elements of
video (such as DCT coeffs)– Re-assemble the compressed video stream
• No motion estimation during re-assembly• Watermark embedder ensures that alterations to video
stream preserve decodeability
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EPICS Spring 2003 Slide 105
Watermarking Compressed Video• Advantages
– Lower computational cost– Watermark embedder can make informed decisions
based on compression parameters– Do not need to embed with excess strength
• Disadvantages:– Must parse compressed video data during watermark
embedding– Watermark insertion is constrained by allowable
syntax / semantics of compressed video stream
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EPICS Spring 2003 Slide 106
Watermarking Issues• General watermarking issues:
– Capacity, robustness, perceptibility, security– Synchronization– Attacks– Computational complexity
• Compressed domain issues:– Drift compensation– Preservation of data rate
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EPICS Spring 2003 Slide 107
Drift Compensation• Drift occurs when a predictor is modified without
adjusting the residual
3 6 4 2 2 2 4 4 Original Signal
3 +3 -2 -2 0 0 2 0
Predictive (Differential) Coding
Residual
Original Coded Signal
3 +3 -2 -2 0 0 2 0
Watermark +1 +1 Watermarked Signal 3 +3 -1 -2 0 1 2 0 Reconstructed Watermarked Signal 3 6 5 3 3 4 6 6
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EPICS Spring 2003 Slide 108
Drift Compensation
• Must compensate for drift error during watermarking
Original Coded Signal
3 +3 -2 -2 0 0 2 0
Watermark +1 +1 Drift Compensation Signal -1 -1 Watermarked Signal 3 +3 -1 -3 0 1 1 0 Reconstructed Watermarked Signal 3 6 5 2 2 3 4 4
3 6 4 2 2 2 4 4
Original Signal
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EPICS Spring 2003 Slide 109
Preservation of Compressed Data Rate
• Watermark signal is often “noisy” pseudo-random number sequence
• “Noisy” sequences are difficult to encode, causing the data rate of the watermarked video to increase, sometimes substantially
• Compressed domain watermark embedders must control the data rate
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EPICS Spring 2003 Slide 110
Hartung’s Technique
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EPICS Spring 2003 Slide 111
Hartung’s Technique
• Bitstream is parsed to obtain encoded DCT coefficients and motion vectors
– Motion vectors are not watermarked– Spread spectrum signal is inserted into the DCT
coefficients
• Drift compensation
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EPICS Spring 2003 Slide 112
Hartung’s Technique• Not all coefficients are watermarked:
– Zero coefficients– If embedding watermark increases data rate
• Evaluated for high data rate video (4-12 Mbits/s)• Spread-spectrum watermark fairly robust against signal
processing attacks
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EPICS Spring 2003 Slide 113
Hartung’s Technique
• Problems:– Not many coefficients are actually watermarked
because of rate constraints (10% or so)– Thus, watermark may be more vulnerable to removal– Method of bit-rate control may not be applicable for
low-rate video
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EPICS Spring 2003 Slide 114
Langelaar’s Technique
• Watermark key determines block groupings
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EPICS Spring 2003 Slide 115
Langelaar’s Technique
• Low computational cost• Original technique only watermarked I-pictures; has
been extended for P/B-pictures– Not very robust against attack, particularly against
transcoding– Technique may have problems with low-rate video
• Evaluated using high rate video (4-8 Mbits/sec)
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EPICS Spring 2003 Slide 116
Conclusions
• Overview digital video compression• Overview issues when watermarking compressed video• Reviewed two different means for watermarking
compressed video
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EPICS Spring 2003 Slide 117
Temporal Synchronization in Video Watermarking
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EPICS Spring 2003 Slide 118
Synchronization
• Synchronization is necessary for reliable detection of many watermarks
• The detector establishes correspondence between the coordinates of the watermarked signal and the watermark
Watermarked Attacked
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EPICS Spring 2003 Slide 119
Synchronization Attack Example
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EPICS Spring 2003 Slide 120
Synchronization
• Spatial synchronization– Attacks: Scaling, rotation, translation, warping
• Temporal synchronization– Initial synchronization– Re-synchronization after bit/decoding errors– Attacks: Frame deletion, insertion, transposition,
averaging
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EPICS Spring 2003 Slide 121
Synchronization Techniques
• Fast synchronization techniques desired– Avoid computationally expensive searches– Real-time video applications
• Techniques– Sliding correlator– Templates
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EPICS Spring 2003 Slide 122
Sliding Correlators
• Correlate over all spatial and temporal shifts x0, y0, t0
• In general, very expensive search over a large search space
∑∑∑ −−−x y t
ttyyxxYtyxW ),,(),,( 000
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EPICS Spring 2003 Slide 123
Synchronization Techniques• Templates
– Additional signal embedded into video for fast synchronization
– Disadvantages:• Templates must be easily detectable, and thus vulnerable to
attack and removal• Template embedding adds distortion in the watermarked
video, affecting perceptual quality
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EPICS Spring 2003 Slide 124
A Synchronization Framework
• Assume a symmetric video watermarking technique is used
• Embedder uses key K(t) to create the watermark embedded in frame t
– If detector can deduce K(t): Synchronization achieved– If detector cannot deduce K(t): Synchronization lost
• K(t) often corresponds to watermark signal generator state
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EPICS Spring 2003 Slide 125
Temporal Redundancy
• Temporal redundancy: The degree by which the watermark signal can be deduced given the watermark signal in the past
• Synchronization search can be reduced by increasing temporal redundancy of watermark
– May be a security trade-off, as increased redundancy could make watermark more vulnerable to attack
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EPICS Spring 2003 Slide 126
Temporal Redundancy
• Time-invariant watermark– Very high temporal redundancy– Temporal synchronization is trivial (not needed)– Low security
K0 K0 K0 K0
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EPICS Spring 2003 Slide 127
Temporal Redundancy
• Independent watermark for each picture– Many watermarks implicitly use this model– No temporal redundancy– Synchronization is difficult or expensive– High security
K0 K1 K2 K3
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EPICS Spring 2003 Slide 128
Temporal Redundancy
• Repeating (periodic) watermark– Synchronization is trivial– Security better than time-invariant watermark but the
watermark may be deduced by inter-frame correlation
K0 K1 K2 K3
KT-1
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EPICS Spring 2003 Slide 129
A Protocol for Synchronization• Exploits temporal redundancy to allow fast
synchronization (no templates)• Time-invariant, periodic, time-independent watermarks
are special cases• Video-dependent
• Embedder: Temporal redundancy is controlled by period and repeat parameters
• Detector: Uses priority queue to perform and maintain synchronization
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EPICS Spring 2003 Slide 130
Watermark Embedding Protocol
Input Frame X(t)
Watermarked Frame
Y(t)
Finite State Machine
Temporal Redundancy
Control
Frame Analyzer
Watermark Key for Next Frame
K(t+1)
Feature Values Watermark W(t)
Watermark Generator
Embed
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EPICS Spring 2003 Slide 131
Temporal Redundancy Control
• a (period) = # of frames watermarked before resetting the state machine to initial state and watermark key to the initial key K0
• ß (repeat) = # of frame watermarked by identical watermark before watermark key is changed
a=8, ß=2 0 1 2 3 4 5 6 7 8 9 10 11 12 13
time
Numbers indicate frame index
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EPICS Spring 2003 Slide 132
Frame Analyzer
• Analyzes the watermarked frames, output is a vector of feature values
– Used with the state machine to generate the next watermark when needed
– Allows the watermark to be video dependent
– Features should be robust and not change in value unless significant change occurs to image
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EPICS Spring 2003 Slide 133
State Machine
• Each state describes how to generate next watermark K(t+1) from the current watermark K(t)
• The state machine itself has redundancy because possible number of next states from a given state is finite
State 0 Ki = K0
State 1 Ki+1 =Ki + c1X
State 2 Ki+1 = ~Ki
State 3 Ki+1 = Ki+c2Y
State 4 Ki+1 = Ki+c3Z
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EPICS Spring 2003 Slide 134
Watermark Detection Protocol
Test Picture Z(t)
Frame Analyzer
Watermark Detector
Watermark
Memory (Queue)
State, Key
Watermark Generator
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EPICS Spring 2003 Slide 135
Watermark Detection Protocol• Detector does not know a, ß• The detector tries the following keys:
– The initial key K0
– Every key value stored in the queue
• If no watermark found: Queue not updated• If watermark found:
– Move (state,key) to top of queue– Use state machine, features to find (next state, next
key) and insert into top of queue
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EPICS Spring 2003 Slide 136
Experimental Setup• Uncompressed video (4:1:1 352x288 CIF)• Attacks
– Frame dropping– Frame insertion
• Inserted frames are not watermarked
– Local frame transposition– Frame averaging– Combined attack
• Ten trials for each attack, average detection results are shown
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EPICS Spring 2003 Slide 137
Experimental Setup
• Watermark: Spatial domain Gaussian• Detection: Correlation detector• State Machine: Previously shown• Features:
Feature X = Quant[ Mean( Pixels in X )]
Feature Y = Quant[ Mean( Pixels in Y )]
Feature Z = Quant[ Mean( Pixels in Z )]
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EPICS Spring 2003 Slide 138
Frame Drop
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EPICS Spring 2003 Slide 139
Frame Insertion
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EPICS Spring 2003 Slide 140
Frame Transposition
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EPICS Spring 2003 Slide 141
Frame Transposition II
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EPICS Spring 2003 Slide 142
Frame Averaging
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EPICS Spring 2003 Slide 143
Combined Attack
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ueCombined Attack (10% Insert, 10% Transpose, 10% Drop, Averaging)
Not Attacked Deletion Insertion Transpos ition
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EPICS Spring 2003 Slide 144
Combined Attack
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Redundancy ( Period [pictures] / Repeat [pictures] )
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5% 10% 25%
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EPICS Spring 2003 Slide 145
Improvements
• Improving security– Current weakness: Correlation can find frames
watermarked with K0
– Resynchronization watermark does not have to be K0. Can be video-dependent:K = f(K0, X, Y, Z)
– State machine: Ad-hoc design
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EPICS Spring 2003 Slide 146
Improvements
• Improving robustness– Change watermark based on feature values and not
on a, ß parameters. Can improve picture deletion and averaging performance
– Find better or more robust features– Find better means of adding temporal redundancy
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EPICS Spring 2003 Slide 147
Conclusions and Future Work• General method of temporal synchronization by
redundancy in the watermark• A small search is performed instead of using templates,
but technique does not preclude the use of templates
• Investigate more appropriate attack models• Investigate features for compressed video
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EPICS Spring 2003 Slide 148
Current Research Issues
• Theoretical Issues– capacity and performance bounds– models of the watermarking/detection process
• Robust Watermarks– linear vs. nonlinear– scaling and other geometric attacks – watermarking analog representations of content– new detection schemes– what should be embedded (watermark structure)
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EPICS Spring 2003 Slide 149
Audio Watermarking
Use of techniques similar to images– perceptual models are better developed– attacks are different
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EPICS Spring 2003 Slide 150
“Watermarking” Standards
• Data Hiding Subgroup (DHSG) of the Copy Protection Technical Working Group (CPTWG)
– two groups have proposed systems for watermarking video used in DVD
– watermark will be second level of security after encryption
– the watermarks are relatively easy to defeat by difficult to remove
• SDMI (digital audio)• MPEG-4/MPEG-21
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EPICS Spring 2003 Slide 151
Watermarking: Legal/Political Issues
• Watermarking technologies have not been tested in court– is watermarking the “feel good” technology of
multimedia?• Might one be better off just doing timestamping and/or
other forms of authentication?• What does it mean when a watermarking technique
survives an attack (verification based on statistical tests)• Watermarking may always be the secondary security
method
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EPICS Spring 2003 Slide 152
Legal Issues
• When one says: “My watermarking withstands the X attack!”
– What does it mean? (Has the watermark been damaged?)
– It is legally defensible?– Nearly all watermarks require statistical tests for
verification
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EPICS Spring 2003 Slide 153
Unauthorized Distribution and Illegal Copies
• Unauthorized Distribution– You took my image from my web site!– You are selling my image from the CD-ROM you
bought from me!• Who owns it? ⇒ hash and timestamp• Is your image the same as mine? (derived work)
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EPICS Spring 2003 Slide 154
Conclusions
• The “secure” multimedia system is evolving• Simple add-ons will not work (not like the text-based
systems)• Exploit the unique nature of the type of data • Digital watermarking is crucial to secure networked
multimedia systems• Time stamping is important• New techniques tolerate changes to images, and are
compatible with compression
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EPICS Spring 2003 Slide 155
Conclusions
• Watermarking is still an interesting research area with many interesting problems
– where will it be useful? – will watermarking only be used a second-tier security
system? – will there be significant theoretical developments?
• Is watermarking the “feel good” technology of multimedia?
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EPICS Spring 2003 Slide 156
Reference
R. B. Wolfgang, C. I. Podilchuk, and E. J. Delp, “Perceptual watermarks for digital images and video,” Proceedings of the IEEE, Vol. 87, No. 7, July 1999, pp. 1108-1126.
Available at:ftp://skynet.ecn.purdue.edu/pub/dist/delp/watermark-
proceedings/