Digital Watermarking. Introduction Relation to Cryptography –Cryptography is Reversibility (no...
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Transcript of Digital Watermarking. Introduction Relation to Cryptography –Cryptography is Reversibility (no...
DigitalWatermarking
Introduction
• Relation to Cryptography– Cryptography is
• Reversibility (no evidence)• Established
– Watermarking (1990s)• Non-reversible (noise)
– Information Hiding• Covert communication channel (steganography)
Digital Watermarking Media
• Video
• Audio
• Images– Our discussion will focus on this.
WatermarkingAlgorithm
WatermarkedImage
OriginalImage
WatermarkBlock Diagram
of image watermarking
Applications
• Copyright– The objective is to permanently and unalterably mark the image
so that the credit or assignment is beyond dispute.
• Digital Rights– A file may only be used by users with a license that matches the
watermarked signature.
• Information Hiding– Foil counterfeiters
• Revision History– Tamper detection
• Meta-tagging– Store keywords, descriptions, time along with images.
Criteria
• Main Criteria– Capacity– Payload – Computational Complexity– Transparency– Robustness
Require optimum relationshipRequire optimum relationship
Capacity
• The ability to detect watermarks with a low probability of error as the number of watermarks in a single image increases.
Payload
• The amount of information that can be legitimately stored within a data stream– Dependent on host medium– JPEG example
Computational Complexity
• Difficulty in process of watermark extraction– Realtime?
Transparency
• Transparency refers to the perceptual quality of the data being protected.– Watermark should be invisible over all image
types as well as local image characteristics.
• Need to consider perceptually insignificant portion of host image for insertion for maximum transparency
Robustness
• Resistance to attacks on the watermark– Attack – an operation performed on the image that
compromises the watermark– Active, Passive, Collusion, Forgery– Blind vs. Nonblind
• Use of non-robust watermarks– eg. tamper detection
Approaches and Implementation
• Two Types of Encoding– Spatial watermarking (spatial domain)– Spectral watermarking (frequency-domain)
• Many types due to variety of transforms• Adjustments made in frequency domain• More robust
Spatial-Domain Implementation
• Low-level Encoding
• Use of Image Analysis Operations– eg. Edge Detection/Color Separation
• Cons– Easily Attacked (Cropping)
Frequency-Domain Implementation
• Algorithm– Decomposition of image– Addition of Watermark
• Possibly encoded/encrypted
– Re-composition of Image
Frequency-Domain Implementation (Discrete Cosine Transform)
• Discrete Cosine Transform (DCT)– Used in today’s standard JPEG compression
• Relation to DFT• Compression explained by previous groups
– Image divided into non-overlapping blocks– Each block is DC transformed– Block coefficients are quantized through a special
algorithm
• Not ideal for human visual system
Frequency-Domain Implementation (Wavelet Transform)
• Wavelet Transform– Based on Short Time Fourier Transform
(STFT)– Becoming more common in compression
techniques• Better model of Human Visual System than DCT
Examples of Wavelets
Frequency-Domain Implementation(Common Wavelet Transform Algorithm - Decomposition)
Filter Bank Decomposition (10 Bands)
Frequency-Domain Implementation (Wavelet Transform Algorithm - Overview)
Watermarked Image
EncodedWatermark
Frequency-Domain Implementation (Cortex Transform)
• Cortex Transform– Recent– Mimics human visual system
• Corresponds to known structure of human eye
– Has its own disadvantages• Computational complexity – requires much more
data!
Other Issues
• Just Noticeable Difference (JND)– Threshold based on Human Visual System
• Adjustment in Frequency• Adjustments in Intensity
– Important impact on transparency
• Spatial adjustment of Frequency-Domain Watermark
Spread Spectrum
• Used to fulfill transparency criterion• The watermark in is based on spread spectrum
communications– Delivers narrowband data through a noisy channel, by
modulating each data symbol with a wideband (but very low amplitude) signal.
– The data is a single bit – a yes or no decision on whether the given watermark is present.
– The channel is the image data itself– The wideband signal is the watermark.
Color Images
• Scheme nearly identical to grayscale– R/G/B channels
• Each color plane treated as a separate image
– Luminance/Chrominance channels• Luminance = intensity• Chrominance = color
Resources• ftp://skynet.ecn.purdue.edu/pub/dist/delp/watermark-proceedings/paper.pdf• http://www.cosy.sbg.ac.at/~pmeerw/Watermarking/• http://www.cosy.sbg.ac.at/~pmeerw/Watermarking/MasterThesis/• http://www.eso.org/projects/esomidas/doc/user/98NOV/volb/node308.html• http://www.jjtc.com/Steganography/• http://www.mathworks.com/matlabcentral/files/3508/digital%20watermarking.pdf• Mihcak, Mehmet Kivanc. “Information Hiding Codes and Their Applications to Images
and Audio”, PhD Thesis. 2002.