claudia schremmer / university of mannheim / germany digital image watermarking claudia schremmer...

Claudia Schremmer / University of Mannheim / Germany

Digital Image Watermarking

Claudia SchremmerUniversity of Mannheim /

Germany

September 2001

Teleseminar on Electronic CommerceIntroductory Week: 24.-28.9.2001 in Nice

Claudia Schremmer / University of Mannheim / Germany page 2

Overview

• 1. Introduction

• 2. Digital Watermarking

• 3. Attacks

• 4. Algorithm of Zhao/Koch

• 5. Conclusion

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PreludeP

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Page from a prayer book showing the baking of unleavened bread (15th century) from the Illuminated Haggadah Exhibit.


1. IntroductionIn former times...

• Traditional content media yield degraded content when copied:– paper documents,

– analogue recordings,

– celluloid film,

– canvas paintings, and

– marble sculptures

• Or they require expensive and specialized equipment to produce high-quality copies.

• Thus, the technical burden on traditional content creators for protecting their material has been minimal.

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Nowadays...

• High quality (even: identical) copies are easy to produce.

• There is apparently no limit to the value that can be added by creating and providing access to digital content.

• Creators and owners of (digital) content need ways to protect their property rights in the increasingly digital world.

• Distributors and recipients of content need ways to fully utilize the rich potential of their digital assets.

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Example

• Is this photo original?

• Is it fake / has it been modified?

• Who owns the right?

• Are we allowed to use this photo in our seminar?

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Robbie Williams


Definition1.

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Aspect ofSecurity

Description Example

Access Control Control of access toa system andlimitations to systemfunctions and data

Firewall

Authentification Proof of identity ofthe author andgenuineness of adocument

Digitalsignature,digitalwatermarks

Confidentiality Ensures thatunauthorized userscannot access data

Encryption

Integrity Proof that the dataare unaltered

Digitalsignature

Copyright Protects intellectualproperty

DigitalWatermarks


2. Digital Watermarking

• Digital watermarks are fixed to the underlying data.

• In contrast to the traditional way of watermarking paper or money, digital watermarks rely on steganography:

A digital watermark is included into a digital mediumthe way that the user does not perceive it.

Thus, the communication is secret.

• Two ways of secret communication:– The copyright information is transferred via a secret key while the

algorithm is known. Thus, only the user can access the information.

– No secret key exists, but the algorithm is kept secret.

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Practical Approach

• Substitutional steganography:A noisy component of the digital message is substituted by an encrypted secret message.

• Constructive steganography:The secret message is not included by means of replacement, but by means of reproduction of noisy signals, based on the model of the original noise.

Existing watermarking algorithms are based on the substitutional steganography.

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Types of watermarks

Subdivision of unperceptible (steganographic) watermarks:

– Unperceptible robust: Insertion of hidden information, copyright information, authentification, and meta information.

• Based on a modification of the pixel (for an image), example: modification of an amplitude in the blue channel.

• Or based on a modification in the frequency domain, example: after DCT or wavelet transformation.

– Unperceptible fragile: Aim to proof the integrity of a document. They are destroyed on purpose in order to detect manipulation.

• Based on content related algorithms like the digital signature.

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3. Attacks: unintentional

There are a number of unintentional and intentional attacks:

• Unintentional attacks:– Image: compression, transcoding, printing/scanning, filtering,

noise, geometric transforms, cropping, compositing/mosaicing,...

– Video: AD/DA conversion, compression, transcoding, text/logo insertion, geometric transformations, jitter, cropping,...

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Attacks: intentional

• Intentional attacks:– watermark removal/interference:

• denoising, compression, quantization, remodulation, blurring, averaging,...

– Desynchronization (detector disabling):

• cropping, affine and projective transforms, jittering, mosaicing, collage,...

– Cryptographic:

• key determination (brute force), Oracle attack (i.e., generate unmarked data by trial and error)

– Protocol:

• copy attack, printing/rescanning,...

Watermark research must include work on attacks!

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Example (I)3.

Att

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original

original

wavelet compr.94% crumple & scan

warp mosaic


Example (II)3.

Att

acks composition of wavelet compressed house and warped bear


Constraints

• Human visual perception shall not be disturbed by the watermarks.

• The capacity for watermarks differs according to the number of bits allocated.

• Robustness regards the performance towards intentional and unintentional attacks.

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visibility

robustnesscapacity


4. Algorithm of Zhao/Koch

Jian Zhao and Eckhard Koch: Embedding robust labels into images for copyright protection. In: Proc. of the International Congress on Intellectual Property Rights for Specialized Information, Knowledge and New Technologies. Vienna, Austria, August 1995.http://www.mediasec.com/know/papers.html

This is the initial paper which initialized a whole new research area.

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Outline of the Zhao/Koch Algorithm

The paper of Zhao/Koch describes a pool of novel steganographic methods for secure and robust insertion of watermarks into digital images. The included watermark is neither detectable, nor movable/relocatable, nor changeable. Furthermore, it survives attacks which do not strongly the image quality, such as lossless compression, low pass filtering (i.e., smoothing) and transcoding.

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Fundamentals (I)

• System for Copyright Protection: SysCoP

• The system supports gray value, color, and binary images.

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Labelled Image (y’)

Position Sequence GeneratorTs(y, Uk)

Label Embedding System

Position Sequence (PS)

Image Data (y)

User Key (Uk)

Label Code (C)

write a watermark

• Two steps to write a watermark:– Generation of a pseudo-random sequence of positions in order

to select DCT-encoded blocks in which the code is embedded. Here, this step is accomplished by the function

– Embedding of the code into the block selected by the sequence of positions.

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Fundamentals (II)

• The reading process reverses the writing process:– Reading the code from the block selected by the sequence of

positions.

– Retrieve the label inserted.

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Position Sequence GeneratorTs(y, Uk)

Label RetrievalSystem

Position Sequence (PS)

Image Data (y)

User Key (Uk)

Embedded Code (C)

read a watermark


Remarks

• In the following: consideration of gray value images only.

• A block is of size 8x8 pixels. It can be contiguous or distributed. – A contiguous block is a square part of an image.

– A distributed block is a random agglomeration of 64 pixels of the image.

• It is difficult to determine an optimal function . Therefore, only the width and the height of an image are actually used to generate the block positions.

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Framework(write)

Algorithm: Framework(write).– (1) If i n, return.– (2) Randomly select a block b, using the position sequence

generation function Ts(Uk, y).

– (3) If b exists already in B, go to (2), otherwise add b to B.

– (4) Call check_write(b, ci) to check weather b is a valid block: if this function returns False, go to (2).

– (5) Call write(b, ci) to embed a bit ci to the block b.

– (6) Increment i, go to (1).

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– The code C to be embedded is represented by its binary bit sequence:

– i marks the current bit in the sequence

– B is the set of blocks of which a block is randomly selected.

– The initialization is: i=0, B={}.

ncccC ...,, ,10


Framework(read)

Algorithm: Framework(read).– (1) If i n, return.

– (2) Randomly select a distributed or a contiguous 88 block b, using the position sequence generation function Ts(Uk,y).

– (3) If b exists already in B, then go to (2), otherwise add b to B.

– (4) Call check_read(b, ci) to check weather b is a valid block: if this function returns False, go to (2).

– (5) Call read(b) to retrieve a bit from the block b.

– (6) Increment i, and go to (1).

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JPEG-based Code Insertion

In the framework, the following algorithms are still undefined:– check_write(b, ci)

– write(b, ci)

– check_read(b, ci)

– read(b, ci)

They depend on the coding format of the image. We restrict our future considerations to JPEG-encoded images:

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Image pre-processor

Pixel

Block

Image encoder

DCT

Quantization Entropy encoding:

run lengthHuffmanarithmetic


Visual Perception

• General idea: Insert watermarks in positions where the human visual system does not or nearly not perceive a modification of the coefficients.

• Here: Insertion in the smooth background areas, i.e., in the white areas of the right picture.

• The coefficients concerned are of the middle frequencies in the DCT-transformed domain.

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Possible Positions

Statistic and empirical research has defined possible positions to include a code into a block.

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0 1 2 3 4 5 6 70 2 31 9 10 112 16 17 1834567

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Demonstration4.

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5. Conclusion

• We have not...– ... detailed any precise algorithm.

– ... discussed other media than images. For audio, this would e.g. mean entering the field of research on human audible properties.

– ... discussed the various attacking algorithms like e.g. STIRMARK.

• We have...– ... given an introduction and an overview on the purpose of digital

watermarking.

– ... outlined the challenge of creating a robust algorithms to unintentional and intentional attacks.

– ... restricted our considerations on images.

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Info

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Information...

Claudia SchremmerUniversity of Mannheim, GermanyDepartment Praktische Informatik IVhttp://www.informatik.uni-mannheim.de/informatik/[email protected]

claudia schremmer / university of mannheim / germany digital image watermarking claudia schremmer...

Documents

digital watermarking

digital message

introduction slide

digital medium

digital assets

digital world

prelude page

nice slide