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Chapter 7: Digital Watermarking and Copy(right) Protection

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Digital recording

The biggest advantages of digital recording: no quality degradation over time, transmission,

copying; contents can be transmitted and “sold” over the

digital network

Problem: difficult to guard against piracy

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Digital watermarking, cryptography, and steganography

One way to protect your content from being stolen is to encrypt it. You make sure that only an intended (paid) recipient has the key to decrypt the content.

Unfortunately, cryptography does not prevent the recipient from making copies of the content and re-sell them.

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Hardware key

To prevent re-distribution, we can encrypt the file using a key that is dependent on the user’s hardware e.g., a palm device or a PC has a unique processor id

Before a user download a (paid) file, he will have to send his machine’s id to the server. The server then encrypts the file based on the id.

To use/process/read the file, the user has to download a special program, which will access the hardware id and use that to decrypt the file. There should be no other way to read the file content.

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Hardware key

shop

user

hardware key (k), money

k

shop

user

encrypted file, F’, reader program P

k

F userk P F’

user’k’ P F’

another user obtaining P and F’ would not be able to read the file since he does not have the rightprocessor id.

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Hardware key

Disadvantages: a user can only read the file on one machine only users cannot “upgrade” their hardware without

losing the paid content if the hardware is broken, the file can no longer

be read

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Digital watermarking, cryptography, and steganography

Another possibility is to hide some secret information into your content to prove your ownership.

Ben’s photo It is my photo, not

yours

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Steganography

literally means “covered writing” refers to hiding information in ways that

prevent the detection of hidden messages used to communicate information without

letting others even notice that a communication is being taken place

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Steganography

Example: A German spy sent the following “harmless” message in WWII 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.

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Steganography

If we extract the 2nd letter in each word, we get: 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.

Or the secret message: Pershing sails from NY June 1.

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Another example

“This hidden article needs keeping safe from other renegade Yeomen of unscrupulous repetition awaiting to theorize every new technological idea of nonsense!”(taken from DataMark Technology)

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A third example

AND

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Digital watermarking

Similar to steganography, watermarking is about hiding information in other data. The difference is that a watermark should be somewhat resilience against attempts to remove it.

There are watermarking techniques for embedding information in text image audio video

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Terminology

cover data (image) – data (image) to be protected

watermark – secret data to be hidden in a cover

watermarked data – cover data + watermark

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Watermarking applications

Ownership assertion – a rightful owner can retrieve the watermark from his content to prove his ownership.

Fingerprinting – an owner can embed a watermark into his content that identifies the buyer of the copy (c.f. serial number). If unauthorized copies are found later, the owner can trace the origin of the illegal copies.

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Watermarking applications

Authentication – the creator of a content can embed a fragile watermark into the content to provide a proof of authenticity and integrity. Any tampering of the original can be detected.

Visible watermarking – a visible watermark (an image) can be embedded on a “preview” to destroy its commercial value.

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A visible watermark

taken from IBM research magazine

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Invisible watermark

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LSB substitution

Let wi be the ith bit of the watermark

Let pj be the jth pixel of a cover image

LSB substitution simply converts the least significant bit of pi to wi.

Since only the LSBs are changed, the modification does not cause much perceptible noise to the cover.

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LSB substitution with a secret key

If you want to enhance the watermark secrecy, we can insert the watermark bit at random pixels, dependent upon a key.

A user selects a secret key, k. A sequence of pseudo-random numbers ri’s are generated using k as the seed.

Example: pick a number m and a number a r1 = (k mod m) + 1 ri = (a * ri-1 mod m) + 1

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LSB substitution with a secret key

suppose the numbersequence is:3 6 2 3 7 1 3 2 …

only if one knows thekey does one knowhow to retrieve thewatermark

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Robustness

LSB substitution is not robust against attempts to remove the watermark. A simple modification of the image (e.g., by

setting all LSBs of all pixels to ‘0’) can wipe out the watermark.

Transcoding (such as converting the image into lossy JPEG file) can also remove the watermark effectively.

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A DCT-based technique

Given a cover image, first decompose it into a number of 88 blocks.

If the watermark contains n bits, use a secret key to locate n blocks at “pseudo-random” locations.

The ith block Bi is used to carry the ith bit of the watermark, wi.

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A DCT-based technique

Compare the DCT coefficients: u3v0 and u2v2, try to enforce the following: if u3v0 > u2v2 then wi = 1; wi = 0 otherwise

if the original values of u3v0 and u2v2 do not follow the above, swap them

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DCT-based watermarking

233.1 0.3 -9.8 -7.9 2.1 -0.1 -3.7 1.1

-25.5 -15.9 -3.5 -6.4 -2.9 2.1 -0.7 -1.5

-12.3 -8.5 -0.3 0.1 0.2 0.0 -1.1 -0.2

-6.4 -2.3 -0.4 2.2 0.9 -0.6 0.2 0.4

1.9 -2.2 -0.8 4.3 -0.1 -2.5 1.6 1.5

5.2 -2.0 -1.6 3.4 -0.8 -1.0 2.4 -0.6

2.0 -2.1 -3.3 2.1 -0.5 -0.6 2.3 -0.4

-0.6 0.5 -5.6 0.3 1.9 -0.2 0.2 -0.2

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DVD Region Code

8 regions:1: U.S., Canada, U.S. Territories2: Japan, Europe, South Africa, and Middle East (including Egypt)3: Southeast Asia and East Asia (including Hong Kong)4: Australia, New Zealand, Pacific Islands, Central America, Mexico,

South America, and the Caribbean5: Eastern Europe (Former Soviet Union), Indian subcontinent, Africa,

North Korea, and Mongolia6: China7: Reserved8: Special international venues (airplanes, cruise ships, etc.)

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DVD Region Code

A region code (an 8-bit byte) is recorded on a disc.

A player checks if its region code matches that of a disc.

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DVD copy protection

Traditional recording media (e.g., audio tape, VHS tape) for audio and video are analog.

Piracy is not too big of a concern because quality degrades with each copy generation.

With digital recording and high-resolution video, DVD copy protection was a big issue to the movie industry.

In fact, it took about 2 years after the invention of DVD to put DVD movies on the shelf. Part of it is due to the development of a reasonable copy protection scheme.

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CSS

Content Scrambling System proposed by the Copy Protection Technical

Working Group for DVD (CPTWG), IBM, Intel, Matsushita, Toshiba.

The idea: Alice sells Bob a video, in order for Alice to prevent Bob

from re-disseminating the video to others, Alice tries to make sure that Bob only accesses the video data on a trusted (or complaint) device.

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Trusted devices

A trusted device is manufactured by a trusted manufacturer.

A manufacturer is trusted if it joins the Copy Control Association (CCA)

A trusted device is given a (secret) player key ki. The trusted manufacturer has to sign an agreement

with CCA, basically barring it from making devices that could undermine the copy protection mechanism.

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CSS

In most cases, a DVD (video disk) is protected by the CSS scheme. Intuitively, the video content is encrypted using a disc key, k.

In the lead-in area of a CSS-protected DVD, the disk’s key k is encrypted about 400 times, each using a different player key.

A DVD player with the ith player key will read the ith entry of the key block. This entry is then decrypted using the player key ki to obtain the disk key k.

The video content is then decrypted on-the-fly while the movie is played.

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CSS

An off-the-shelf DVD writer will not copy the key block in the lead-in area, hence even the file on the DVD is copied, the copy will not have the key block. This makes the disk non-playable.

However, it is still possible that people make pirated copies with the lead-in key block.

Also, if someone decrypts a video, he can make pirated copies without CSS (i.e., not encrypted)

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CSS

Another possibility is to go for a digital-analog-digital route. One can feed the analog output (AV or components) from a compliant player to an MPEG encoder. The encoded MPEG file is then written on DVD disks (without CSS).

legalDVD

rgb

complaint player

MPEGencoder

DVDwriter

piratedDVD

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Is VCR illegal?

In the States, videotaping a TV program for time-shifting purposes does not constitute copyright infringement.

However, a person cannot make a copy of the copy. A digital video stream can be obtained from

DVD (may not be copied) Digital TV broadcast (can be copied once)

We need a mechanism to distinguish the 2 cases copy generation management system (CGMS)

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CGMS

A CGMS is a pair of bits in the header of an MPEG stream: copy_freely copy_never copy_once copy_no_more

When a compliant DVD recorder is given a copy_once video, it will change the CGMS bits to copy_no_more in the DVD copy it writes.

A complaint DVD recorder will refuse to write a video whose CGMS bits are copy_never or copy_no_more.

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CGMS

The CGMS bits in the MPEG header is not very secure. It can be easily removed.

Suggestions are there that the video should be watermarked with the CGMS bits – to make them very difficult to remove.

Watermarking can also help preventing piracy. If a compliant player detects a watermark (copy_never, copy_no_more) from a video that is itself un-encrypted (i.e., without CSS), the player should refuse to play.

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MacroVision

Analog Protection System (APS) prevent recording a DVD movie on

consumer tapes. A player with APS modifies the analog

signal output (e.g., by adding pulses to the vertical blanking signal). This confuses most VCR. The recorded pictures are significantly distorted.