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-selling 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
F
shop
hardware key (k), money
shop
encrypted file, F’,
reader program P
user
k
user
k P F’
user
k
user’
k’ P F’
another user obtaining P and F’
would not be able to read the file
since he does not have the right
processor 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 byproducts, 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 byproducts, 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 content destroys the fragile
watermark and thus 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
17
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 pseudorandom 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 number
sequence is:
36237132…
only if one knows the
key does one know
how to retrieve the
watermark
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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 88 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
0.3
-5.6
1.9 -0.2 0.2 -0.2
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DVD Region Code
 8 regions:
1: U.S., Canada, U.S. Territories
2: 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 Caribbean
5: Eastern Europe (Former Soviet Union), Indian subcontinent, Africa,
North Korea, and Mongolia
6: China
7: Reserved
8: 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 has joined 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.
Lead-in
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CSS
 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.
copy
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CSS
 However, with special hardware, 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).
unencrypted
decrypt
and write
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CSS
 Another possibility is to go for a digital-analogdigital 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).
unencrypted
complaint player
legal
DVD
r
g
b
MPEG
encoder
DVD
writer
pirated
DVD
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Is VCR illegal?
 In the States, videotaping a TV program for timeshifting 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 unencrypted (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.
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