T325: Technologies for digital media Second semester – 2011/2012 Block 2 – Intellectual Property...

Arab Open University – Spring 2012

T325: Technologies for digital media Second semester – 2011/2012Block 2 – Intellectual Property and Security Issues

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INTELLECTUAL PROPERTY RIGHTS

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Introduction

• Digital technologies made easier the production and distribution of works (music, films, etc.)

• Large proportion of incomes of entertainment industry comes from selling copies of digital work

• Entertainment industry feels threatened by illegal copying and distribution

• The way intellectual property rights are interpreted and enforced will have a significant impact on how digital works are distributed in the future, both in an electronic form or in a storage medium

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Introduction

• Intellectual property (IP) takes several different forms• IP-Related laws attempt to balance the interests of

different parties.• There is no universally agreed balance• Laws change to reflect the changes in the views of

dominant players and changes in technologies.• Intellectual property (IP) can allow you to own things

you create in a similar way to owning physical property.

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What is Intellectual Property (IP)?

• The term ‘intellectual property’ is commonly used to refer to the actual invention or creative work, and also to the rights associated with the works.

• Types of intellectual property• Copyright : protects material, such as literature, art, music,

sound recordings, films and broadcasts.• Designs : protect the visual appearance or eye appeal of

products.• Patents : protect the technical and functional aspects of

products and processes.• Trademarks : protect signs that can distinguish the goods

and services of one trader from those of another.

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Intellectual Property vs. Physical Property

• Comparison of Intellectual Property to physical property is highly contentious.

• knowledge is non-rivalrous.• If I have an idea and I tell it to another person, then I can still

have that idea -- we are not rivals in the possession of the same idea.

• This is not the case with rivalrous objects; if I give someone a pair of my shoes, then I can no longer wear those same shoes.

• Intellectual property is treated in law differently to physical property: • Certain exclusive rights awarded to owners of intellectual

property are time limited, after which the works are considered to be in the public domain.

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Definition of intellectual property

• Intellectual property rights can be described as an attempt to establish a trade-off between rewarding individuals and benefiting society

• The owners of intellectual property can be organizations, not only individuals

• Three main stakeholders: • Creators, commerce and the public.

• Intellectual property is characterized as a bundle of rights.• Examples of the rights associated with a novel : to produce

a printed edition for sale in particular countries, to translate it, to film it, to adapt it for the stage, and so on.

• The word ‘instrument’ describes the recognized forms of intellectual property.

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Definition of intellectual property

• Rights can be disposed• Example: pass some of these rights to a publisher through a

signed contract• Economic exploitation of the novel is only possible by

someone who holds the right to do so.

• Rights can be fragmented and traded• Allows many freelance creators (novelists, lone inventors or

software creators) to generate money from their work

• Copyright between Employee and Employer• contracts of employment usually state that the rights in any

creative work done as part of their employment are automatically the property of their employer.

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Rights and exceptions of copyright

• Exceptions to copyright legislation are generally a recognition that users or consumers should be able to do certain things with copyrighted works, to a limited extent or in special circumstances, that copyright legislation forbids when performed on a larger scale or in other circumstances.

• The exceptions to copyright legislation generally come under the heading of ‘fair dealing’.• research and private study.• criticism, review and news reporting.• incidental inclusion of copyright material.

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More on intellectual property

• The Creative Commons movement has looked for a remedy that does not discard the concept of copyright.

• The movement envisages copyright as a set of explicit permissions rather than an implicit prohibition.

• Most intellectual property rights are covered by civil law, however, Because of the large impact intellectual property infringement has on the economy in the UK, some infringements of intellectual property rights when done on a large scale are criminal offences Selling a fake DVD!

• The term of copyright is at present too long • Some economists argue that the optimal term of copyright is

at most 7 years, other consider a term of 25 years

•

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Arab Open University – Spring 2012 11

• Creators can choose among or a combination of four license elements

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Software piracy -- results of 2006 Global Software Piracy Study,figures for European Union countries


SECURITY

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Introduction

• Now, we shall look at some of the technologies employed to make infringement of copyright more difficult.

• Security of intellectual property involves both access control and copy control.

• Some of the restrictions imposed by copyright holders go beyond the exclusive rights provided by copyright laws licenses

• In most forms of security attack the attackers do not have access to the unprotected versions of the data being protected. However, attackers of copyrighted works may have legitimate access to unprotected versions of works Purchasing a DVD

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Security techniques

• Principles of encryption• Vulnerability to attack• Integrity• Authentication

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Principles of encryption

• An encryption algorithm and all its possible keys, plaintexts and cipher texts is known as a cryptosystem or cryptographic system.

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Cryptography

• Cryptography is the art and science of keeping messages secret.

• Requirements for cryptography: It should be computationally infeasible to derive:• Plaintext from the cipher text without knowledge of the

decryption key• Cipher text from the plaintext without knowledge of the

encryption key.

• Types of cryptographic systems• Symmetric or shared key systems systems• Asymmetric or Public key systems.

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Symmetric key systems

• Sharing a single secret key between the two communicating entities used for both encryption and decryption.

• In practice, the encryption and decryption keys are often different but it is relatively straightforward to calculate one key from the other

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Symmetric key systems - Activity

• Derive a formula for the number of shared keys needed in a system of “n” communicating entities.

• Each entity in the network of “n” entities requires a separate key to use for communications with every other entity in the network, so the number of keys required by each entity is: (n -1)

• But each entity shares a key with another entity, so the number of shared keys for each entity is: (n-1)/2

In a system of “n” communicating entities the number of shared keys required is: n x (n-1)/2

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Symmetric key systems

• Encryption can be done either on blocks of data or on continuous streams

• Two types of symmetric key systems• Block cipher • Stream cipher

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Symmetric key systems – Block cipher

• A block cipher operates on groups of bits: typically groups of 64.

• If the final block of the plaintext message is shorter than 64 bits, it is padded with some regular pattern of 1s and 0s to make a complete block.

• Block ciphers encrypt each block independently• A block of plaintext will always encrypt to the same

block of cipher text provided that the same algorithm and key are used

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Symmetric key systems – Stream cipher

• A stream cipher [unlike a block cipher] generally operates on one bit of plaintext at a time, although some stream ciphers operate on bytes.

• A component called a key stream generator generates a sequence of bits, usually known as a key stream.

• In the simplest form of stream cipher, a modulo-2 adder (exclusive-OR or XOR gate) combines each bit in the plaintext with each bit in the key stream to produce the cipher text.

• Stream ciphers can be classified as • Synchronous • Self-synchronizing

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Symmetric key systems – Stream cipher23

Encryption

Decryption


Synchronous stream cipher

• In a synchronous stream cipher, the key stream output is a function of a key, ONLY, and is generated independently of the plaintext and the cipher text.

• A single bit error in the cipher text will result in only a single bit error in the decrypted plaintext

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Self-synchronising cipher

• In a self-synchronizing cipher, the key stream is a function of the key and several bits [n] of the cipher output.

• Because the key stream outputs depend on the previous “n” bits of the plaintext or the cipher text, the encryption and decryption key stream generators are automatically synchronized after “n” bits.

• However, a single bit error in the cipher text results in an error burst with a length dependent on the number of cipher output bits used to compute the key stream.

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Self-synchronising cipher26


Symmetric key systems: Examples of Commercial systems

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Asymmetric key systems

• Asymmetric or public key systems are based on encryption techniques whereby data that has been encrypted by one key can [ONLY] be decrypted by a different, seemingly unrelated, key.

• One of the keys is known as the public key and the other is known as the private key related to each other mathematically The relationship is complex it is computationally infeasible to calculate one key from the other.

• Each communicating entity will have its own key pair:• private key will be kept secret • public key will be made freely available

• Using asymmetric key systems with “n” communicating entities, the number of key pairs required is “n”.

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Asymmetric key systems: Examples of Commercial Systems

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Vulnerability to attack

• All the symmetric and public key algorithms listed previously share the fundamental property that their secrecy lies in the key and not in the algorithm.

• This is generally known as Kerchoff’s Principle, after the Dutchman who first proposed it in the nineteenth century.

• This means that the security of any system using encryption should not be compromised by knowledge of the algorithm used.

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Cryptanalysis

• Cryptanalysis is the science of breaking a cipher without knowledge of the key (and often the algorithm) used.

• Its goal is either to recover the plaintext of the message or to deduce the decryption key so that other messages encrypted with the same key can be decrypted.

• One of the more obvious attacks is to try every possible key (i.e. the finite set of possible keys, known as the key space) until the result yields some intelligible data.

• This kind of attack is known as a Brute Force Attack. • Clearly, the greater the key space, the greater the immunity to

a brute force attack.

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Cryptanalysis - Activity

• Assuming you could process 1012 key attempts per second, calculate how long it would take to search the keyspace of a 56-bit key. Compare this with the time needed to search the keyspace of a 128-bit key.

• A keyspace of 56 bits provides 256 = 7.2 x 1016 possible keys. At a rate of 1012 keys per second it would take approximately 7.2 x 104 s or about 20 h to try every key.

• A keyspace of 128 bits provides 2128 = 3.4 x 1038 possible keys. This would take approximately 3.4 x 1026 s or about 1019 years. (Note: the lifetime to date of the universe is thought to be of the order of 1010 years!)

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Hybrid systems

• Which of the two types of cryptographic systems is better : Symmetric or Asymmetric?

• A major advantage of asymmetric key systems over symmetric key systems is that no exchange of a secret key is required between communicating entities.

• In practice, public key cryptography is rarely used for encrypting [entire] messages for the following reasons:• Security: it is vulnerable to chosen plaintext attacks.• Speed: encrypting data with public key algorithms generally

takes about 1000 times longer than with symmetric key algorithms.

• Instead, a combination of symmetric and asymmetric key systems is often used

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Integrity

• Integrity: relates to assurance that there has been no unauthorized modification of a message and that the version received is the same as the version sent

• Why encryption alone does not provide a completely workable solution for integrity ?• Encryption process carries overheads in terms of resources and

for some applications it is preferable to send data in the clear.• Some network management protocols separate the

confidentiality and integrity functions, so encryption is not always appropriate.

Concise identity of the original message is needed to check against the received message to reveal any possible discrepancies between the two Message digest

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Integrity

• Message digest: consists of a small, fixed-length block of data, also known as a hash value, which is a function of the original message.

• The hash value is dependent on all the original data it will change even if only one bit of the data changes

• The hash value is calculated by applying a mathematical function, known as a hash function, which converts a variable-length string to a fixed-length string.

• It is easy to compute a hash value for a given input string, but extremely difficult to deduce the input string from the hash value one-way hash functions

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Authentication

• A message authentication code is similar to a one-way hash function and has the same properties, but the algorithm uses the additional ingredient of a secret key, and therefore possession of the key to perform the check is necessary.

• Authentication: the process of verifying an identity claimed by or for a system entity

• Authentication is needed to provide some assurance about the source of a message: did it originate from the location it appears to have originated from?

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Authentication

• Some additional assurance is required that irrevocable binds together the true identity of a person with a public key.

• This assurance can be provided by a trusted third party, known as a certification authority independent companies (such as Verisign)

• The certification authority knows only the public key of the entity and not the private key, which should of course be kept secret at all times.

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http://www.verisign.com/


Digital certificate

• Certification authorities form part of what is known as a Public Key Infrastructure (PKI)

• PKI: is a combination of services and encryption techniques that together are used to protect the security of data over networks.

• A PKI includes• a registration authority, which checks and verifies the

credentials of a user before a digital certificate can be issued• a certification authority that issues and verifies digital

certificates • directory services for the publishing of public keys and

certificates • certificate management and key management services.

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Public key infrastructure39


Media players

• Three examples • DVD player for pre-recorded DVDs: Content Scrambling

System (CSS)• Digital video broadcast (DVB) conditional access• Windows Media Player.

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Content Scrambling System (CSS)

• Each DVD player has a set of secret player keys that can be used to play any DVD• 409 player keys is provided• Has a table stored in a hidden area that has encrypted

versions of the disc key

• In addition, each disc has stored another version of the disc key encrypted with the disc key itself

• The DVD player will select one of its player keys and decrypt the corresponding entry in the table If the result is the value of the disc key DVD player has correctly obtained the disc key

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Content Scrambling System (CSS)

• Each disc also contains an encrypted version of the title key for each work stored on the disc encrypted with the disc key Once the title key has been found it can be used to decrypt the actual content of each sector of the disc (MPEG-2 version of a film).

42Arab Open University – Spring 2012


Digital Video Broadcasting Conditional Access

• Some digital television broadcasts are encrypted for pay TV application require smart cards

• The most common standard is digital video broadcasting conditional access (DVB-CA).

• The scrambling system used to encrypt the MPEG-2 encoded video signal is not a public system and information is available through non-disclosure licences, which is contrary to the Kerchoff principle

• The scrambling is based on a secret key and can only be de-scrambled with knowledge of the value of this key.

• A major difficulty is distributing this key securely to valid subscribers

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Arab Open University – Spring 2012 44Digital Video Broadcasting Conditional Access


• Because of the characteristics of their personal keys it may be possible to group subscribers together and the same encrypted service key can be decrypted by all the subscribers in a group, thereby reducing the number of transmissions necessary.

• The MPEG data for pictures, sound and data are multiplexed with the encrypted entitlement messages and are fed into the scrambler.

• However, only the protected picture, sound and data messages are scrambled, otherwise the receivers will not be able to access free broadcast programmes and the encrypted entitlement messages

45Digital Video Broadcasting Conditional Access


Windows Media Player

• The Microsoft version of DRM is incorporated in Windows Media files.

• These files have the file extension ‘.wma’ for audio, ‘.wmv’ for video, and ‘.asf’ for streaming audio and video.

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Basic process for delivering package Windows Media files

and issuing licences


Windows Media Player

• Uses a proprietary system to package the digital work in a secure container, and users cannot access the digital work in a container without obtaining a licence.

• The secure container comprises an encrypted version of the digital work, information about the work and how a licence can be obtained to access the work.

• The licence includes a decryption key to access the digital work and any conditions on users’ access to the work, e.g. any limitation on how long they can access the work.

• The licences issued to users may include information about their computers and thereby limit access to individual machines.

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DIGITAL RIGHTS MANAGEMENT

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Introduction

• DRM is considered as a set of technologies that controls the access and use of digital works

• DRM should not be thought of simply as a collection of technologies.

• The success of a DRM system depends on its technology, intellectual property laws and business model of the application

• DRMs have two functions:• Accountant: DRM ‘watermarks’ can track usage

electronically to report information back to collecting societies to ensure distribution of royalties is fair

• Policeman: DRM can limit the access to content, for example, to prevent copying.

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Introduction

• DRM will never be entirely effective, but can remove temptation for the ‘casual copier’.

• DRMs can almost invariably be circumvented, a legal framework is needed to prevent unauthorized copying at a commercial scale

• (Report of All Party Internet Group (APIG) on DRM published in June 2006)

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Problems with DRM solutions

• People incentives• Will people buy digital works that have technological restrictions

on their use rather than obtaining a free version that does not have any technological restrictions?

• Price discrimination• Non-implementation of fair dealing exceptions

• DRMs can prevent uses permitted under fair dealing exceptions, and DRM tools do not necessarily expire when the copyright expires.

• Collateral damages• DRMs can damage users’ computers and can put limits on what

users can and can’t do with the products

• Non implementation of accessibility features• DRMs can prevent activities permitted under the exceptions


DRM and Copyright laws

• Difference between DRM and Copyright laws approaches• Copyright law ‘everything that is not forbidden is

permitted’• DRM ‘everything that is not permitted is forbidden’

• DRM has been strengthened by recent changes to the law.• The US Digital Millennium Copyright Act of 1998 and the

2001 EU Copyright Directive make it illegal to bypass DRM technology, even if the person doing so would otherwise have the legal right to access the information behind the digital fence.

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DRM Technologies - Obstacles

• DRM technologies faces a number of obstacles• Bypassing DRM technologies: it has become commonplace

for new DRM to be cracked (albeit illegally) shortly after its release and the circumvention codes to be widely distributed on the internet

• Consumer non-satisfaction: consumers may become increasingly frustrated with DRM that restricts usage and presents problems (i.e., as hardware and software need changing or updating).

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ACTIVITY 3.2: THE DARKNET AND THE FUTURE OF CONTENT PROTECTION (ARTICLE)


Darknet

• A darknet is collection of networks and technologies used to share digital content.

• It is not a separate physical network but an application and protocol layer riding on existing networks.

• Examples: peer-to-peer file sharing, CD and DVD copying, and key or password sharing on email and newsgroups.

• Two types• Fully distributed • Centralized

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Search

Web Search Engine

FTPhttp

HttpHttp

FTP

PC

PCPC

PC

PC

TCP / Napster Protocol

PC

PC

PC

PC

PC

NapsterSearch Engine

PC

PC

PC

PC

PC

PC

PC

PC

PC

PC

PC

gnutellaSearch Engine

PC

PC

PC

TCP/UDPGnutella Protocol

(a) - “Sneaker Net” (b) - The World Wide Web

(c) - Napster

(d) - Gnutella


Darknet Technological and infrastructure requirements

• Input: facilities for injecting new objects into the darknet• Transmission: distribution network that carries copies of

objects to users• Output: ubiquitous rendering devices, which allow users

to consume objects• Database: search mechanism to enable users to find

objects• Storage: that allows the darknet to retain objects for

extended periods of time. • Functionally, this is mostly a caching mechanism that

reduces the load and exposure of nodes that inject objects.

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Fully distributed darknets

Fully distributed

• Strengths• No single point of failure

• Weaknesses• Free riding• Lack of anonymity

Centralized

• Single point of failure

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BOBE: Break Once Break Everywhere

• DRM systems: BOBE (break-once, break everywhere)-resistant. • Suppliers anticipate (and the assumptions of the darknet predict)

that individual instances (clients) of all security-systems, whether based on hardware or software, will be subverted.

• If a client of a system is subverted, then all content protected by that DRM client can be unprotected.

• If the break can be applied to any other DRM client of that class so that all of those users can break their systems, then the DRM-scheme is BOBE-weak.

• If, on the other hand, knowledge gained breaking one client cannot be applied elsewhere, then the DRM system is BOBE-strong.

• DRM renewability: Vendors recognize the possibility of exploits, and build systems that can be field-updated

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Activity 3.10: “If piracy is the problem, is DRM the answer?”

• DRM technology components• License evaluation: software or hardware within the system

that evaluates the license against a requested action, determines if that action conforms to the terms of the license, and either allows or blocks that action from occurring.

• Authentication component to identify the licensee: The licensee could be a human user or a piece of hardware or software.

• A way to associate licenses with content.

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Activity 3.10: “If piracy is the problem, is DRM the answer?”

• Two types of content :• Managed content : content that is associated with a license

using some technological means• Unmanaged content: content that does not have a license

associated with it

• If users can somehow convert a managed piece of content into an unmanaged form, then they can use it in unlimited ways. In particular, they can share it with other unauthorized users Illegitimately transformed content Dissociated content.

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DRM techniques

• Secure Container Methods• Distributing and storing content in an encrypted form and

protecting it indirectly by managing the keys used to decrypt the content

• Problem: convert managed content to dissociated content The Analog hole

• Fuzzy Hashing • fuzzy hashing attempts to compute an identical hash for two

pieces of content if they are perceptually equivalent.

• Ordinary vs. Draconian• Ordinary DRM: devices handle both managed and unmanaged

content simultaneously. • Draconian DRM, in which devices that handle managed

content do not handle unmanaged content at all


MPEG-21 Rights expression language

• Based on XML (extensible markup language)

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MPEG-21 Rights expression language

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DIGITAL WATERMARKING

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Introduction

• Digital watermarking refers to techniques for embedding marks in digital works.

• Digital watermarking does not make the encryption of digital work more secure, but it does offer additional protection of tying copyright information to the content in a secure way.

• It may even be possible for the watermark to survive digital to analogue conversion.

• Digital watermarking has applications other than copyright protection.

• Watermarking is not a steganography technique

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Introduction

• Watermarking vs. Steganography• Steganography: attempts to hide the existence of messages

and is independent of the cover work • The information contained in a watermark (the watermark

payload) is generally related to the work in which it is embedded.

• The presence of a watermark is generally known, but the techniques employed are robust to attempts to remove it.

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Watermarking digital media

• Watermarking insertion

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• Watermarking extraction

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• Watermarking detection

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Requirements for digital watermarking algorithms

• Imperceptibility (or fidelity) of the watermarking process: a human listener or viewer should not [be] able to hear or see whether the cover signal has been watermarked.

• High capacity: The capacity should be high enough for the intended application of the watermarking scheme.

• Reliability: false negative and bit error rates should be low• Robustness• Tamper resistant (robust to attacks) • Security should only rely on the secret of keys (following

the well known Kerckhoffs’ principles). • Cost constraints of the application

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• The watermark payload should be spread over a digital work• For example, a payload of 70 bits can be embedded in each

1 s fragment of a video stream. • If we assume that a video stream is operating at 1 Mbit/s,

then, in effect, each watermark bit is spread over approximately 14 000 bits of the original

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Attacks on watermarks

• Unauthorised embedding• Commercial broadcasting • Fake authenticity: insert a fake watermark after modifying the work

• Unauthorised detection/extraction• In some applications it is desirable that the presence of a watermark

cannot be detected without the owner’s authorisation (Example: medical records)

• Unauthorised removal• It is possible to distinguish between two types of attack by this

unauthorised action: masking and elimination. • A masking attack modifies a watermarked work in an attempt to fool

a detector that a watermark is not embedded• An elimination attack attempts to identify the watermark and remove

it so completely that the resultant work cannot be distinguished from normally occurring unwatermarked works

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Security of watermarks

• Known attacks against digital watermarking• Copy attack• Pathological distortion attack• Sensitivity analysis attack• Ambiguity attack

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TEST YOUR KNOWLEDGE(QUESTIONS FROM PREVIOUS EXAMS)


Multiple Choice questions

• Hashing a message and encrypting the message digest with the sender private key ensuresa. Confidentiality

b. Integrity

c. Non replay

d. Availability

e. All of the above

• Which of the following intellectual property aspects cannot be categorized as industrial propertya. Patents

b. Trademarks

c. Copyright

d. Designs

e. None of the above

(Final Exam – Fall 2011)



• Which of the following is incorrecta. RSA is an asymmetric key system

b. DES is a symmetric key system

c. Cryptanalysis is the science of breaking a cipher without the knowledge of the key

d. Hybrid cryptosystems have the security of asymmetric and the speed of the symmetric key systems

e. MD5 is a hybrid encryption algorithm

• The principle that the secrecy of a cryptosystem lies in the key and not in the algorithm is known asa. De Morgan’s principle

b. Moore’s principle

c. RSA’s principle

d. Kerchoff’s principle

e. Schneir’s principle

(Fall 2010 – Final Exam)



• The process of verifying an identity claimed by or for a system entity is calleda. Non repudiation

b. Confidentiality

c. Authentication

d. Identification

e. Integrity

• Which of the following is not included in a public key infrastructure (PKI)?a. Certification Authority

b. Registration authorities

c. Directory services

d. Authentication authority

e. Certificate management and key management services

(Fall 2010 – Final Exam)



• When the copyright owner of a work is either unknown or cannot be traced, the work is calleda. Lost

b. Orphan

c. Untraced

d. Unknown

e. Copy-left

• Which of the following is an exclusive right for copyright owners in UKa. Copy the work

b. Issue copies of the work to the public

c. Perform, show or play the work in public

d. Make an adaptation of the work

e. All of the above



• The category of intellectual property that protect the technical and functional aspects of products and processes is known asa. Trade marks

b. Copyright

c. Designs

d. Patents

e. None of the above


Short questions

• Explain the principle of hybrid cryptographic systems and show their advantages against pure symmetric and asymmetric systems. (Final Exam – Fall 2011)

• What are the four categories of Intellectual Property? Explain each of them in one or two sentences.

• Explain the principle of symmetric and asymmetric encryption systems. Compare them in terms of speed and security

• What is a hash function? What do we mean by one-way hash function? And collision free hash function?

• Explain the difference between stream cipher and block cipher• There are two main requirements for cryptography, explain

both of them.


Long Essay questions / Exercises

• Explain the principle of content scrambling system (CSS) used in pre-recorded DVDs. In your answer, highlight the use of the following keys: player keys, disc key and title key. How they are stored and used to encrypt and decrypt the content of the DVD. (Final Exam – Fall 2011)

• Security techniques relevant to technological protection of intellectual property include encryption and authentication, explain how they are applied (Final Exam - Spring 2010)

• Assuming you could process 10^12 key attempts per second, calculate how long it would take to search the key space of a 56-bit key. Compare this with the time needed to search the key space of a 128-bit key



• The computing power of a certain machine is the number of instructions it can execute per unit of time. The computing power of nowadays personal computers (2011) is around 4500 MIPS (core i5 Intel processor or equivalent) where MIPS stand for Millions Instructions per Second. In this exercise we will consider such computing power in the calculations. Suppose that an algorithm to verify one 56-bit DES key needs around 600 elementary instructions. Suppose that we have a couple of clear text and encrypted text using DES and that we want to find the encryption key using brute force attack; which means by testing all the keys once after another.

• We suppose that all keys are equally probable.• Note: large numbers can be expressed in the form of powers of 2

i. What time it takes for the machine mentioned above to test a key? (3 marks)

ii. What is the number of instructions necessary to find a DES key? Consider the worst case scenario where all keys should be verified. (3 marks)

iii. What time it takes if the computing power of one billion (109) PCs worldwide are grouped for this task? (we suppose that all the PCs have the same computing power mentioned above) (2 marks)

iv. Calculate the time it takes to find a 112 bits triple-DES key supposing that the number of instructions to test one 3-DES key is the double of a DES key. (2 marks)



• Consider the following scenario :• Alice challenges Bob by sending him some random number.• Bob encrypts the random number using his own private key

and sends the result to Alice.• Alice decrypts the message using Bob’s public key (suppose

that Alice has confidence that the public key does indeed belong to Bob).

• What security service can be assured by this exchange? Explain your answer.

T325: Technologies for digital media Second semester – 2011/2012 Block 2 – Intellectual Property...

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