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Protection of Multimedia Data in Distribution and Storage - Introduction, Encryption, Watermarking

digital content image key

Ahmet M. Eskicioglu
Department of Computer and Information Science
Brooklyn College of the City University of New York

Definition: Multimedia data needs to be protected from unauthorized duplication and consumption, from unauthorized disclosure and misuse, and from unauthorized use and exploitation.


Multimedia can be defined as a combination of different types of media (e.g., text, images, audio, video, and graphics) to communicate information in a given application.

Recent advances in digital technologies have drastically increased the capacity of both data channels and storage:

  • When compare with floppy disks and CDs, the capacity of DVDs, digital tapes, and hard disk is much larger. Personal computers can be configured with a processor speed of 3 GHz, main memory of 2 GB, and a hard disk of 500GB. DVDs manufactured with a double sided format, each side having a dual layer, have a data capacity of 17GB. Digital Linear Tapes (DTLs) come with a storage space above 200GB for compressed data.
  • Asymmetric digital subscriber line (ADSL) is a new technology that allows more data to be sent over existing copper telephone lines, supporting data rates up to 9 Mbps when receiving downstream data. Very High Speed Digital Subscriber Line (VDSL) transmits data in the 13 Mbps – 55 Mbps range over short distances, usually between 1000 and 4500 feet.

The transition from analog to digital technologies started in 1990s. With the higher capacity of storage devices and data communication channels, multimedia content has become a part of our daily lives. This type of data is now commonly used in many areas such as education, entertainment, journalism, law enforcement, finance, health services, and national defense. The lowered cost of reproduction, storage, and distribution has Page 716  added an additional dimension to the complexity of the problem. In a number of applications, multimedia needs to be protected for several reasons. Table 1 includes three applications where the data should be protected.

Encryption and watermarking are two groups of complementary technologies that have been identified by content providers to protect multimedia data. Watermark embedding and detection are sometimes considered to be analogous to encryption and decryption.

  • Encryption makes the content unintelligible through a reversible mathematical transformation based on a secret key . In secure multimedia content distribution, the audio/visual stream is compressed, packetized and encrypted. In symmetric key encryption, which is commonly used for protecting multimedia elements, each encryption transformation E K is defined by an encryption algorithm E and a key K. Given a plaintext M, the transformation produces the ciphertext C = E K (M). Each decryption transformation D K is defined with a decryption algorithm D and K. For a given K, D K = E K -1 such that D K (E K (M)) = M. One of the most challenging problems in distribution architectures is the delivery of the decryption key.
  • Watermarking (data hiding) is the process of embedding data into a multimedia element such as image, audio or video. The embedding transformation E K is defined by an embedding algorithm E and a key K . In watermarking, the usual approach is to use a symmetric key although there is a recent trend to use asymmetric techniques. Given a cover image I and a watermark W, the transformation produces the watermarked image I w = E K (I,W)- Each detection (or extraction) transformation D K is defined with a detection (or extraction) algorithm D and K. For a given K and the watermarked image I w , the watermark is either detected (or extracted): W = D K {I W ) .


Figure 1 shows five primary means of multimedia delivery to consumers: satellite, cable, terrestrial, Internet and prerecorded media (optical and magnetic).

For end-to-end security from the source to the final destination, the most important requirements are:

  • Secure distribution of multimedia content
  • Secure distribution of access keys
  • Authentication of source and sink consumer devices in home networks
  • Association of digital rights with content
  • Manufacturing of licensed devices that have the protection technology
  • Renewability of secure solutions

In the last 10 years, three industries (consumer electronics, information technology, and motion picture) have been working on solutions for protecting copyrighted multimedia content. Some of the key players with interest in developing or implementing secure solutions are ATSC (Advanced Television Systems Committee), CableLabs, CPTWG (Copy Protection Technical Working Group), DVB (Digital Video Broadcasting) Organization, DVD Forum, EIA (Electronics Industries Association), IETF (Internet Engineering Task Force), MPAA (Motion Picture Association of America), MPEG (Moving Pictures Expert Group), North American Broadcasters Association (NABA), RIAA (Recording Industries Association of America), and SCTE (Society of Cable Television Engineers).

In digital distribution networks, copyrighted multimedia content is commonly protected by encryption:

  • Cable, satellite, and terrestrial distribution : A conditional access (CA) system provides the encryption technology to control access to digital television services. Digital content (“program”) is compressed, packetized, encrypted and multiplexed with the entitlement messages. Two types of entitlement messages are commonly used associated with each program: The Entitlement Control Messages (ECMs) and the Entitlement Management Messages (EMMs). ECMs carry the decryption keys (“control words”) and a short description of the program while EMMs specify the authorization levels related to services. The programs are usually encrypted using a symmetric cipher such as the Data Encryption Standard (DES) or any other public domain or private cipher. The CA providers often protect the ECMs privately although public-key cryptography and one-way functions are useful tools for protecting access keys. Authorized users can use the appropriate decoder to decrypt the programs. Because of their secure features, smart cards are a good option for set-top boxes.
  • Internet distribution: Digital Rights Management (DRM) refers to the protection, distribution, modification, and enforcement of the rights associated with the use of digital content. The primary responsibilities of a DRM system include secure delivery of content, prevention of unauthorized access, enforcement of usage rules, and monitoring of the use of content. A customer obtains an encrypted file from a server on the Internet for viewing purposes. To be able to decrypt the file, a license (that contains the usage rights and the decryption key) needs to be downloaded from a clearing house. A major responsibility of the clearing house is to authenticate the customer based on his credentials. The client device should have a player that supports the relevant DRM system to play the file according to the rights included in the license. Superdistribution is a process that allows a customer to send the encrypted file to other people. However, as licenses are not transferable, each new customer has to purchase another license for playback. Today, interoperability of DRM systems is a major problem.
  • Distribution in digital home networks: A digital home networks is a cluster of consumer electronics devices (e.g., DTV, DVD player, DVCR, and STB) that are interconnected. The multimedia content is encrypted in transmission across each digital interface, and on storage media. The technical solutions developed in recent years are listed in Table 2. In a digital home network, multimedia content moves from one device to another for storage or display. These devices need to authenticate each other to make sure that they are equipped with the licensed protection technology.


A digital watermark is a pattern of bits inserted into a multimedia element such as a digital image, an audio or video file. The name comes from the barely visible text or graphics imprinted on stationery that identifies the manufacturer of the stationery. There are several proposed or actual watermarking applications: broadcast monitoring, owner identification, proof of ownership, transaction tracking, content authentication, copy control, and device control. In particular, watermarks appear to be useful in plugging the analog hole in consumer electronics devices

The components of a watermark embedding/detection/extraction system are depicted in Figure 1. A watermarking system consists of watermark structure, a marking algorithm that inserts some data into multimedia and an extraction or detection algorithm that extracts the data from, or detects the data in, a multimedia element.

In applications such as owner identification, copy control, and device control, the most important properties of a watermarking system are perceptual transparency, robustness, security, high data capacity, and unambiguousness. The relative importance of these properties depends on the requirements of a given application.

  • Perceptual transparency: An embedded watermark should not introduce a significant degree of distortion in the cover image. The perceived degradation of the watermarked image should be imperceptible.
  • Robustness: Robustness refers to the ability to detect the watermark after normal A/V processes or intentional attacks. A watermark can still be detected or extracted after the image has undergone some common signal processing operations. These operations include special filtering, lossy compression, printing/scanning, and geometric distortions such as rotation, translation, cropping, and scaling.
  • Security: Security is the ability to resist unauthorized removal, embedding, or extraction. A hostile attack is any process specifically intended to thwart the watermark’s purpose.
  • Capacity: Data capacity can be defined as the amount of data that can be embedded. A watermarking system should be able to embed relatively high amount of data without affecting perceptual transparency.
  • Unambiguousness: The watermark should unambiguously identify the owner. It is desired that the difference between the extracted and the original watermark is as low as possible. For accuracy of identification, the system should exhibit a graceful degradation irrespective of the type of attack.

Several criteria can be used to classify image watermarking systems. Five of such criteria are the type of watermark, the type of domain, the type of watermarking scheme, type of algorithm, and the type of information needed in the detection or extraction process. The classification according to these criteria is listed in Table 3. In general, systems that embed the watermark in the pixel domain are simpler but are less robust to image manipulations. On the other hand, frequency domain watermarking techniques are more complex and robust.

Embedding multiple watermarks in a transform domain using the coefficients in several frequency bands drastically increases the overall robustness of a watermarking scheme. For one group of attacks, detection or extraction in lower frequencies is better, and for another group of attacks, detection or extraction in higher frequencies is better. Since the advantages and disadvantages of low and middle-to-high frequency watermarks are complementary, embedding multiple watermarks in an image (namely, one in lower frequencies and the other in higher frequencies) would result in a scheme that is highly robust with respect to a large spectrum of image processing operations.

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