Protecting Content with Digital Rights Management


As the availability of online music, books, films, and other types of digital media has increased, so have the concerns of content owners, artists, and publishers who own the copyrights to the material. In response to these concerns, Microsoft developed DRM technology to be used in conjunction with Windows Media Encoder and Windows Media Player.

DRM enables content owners to package their Windows Media-based files by encrypting the content with a key. The key is a piece of data that locks and unlocks the content. To play the encrypted file or stream, the user needs to obtain a separate license that contains this key. The license enables content owners to set rights determining how users can use the protected content. For example, a content owner could set an expiration date for the license or allow users to play the content a given number of times, after which the user would need to acquire a new license. Licenses are typically issued by a license provider, who provides a licensing service for one or more content owners. Licenses are bound to the computer to which they were issued, and so cannot be copied or shared.

The DRM process is described in the following download scenario. A content owner encodes source content to create a Windows Media file, protects it using DRM, then distributes the protected file to users on a Web site. A user downloads the file and then plays it using a player that supports DRM. The player, detecting that the file is protected, connects to the license provider’s Web site to acquire a license. Once the user has agreed to the terms of the license, the license is issued. The user can then play the content according to the terms of the license.

Figure 2.8 shows the basic DRM process.

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Figure 2.8: Encrypting, licensing, and playing a protected file.

In previous releases of Windows Media Technologies, the process of encrypting content required the use of the Microsoft Windows Media Rights Manager SDK, and was performed after the content had already been encoded into a Windows Media file. In Windows Media 9 Series, real-time DRM is available with Windows Media Encoder and the Windows Media Encoder SDK; that is, you can protect and encode content in the same process. Now, in addition to developers using the Windows Media SDKs, users of Windows Media Encoder can also protect their content.

However, protecting content is only the first half of the DRM process; licenses must also be issued. License providers use the Windows Media Rights Manager SDK to create a licensing service, and Windows Media Encoder users can use one of these services to issue licenses for their content.

Encoding and Protecting Content

Once you have a DRM profile, you are ready to protect your content. Using Windows Media Encoder 9 Series, you can protect files or streams as a step during the encoding process.

Figure 2.9. shows how you select the DRM profile for protecting your content during the encoding session.

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Figure 2.9: Protecting content with DRM during the encoding process.

Once you have configured the encoding session and selected a DRM profile, you can apply the settings you specified and start encoding. During this process, the key to lock the content is created, the content is encrypted with that key, and DRM-specific information is added to the content header.

At this time, a key ID is also generated. The key ID is used in the encryption algorithm, along with other secret values, to generate the key. The key ID is stored in the content header and is extracted later by the license provider to generate the correct license. While you can specify a key ID, it is recommended you let Windows Media Encoder generate a unique value each time you encode.

Playing Protected Content

After content has been encoded and protected, the user can begin to play it by using player software that supports DRM, such as Windows Media Player 9 Series. When the player detects that the content is protected, it searches the computer for a license. If one is not found, the player opens the URL to the license provider’s service to request a license. The licensing service issues a license as described in the next section. Then the user can play the content according to the rights that are specified in the license.

Depending on whether the license allows it, the user can transfer protected files to portable devices that support DRM. The license for a particular protected file may allow a limited number of transfers, or it may disallow playback on portable devices altogether.

Generating and Issuing Licenses

When a user tries to play protected content, the player sends a license request to the license provider’s service. This license request includes the content header, which contains the key ID. The license provider uses the key ID and the rights specified by the content owner to generate the license for the content.

A license contains the following information:

  • The key to unlock the protected file or stream.

  • The rights and conditions of the license.

  • Priority of the license with respect to other licenses for the same content.

  • Custom attributes (name-value pairs), which might include the date the license was generated or a description of the license.

Once the user has met the requirements for the license (for example, providing payment or registration information), the license is issued.

Licenses can be issued silently or non-silently. Issuing the license silently makes sense when you have no need for the user to provide any information in order to acquire the license. The license can be issued in the background while the user is completing the transaction, such as purchasing or downloading a song. The user might notice a slight delay before being able to play the downloaded file.

Issuing a license non-silently is best when you need information from the user before issuing the license, or if you want the user to be aware of the license acquisition process. This also ensures that users see the terms of the license before they proceed with downloading their content.




Microsoft Windows Media Resource Kit
Microsoft Windows Media Resource Kit (Pro-Resource Kit)
ISBN: 0735618070
EAN: 2147483647
Year: 2005
Pages: 258

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