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Distribute Media Securely with Microsoft's Digital Rights Management

Digital Rights Management (DRM) is a solution to online media piracy that requires a user to obtain server-distributed licenses to unlock encrypted content. Learn how to employ this security model using Microsoft's DRM software for Windows Media.




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The music industry is terrified of the Internet, which it perceives as a loosely controlled world where consumers freely distribute the content in which they have invested millions. The success of P2P piracy confirms that the public are aware of how difficult it is to successfully prosecute or prevent piracy or "free online media." But eventually, content owners must learn how to use the Internet as a genuine business channel, and consumers must accept that they have to pay for their entertainment. The road towards realizing the possibilities of Internet distribution for digital media leads towards Digital Rights Management (DRM).

DRM lets content owners and distributors push their media securely to users, assured that the flexible DRM infrastructure will protect their assets from runaway distribution. DRM's flexibility and security help open up digital distribution to all degrees of content ownership. No large hardware investments are required, and the software is free. The required components require only modest system resources and handle distribution via a web server. With such a small investment necessary to start digital media sales, large entertainment corporations and independent record labels can rapidly develop an Internet presence and compete on a level playing field.

Overview of Microsoft Digital Rights Management
Microsoft is leading the DRM charge in two areas. The first is the introduction of the Secure Audio Path (SAP) into the ME and XP operating systems kernel. With older Windows 2000 and 98 environments, the DRM client decrypts secured content before the Media Player receives the audio stream, meaning that it's possible to intercept the stream after it's decrypted, but before it's played. However, a SAP-compliant operating system maintains media encryption until just before the stream reaches the sound card, making it virtually impossible to digitally route the stream to third-party applications. You can apply SAP settings using the DRM SDK.

The second area is the release of the adaptable Windows Media DRM COM components. These components work in two stages, packaging and licensing (see Figure 1). After the music or video is in a Windows Media format, you can program the components to package the media using a combination of keys and unique identifiers. After the securing the media, the distributor has the option of making the media available as is or adding user-specific attributes at run time, just before a user downloads a track. These user-specific attributes help identify individual instances of downloaded media. By combining this technique with business logic, distributors can restrict users from reacquiring licenses.

Figure 1: Flowchart of Packaging and Licensing

In the licensing stage, the distributor has two methods to consider. The most common method, as used in the sample code that accompanies this article, is to issue a license to users the first time they play a secured media file. The first usage triggers the Media Player to request a license from the license acquisition URL stored in the header of the encrypted content. The target server generates a license on demand, using machine and player-specific details known as a challenge. The unique qualities of the user's challenge ensure that the license is created exclusively and its use is restricted to the requesting machine (and possibly any portable devices to which transfer might be permitted). Media Player stores this license in a .lic file that is verified each time the media is played, it's not a simple one-time decryption of the secured media. This circumvents the potential for licensed content to be decrypted and passed freely onto friends or a P2P network. The second licensing method of licensing involves scripting the RMGetLicense object, included with Media Player 7. This object lets you to deliver licenses silently through a web site. However, I do not recommend this method of license distribution. It makes error handling extraordinarily difficult, and you must be able to guarantee your users will be browsing with Internet Explorer because RMGetLicense is an ActiveX control.

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