How License Chaining Works
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How License Chaining Works
A simple license is bound to the computer or device, and contains the content key to unlock the content. In a license chain, the leaf license contains the content key, and the root license is bound to the computer or device. The content key can be decrypted only by the root license. So, to play protected content, the media player must discover and use both elements in the license chain.
To link the elements in a license chain, each leaf license contains an uplink ID, whose value is the key ID of the next license in the chain, the root license. Each leaf element contains a key that has been encrypted by the private key of the root license.
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The preceding diagram shows how a media player establishes the license chain to get the content key:
- The key ID in the protected content points to the leaf license. The leaf license contains an uplink ID that points to the next license in the chain, the root license.
- The root license private key decrypts the content key in the leaf license. The media player can then play the protected content.
Each license in the chain must contain a valid set of rights. The media player evaluates the rights in both licenses in the chain as follows:
- An action is permitted only if the right is present in both the root and leaf licenses. For example, to play content, the right AllowPlay must be included in both licenses.
- If a root license includes any playback or copy restrictions, all restrictions in the leaf license are ignored.
- If both root and leaf licenses include counts, the counts in both are decremented when the license is used. In cases such as license synchronization when only the root license is used, only the count in the root license is decremented.
- To allow content to be burned as part of a playlist, the right AllowPlaylistBurn must be granted in both the root and the leaf licenses. However, the rights PlaylistBurnTrackCount and MaxPlaylistBurnCount are supported only in a leaf license.
- The AllowTransferToSDMI and AllowTransferToNonSDMI rights are not included in chained licenses, and therefore the content cannot be transferred to devices that are based on Portable Device DRM (only devices that are based on Windows Media DRM for Portable Devices support chained licenses). If you want to support devices based on Portable Device DRM, issue simple licenses instead.
See Also
- Generating and Issuing Licenses
- Issuing a License in a License Chain
- Specifying a License Chain When Protecting Content
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