As digital content transmissions continue to proliferate through various aspects of day to day life, the issues surrounding protection of the content become increasingly important. Content may be transmitted to so-called “open” or “closed” platforms. A reference to an open or “untrusted” platform typically refers to a platform that places no security restrictions on applications that may be installed on it. A typical example of an open or untrusted platform is a personal computing (PC) device running Microsoft Windows. Any application written to conform to the Windows specification may be installed on the platform, without any regard to how safe the application may be. Although Microsoft has recently attempted to “close” the operating system, a PC running Microsoft Windows is still considered an open platform.
In contrast, closed platforms typically do not allow users to install arbitrary software. Examples of closed platforms include set top boxes such as TiVO and Apple's iPhone. In the scenario of TiVO, no applications may be installed on the device—it may simply receive, decrypt and render content. Similarly, although a variety of individuals and vendors may write applications that run on Apple's iPhone, only “approved” applications are made installable on the iPhone via Apple's App Store. Closed platforms thus have inherently built-in protection against malicious applications because the applications are trusted, i.e., from an authorized source.
Regardless of the type of platform, media content is typically encrypted for security purposes. Newer media formats and devices recognize discretely encrypted content, i.e., content in which the payload items are discretely encrypted and the navigational headers of the media stream are left unencrypted. As a result, regardless of whether it is an open or closed platform, the navigational headers are accessible without decryption and the encrypted payload portions can be decrypted independently from each other and from the navigational headers.
In contrast, if the media content is monolithically encrypted as is typical for older media formats and devices, the navigational headers and payload of the content stream are typically encrypted en masse. Thus, on those devices, the platform is forced to decrypt the entire content stream in order to access the navigational headers and play the content. While this poses little to no risk on a closed platform where all content and applications are presumed trusted, this scenario creates a significant security problem for untrusted platforms. One popular solution to this issue is for content providers to encrypt only the media payloads while leaving the header data in the clear. This allows the receiving device to access the headers for navigation purposes, while deferring payload decryption and decoding to the device hardware or other trusted environment.