An architecture for protecting premium content in a nonsecure computer environment executes only a small number of code modules in a secure memory. The modules are arranged in a hierarchy of trust, where a module names other modules that it is willing to trust, and those modules in turn name other modules...http://www.google.co.uk/patents/US6976175?utm_source=gb-gplus-sharePatent US6976175 - Hierarchical trusted code for content protection in computers
