What does trust mean? For the purposes of this book, perhaps the simplest definition would be that trusted computing refers to a computer system for which an entity has some level of assurance that (part or all of) the computer system is behaving as expected. The entity may be any one of various things, including the human user of the PC or a program running on a remote machine. The degree of coverage of this assurance, that is, whether it covers all aspects of the system or just some part, and the nature of the entity to which assurance is provided, vary depending on the system and the environment within which it is used.