4.2. Trusted and Untrusted Classes
In the discussion that follows, we make the distinction between trusted and untrusted classes. Generally, an implementation of a security manager allows more operations for trusted classes than for untrusted classes. Whether or not a class is trusted is a complex decision based upon many factors--not the least of which is the release of Java under which the program is running. The default notion of what constitutes a trusted class has changed significantly between releases of Java:
Nothing inherent in the design of the security manager requires security to be enforced as an all-or-nothing proposition for each class. It's possible to write a security manager that gives access to certain parts of the filesystem only to certain classes (even classes that came from the network), or to write a security manager that prohibits classes loaded from the CLASSPATH from performing operations that are normally permitted to classes loaded from the filesystem. A security manager can be as simple or as sophisticated as its author desires, with the result that the security manager can enforce a simple binary yes-or-no policy for operations, or it can enforce a very specialized, very detailed policy. This is true of all security managers in all versions of Java, though as we'll see in Chapter 5, "The Access Controller", one of the prime benefits of Java 1.2 is that it makes it much easier to achieve fine-grained security policies.
However, even though a sophisticated security manager can enforce a very detailed security policy, most implementations of the security manager (especially implementations that occur within popular Java-enabled browsers) assume that a trusted class is one that has been loaded from the CLASSPATH, while an untrusted class is one that has been loaded from a class loader. Furthermore, trusted classes are normally permitted to perform any operation, while an untrusted class is normally subjected to the full extent of the provisions of the security manager.
This dichotomy is essentially the same as the one we normally make between applications and applets: since an application is loaded entirely through the CLASSPATH, all of its classes are considered trusted, and the application can perform any operation that it wants to. On the other hand, the classes that comprise an applet are generally loaded from the network; hence they are considered untrusted and denied any operation that has the potential to violate the browser's security policy.
Beginning with Java 1.1, this distinction became less clear (and Java 1.2 made it even fuzzier): classes now have the ability to be signed, and classes that are signed can be treated as trusted or untrusted. We discuss the rationale behind that idea in Chapter 7, "Introduction to Cryptography" and we fully explore signed classes in the last part of this book; for now, we'll just keep in mind that some classes are trusted and some are not.
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