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Early versions of Java were built without technological help for making privilege decisions. Since 1996, things have changed significantly; so much so that this book required a complete revision. 3, "Beyond the Sandbox: Signed Code and Java 2," discusses the impact of the new privilege system defined by Java 2 on the Java security situation. Not only is the VM itself infused with the capability to create and enforce privilege policies, but the very primitives out of which the new system is constructed have been made available to Java developers. Java now includes support for standard cryptographic algorithms, including SHA, MD5, DES (at least in North America), and SSL. Now what is needed is tools to create and manage security policies that include privilege decisions. Java 2 offers fine-grained access control, but it does not offer a compelling tool for creating, testing, and managing policy (see Appendix C, "How to Sign Java Code"). Lack of such management tools is likely to slow the adoption of Java 2 functionality in the enterprise.
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One of the most common criticisms of early Java security architecture centered on how Java spreads security functionality throughout the code. Unfortunately, the problem of scattershot security has not gone away. Research at Princeton shows that security boundaries (between trusted system code and less-trusted code) are crossed up to 30,000 times per second in a typical applet [Wallach, et al., 1997]. Other evidence can be seen by the effort that Sun undertook when changing the JDK 1.2 API from the beginPrivileged()/endPrivileged() syntax of beta3 to the doPrivileged() syntax of beta4. Over 250 changes were required in the Sun reference VM implementation to make the change. Reliance on a scattershot architecture means that security depends on many different parts working together properly. There is no centralized security system; no single source for security approval. Java implements security features through dynamic type checking, byte code verification, class-loading restrictions, and runtime checks performed by the Access Controller. Each resides in a different part of the Java environment. Such an architecture depends on too many unrelated functions. If all of the securitycritical functions were collected together in one place, that aggregate code could be more easily verified and analyzed. That simple step would satisfy some concerns held by security experts. Some of Java's security policies are dependent on the Java language itself. This is fine if all Java byte code must be created by a Java compiler, but what guarantees does anyone have that byte code has been generated by a Java compiler that plays by the rules There are none, nor should there ever be. There are compilers now in existence that turn Ada and C code into Java byte code. To take such third-party bytecode development efforts away by legislating a particular compiler would go against the spirit of the language. The problem is that the Virtual Machine interpreting Java byte code may allow more functionality than it should. More explicitly, there may be more functionality built in to the byte code than security would dictate (see Figure 6.1). If the Java compiler never creates byte code capable of exploiting such features of the VM, then the architecture would seem to remain safe. Since no one has control over who and what creates Java byte code, system managers should not rely on such a false hope. Someone could write a compiler able to create byte code that seems valid to the VM but breaks rules ordinarily enforced by the Java compiler. Or, someone could create byte code through any number of other means; for example, direct creation with an editor, or creation with a Java byte code assembler (like Jasmin, see www.isbe.ch/ ~wwwinfo/sc/cb/tex/jasmin/guide.html).
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Figure 6.1 If Java byte code is more powerful than Java source code, then the extra functionality in byte code is dangerous. There is some evidence that this is the case.
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One somewhat inefficient (but interesting) solution to this problem has been suggested by Andrew Appel of Princeton. He suggests checking byte code by first decompiling it to Java source, then recompiling the source to byte code. If a compiler you trust does not complain during recompilation, then the original byte code is equivalent to some Java source program, and hence must obey the rules of the Java language. This process is slow, but in certain security-critical instances it pays to be paranoid.
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