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Beyond the Sandbox: Signed Code and Java 2
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Section 2 -- Security Enhancements in JDK 1.1
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JDK 1.1 appeared in the early Spring of 1997 and included a number of improvements and changes to the base Java security model of JDK 1.0.2. Fortunately, none of the material about the base Java security model covered in the last chapter (or for that matter, things you learned from the previous edition of this book) was outdated or replaced; rather, the Java security architecture was changed through a process of enhancement and addition. From a security perspective, the most important changes introduced in JDK 1.1 were the addition of authentication and simple access-control mechanisms that rely on the use of cryptography. Remember, security is much more than just cryptography. Think of cryptography as a means to an end-an important part of the puzzle, but only a part. A side effect of the need for cryptographic functionality inside the model itself was the creation of a crypto API. The crypto API, also introduced with JDK 1.1, provides a basic toolkit of cryptography algorithms that developers can use in their programs.
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The Crypto API
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Today, Java includes a cryptography toolkit that includes both an API and some packages implementing a portion of the functionality behind the API. Classes in the java.security package, the package implementing the cryptographic functionality, have a dual purpose. One purpose is to provide the cryptographic methods that Java's designers used to implement the JDK 1.1 and Java 2 security models. The second purpose is to provide cryptography functionality to Java developers charged with creating secure applications. Parts of a crypto API were released with JDK 1.1. The parts included both one-way hash functions and
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digital signature capability. DES encryption tools were released only as an extension to North American users. Encryption tools and their mathematically related cousins (such as digital signing) change the way Java use policies are managed. Digital signatures, which are discussed next, make it possible to authenticate who has vouched for a piece of code, and potentially check it for tampering. If you decide to trust a particular person, you can set things up so that you automatically trust programs that person signs. (Note that with the right tools, anyone can sign any piece of code. Whether or not a piece of code is written, released, or supported by the person who signed it is not something digital signatures can tell you.) Because the signature is a mechanism for vouching and spreading trust around, if you trust some experts in the field who agree to approve Java programs based on their analysis, you can trust any code that they sign as well. Digital signing paves the way for a true community of trust to develop. We think digital signing is important enough to warrant an entire section itself. See page 88. Beyond digital signatures, the crypto API released with JDK 1.1 includes a couple of other capabilities. One-way hash functions provide a way to fingerprint a program or data so that you can verify that it has not been changed since being created. Fingerprinting hash functions such as MD5 and SHA make distribution over the Net easier to swallow. If you are certain that a program you are downloading from the Net is the original program (and not a Trojan Horse masquerading as the original), you will probably be more likely to use it. Many archives on the Web today make use of MD5. Fingerprinting, also called message digesting, works by performing a one-way hash over a series of bytes. Given a program (which is really just a bunch of ones and zeros), it is possible to compute a hash that ends up being many times smaller than the original program, but (hopefully) represents only that program. The main trick is to avoid collisions, whereby the same fingerprint is computed for different programs, and to come up with a hash function that can't be run in the opposite direction. MD5 and SHA are systems for computing one-way hashes over a binary file. The crypto API provides a way for Java programs to include this functionality. MD5 and SHA are useful when it comes to signing code because the act of signing is actually a complicated function of a secret crypto key and the data to be signed. The math is hairy enough that it is a much better idea to compute it using a program's hash instead of the program itself. Remember, the hash is many times smaller than the program it represents. Figure 3.1 shows the important role that one-way hash functions play in code signing.
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Figure 3.1 How code is digitally signed (A) and digital signatures are verified (B). (A) Signing code takes several distinct operations: (1) a one-way hash calculation is run on a piece of binary code, resulting in a small "thumbprint" of the code; (2) the hash is signed using the signer's private key; (3) the signed hash and the original binary code are placed together (potentially along with other signed and unsigned code) in an archive JAR. Now the JAR can be shipped around as mobile code. (B) Validating signed code also takes several steps: (1) a piece of binary code and its associated signed hash are removed from the JAR; (2) a new hash is calculated using the same one-way hash algorithm that the signer used to create the signed hash; (3) the signature carried by the signed hash is cryptographically validated with the signer's public key (possibly with reference to certificate authorities and trust chains); (4) if the signature checks out, the now decrypted original hash is available for comparison with the new hash. Though all three Java code signing schemes (Sun, Microsoft, and Netscape) share these two processes, there are enough differences that the systems do not inter-operate. See Appendix C for examples of how to sign Java code under each implementation.
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Another function that appeared as part of the crypto API (at least in the package available only in the United States, and known as the Java Cryptography Extension, or JCE) was DES encryption. DES, an
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acronym for Digital Encryption Standard, is a venerable old encryption algorithm that can in some cases be deciphered (given enough effort and a small enough key). DES is certainly much more secure than plain text, but does not provide the best available security. In 1998, the EFF created a special-purpose machine to crack DES messages. The purpose of the machine was to emphasize just how vulnerable DES really is. (For more on the DES cracker, see www.eff.org/descracker/.) Most Unix machines use a variant of DES to encrypt user passwords stored in the /etc/passwd file. If 56bit (or smaller) keys are used for DES, then the U.S. government will allow its export and use outside the United States. There is also a variant called triple DES that effectively has a 112-bit key, which will be safe against brute-force searching for a long time. The ease of "breaking'' DES is directly related to the length of its key.
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