java核心技術(shù) 卷ⅱ高級(jí)特性

1、英文翻譯資料 英文翻譯資料A.英文原文Core Java, Volume II--Advanced FeaturesWhen Java technology first appeared on the scene, the excitement was not about a well-crafted programming language but about the possibility of safely executing a

2、pplets that are delivered over the Internet (see Volume I, Chapter 10 for more information about applets). Obviously, delivering executable applets is practical only when the recipients are sure that the code can't w

3、reak havoc on their machines. For this reason, security was and is a major concern of both the designers and the users of Java technology. This means that unlike other languages and systems, where security was implemente

4、d as an afterthought or a reaction to break-ins, security mechanisms are an integral part of Java technology.Three mechanisms help ensure safety:? Language design features (bounds checking on arrays, no unchecked type co

5、nversions, no pointer arithmetic, and so on).? An access control mechanism that controls what the code can do (such as file access, network access, and so on).? Code signing, whereby code authors can use standard cryptog

6、raphic algorithms to authenticate Java code. Then, the users of the code can determine exactly who created the code and whether the code has been altered after it was signed.Below, you'll see the cryptographic algori

7、thms supplied in the java.security package, which allow for code signing and user authentication.As we said earlier, applets were what started the craze over the Java platform. In practice, people discovered that althoug

8、h they could write animated applets like the famous “nervous text“ applet, applets could not do a whole lot of useful stuff in the JDK 1.0 security model. For example, because applets under JDK 1.0 were so closely superv

9、ised, they couldn't do much good on a corporate intranet, even though relatively little risk attaches to executing an applet from your company's secure intranet. It quickly became clear to Sun that for applets to

10、 become truly useful, it was important for users to be able to assign different levels of security, depending on where the applet originated. If an applet comes from a trusted supplier and it has not been tampered with,

11、the user of that applet can then decide whether to give the applet more privileges.To give more trust to an applet, we need to know two things:? Where did the applet come from?the proud owner of a billion computers from

12、the time the Earth was formed, each computing a million messages a second, he would not yet have found a message he could substitute.A number of algorithms have been designed to compute these message digests. The two bes

13、t-known are SHA1, the secure hash algorithm developed by the National Institute of Standards and Technology, and MD5, an algorithm invented by Ronald Rivest of MIT. Both algorithms scramble the bits of a message in ingen

14、ious ways. For details about these algorithms, see, for example, Cryptography and Network Security, 4th ed., by William Stallings (Prentice Hall 2005). Note that recently, subtle regularities have been discovered in both

15、 algorithms. At this point, most cryptographers recommend avoiding MD5 and using SHA1 until a stronger alternative becomes available. (See http://www.rsa.com/rsalabs/node.asp?id=2834 for more information.)The Java progra

16、mming language implements both SHA1 and MD5. The MessageDigest class is a factory for creating objects that encapsulate the fingerprinting algorithms. It has a static method, called getInstance, that returns an object of

17、 a class that extends the MessageDigest class. This means the MessageDigest class serves double duty:? As a factory class? As the superclass for all message digest algorithmsFor example, here is how you obtain an object

18、that can compute SHA fingerprints:MessageDigest alg = MessageDigest.getInstance(“SHA-1“);(To get an object that can compute MD5, use the string “MD5“ as the argument to getInstance.)After you have obtained a MessageDiges

19、t object, you feed it all the bytes in the message by repeatedly calling the update method. For example, the following code passes all bytes in a file to the alg object just created to do the fingerprinting：InputStream i

20、n = . . .int ch;while ((ch = in.read()) != -1)alg.update((byte) ch);Alternatively, if you have the bytes in an array, you can update the entire array at once:byte[] bytes = . . .;alg.update(bytes);When you are done, call