Java IO.pdf - Nguyen Dang Binh

Java I/O
you only want to detect significant changes, you have to first filter the insignificant data from
the stream in a predictable fashion before calculating the message digest.
10.2.2 Creating Message Digests
There are no public constructors in java.security.MessageDigest. Instead, you generally
use one of two MessageDigest.getInstance() factory methods to retrieve the appropriate
digest for a particular algorithm.
public static MessageDigest getInstance(String algorithm)
throws NoSuchAlgorithmException
public static MessageDigest getInstance(String algorithm, String provider)
throws NoSuchAlgorithmException, NoSuchProviderException
For example:
MessageDigest sha = MessageDigest.getInstance("SHA");
MessageDigest md2 = MessageDigest.getInstance("MD2", "Cryptix");
Each of these methods returns an instance of a MessageDigest subclass that's configured
with the requested algorithm. These subclasses and the associated MessageDigestSPI classes
that actually implement the algorithms are installed when you install a cryptographic provider.
JDK 1.2 installs the Sun provider, which supplies implementations of SHA-1 and MD5.
Each provider provides a possibly redundant collection of message digest algorithms. The
factory method design pattern used here allows for the possibility that a particular algorithm
may be provided by different classes in different environments. For instance, the SHA
algorithm may be supplied by the sun.security.provider.SHA class in one development
environment and by the cryptix.provider.md.SHA1 class in another. Some standard
algorithm names are listed in Table 10.1. If you request an algorithm none of your installed
providers can supply, a NoSuchAlgorithmException is thrown. Most of the time, you're
content to simply request an algorithm and let any provider that can fulfill your request
provide it. However, if you want to specify a particular provider by name (for instance,
because it's got an especially fast native-code implementation of the algorithm you want), you
can pass the provider name as the second argument to MessageDigest.getInstance(). If
the provider you request isn't found, a NoSuchProviderException is thrown.
MessageDigest has a protected constructor for use by subclasses. If you were writing your
own provider, with your own message digest algorithms and implementations, you'd use this:
protected MessageDigest(String algorithm)
However, as I said earlier, the creation of message digest algorithms and implementations is
harder than it looks and is a task best left to the experts. [3]
3 If you want to design your own one-way hash functions, Applied Cryptography, by Bruce Schneier (John Wiley & Sons, 1995) is a good place to
start. For details about how to implement Java security providers that supply new message digest algorithms, see Chapter 9 of Java Cryptography, by
Jonathan Knudsen (O'Reilly & Associates, 1998).
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