If you want to write your own security provider, you have the option of creating your own message digest engine. Typically, you’d do this because you want to ensure that a particular algorithm like SHA is available regardless of who the default security provider is; if you have a mathematics background, it’s conceivable that you might want to implement your own algorithm.
In order to implement a message digest algorithm, you must provide a
concrete subclass of the
This essentially entails providing an implementation of most of the
public methods we’ve just looked at. Although the public
methods are not declared abstract, they typically do nothing more
than call an internal (protected) method to accomplish their task.
MessageDigest class exists in both Java 1.1
and 1.2, which
is why it extends its SPI (see Chapter 8). For our
example, we’ll directly subclass the
MessageDigest class so that the resulting
example will work under both releases, but remember that in 1.2 you
have the option of extending the
MessageDigestSpi class directly.
There is a single constructor in the
MessageDigest class that is available to
Construct a message digest object. Classes that extend the
MessageDigest class must call this constructor,
as this is the only constructor in the class. As we’ll see,
however, the constructor of the subclass must take no arguments.
In order to write a message digest ...