Class MessageDigest

Class java.security.MessageDigest

Class Members | This Package | All Packages

java.lang.Object
   |
   +----java.security.MessageDigest

public abstract class MessageDigest
extends Object

This MessageDigest class provides the functionality of a message digest algorithm, such as MD5 or SHA. Message digests are secure one-way hash functions that take arbitrary-sized data and output a fixed-length hash value.

Like other algorithm-based classes in Java Security, MessageDigest has two major components:

Message Digest API (Application Program Interface): This is the interface of methods called by applications needing message digest services. The API consists of all public methods.
Message Digest SPI (Service Provider Interface): This is the interface implemented by providers that supply specific algorithms. It consists of all methods whose names are prefixed by engine. Each such method is called by a correspondingly-named public API method. For example, the engineReset method is called by the reset method. The SPI methods are abstract; providers must supply a concrete implementation.

A MessageDigest object starts out initialized. The data is processed through it using the update methods. At any point reset can be called to reset the digest. Once all the data to be updated has been updated, one of the digest methods should be called to complete the hash computation.

The digest method can be called once for a given number of updates. After digest has been called, the MessageDigest object is reset to its initialized state.

Implementations are free to implement the Cloneable interface, and doing so will let client applications test cloneability using instanceof Cloneable before cloning:

 MessageDigest md = MessageDigest.getInstance("SHA");
 if (md instanceof Cloneable) {
     md.update(toChapter1);
     MessageDigest tc1 = md.clone();
     byte[] toChapter1Digest = tc1.digest;
     md.update(toChapter2);
     ...etc.
 } else {
     throw new DigestException("couldn't make digest of partial content");
 }

Note that if a given implementation is not cloneable, it is still possible to compute intermediate digests by instantiating several instances, if the number of digests is known in advance.

See Also:: DigestInputStream, DigestOutputStream