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Class java.security.MessageDigest 

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

Constructor Index

o MessageDigest(String)
Creates a message digest with the specified algorithm name.

Method Index

o clone()
Returns a clone if the implementation is cloneable.
o digest()
Completes the hash computation by performing final operations such as padding.
o digest(byte[])
Performs a final update on the digest using the specified array of bytes, then completes the digest computation.
o engineDigest()
SPI: Completes the hash computation by performing final operations such as padding.
o engineReset()
SPI: Resets the digest for further use.
o engineUpdate(byte)
SPI: Updates the digest using the specified byte.
o engineUpdate(byte[], int, int)
SPI: Updates the digest using the specified array of bytes, starting at the specified offset.
o getAlgorithm()
Returns a string that identifies the algorithm, independent of implementation details.
o getInstance(String)
Generates a MessageDigest object that implements the specified digest algorithm.
o getInstance(String, String)
Generates a MessageDigest object implementing the specified algorithm, as supplied from the specified provider, if such an algorithm is available from the provider.
o isEqual(byte[], byte[])
Compares two digests for equality.
o reset()
Resets the digest for further use.
o toString()
Returns a string representation of this message digest object.
o update(byte)
Updates the digest using the specified byte.
o update(byte[])
Updates the digest using the specified array of bytes.
o update(byte[], int, int)
Updates the digest using the specified array of bytes, starting at the specified offset.

Constructors

o MessageDigest 
 protected MessageDigest(String algorithm)
Creates a message digest with the specified algorithm name.

Parameters:
algorithm - the standard name of the digest algorithm. See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.

Methods

o getInstance 
 public static MessageDigest getInstance(String algorithm) throws NoSuchAlgorithmException
Generates a MessageDigest object that implements the specified digest algorithm. If the default provider package contains a MessageDigest subclass implementing the algorithm, an instance of that subclass is returned. If the algorithm is not available in the default package, other packages are searched.

Parameters:
algorithm - the name of the algorithm requested. See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.
Returns:
a Message Digest object implementing the specified algorithm.
Throws: NoSuchAlgorithmException
if the algorithm is not available in the caller's environment.
o getInstance 
 public static MessageDigest getInstance(String algorithm,
                                         String provider) throws NoSuchAlgorithmException, NoSuchProviderException
Generates a MessageDigest object implementing the specified algorithm, as supplied from the specified provider, if such an algorithm is available from the provider.

Parameters:
algorithm - the name of the algorithm requested. See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.
provider - the name of the provider.
Returns:
a Message Digest object implementing the specified algorithm.
Throws: NoSuchAlgorithmException
if the algorithm is not available in the package supplied by the requested provider.
Throws: NoSuchProviderException
if the provider is not available in the environment.
See Also:
Provider
o update 
 public void update(byte input)
Updates the digest using the specified byte.

Parameters:
input - the byte with which to update the digest.
o update 
 public void update(byte input[],
                    int offset,
                    int len)
Updates the digest using the specified array of bytes, starting at the specified offset.

Parameters:
input - the array of bytes.
offset - the offset to start from in the array of bytes.
len - the number of bytes to use, starting at offset.
o update 
 public void update(byte input[])
Updates the digest using the specified array of bytes.

Parameters:
input - the array of bytes.
o digest 
 public byte[] digest()
Completes the hash computation by performing final operations such as padding. The digest is reset after this call is made.

Returns:
the array of bytes for the resulting hash value.
o digest 
 public byte[] digest(byte input[])
Performs a final update on the digest using the specified array of bytes, then completes the digest computation. That is, this method first calls update on the array, then calls digest().

Parameters:
input - the input to be updated before the digest is completed.
Returns:
the array of bytes for the resulting hash value.
o toString 
 public String toString()
Returns a string representation of this message digest object.

Overrides:
toString in class Object
o isEqual 
 public static boolean isEqual(byte digesta[],
                               byte digestb[])
Compares two digests for equality. Does a simple byte compare.

Parameters:
digesta - one of the digests to compare.
digestb - the other digest to compare.
Returns:
true if the digests are equal, false otherwise.
o reset 
 public void reset()
Resets the digest for further use.

o getAlgorithm 
 public final String getAlgorithm()
Returns a string that identifies the algorithm, independent of implementation details. The name should be a standard Java Security name (such as "SHA", "MD5", and so on). See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.

o engineUpdate 
 protected abstract void engineUpdate(byte input)
SPI: Updates the digest using the specified byte.

Parameters:
input - the byte to use for the update.
o engineUpdate 
 protected abstract void engineUpdate(byte input[],
                                      int offset,
                                      int len)
SPI: Updates the digest using the specified array of bytes, starting at the specified offset. This should be a no-op if the digest has been finalized.

Parameters:
input - the array of bytes to use for the update.
offset - the offset to start from in the array of bytes.
len - the number of bytes to use, starting at offset.
o engineDigest 
 protected abstract byte[] engineDigest()
SPI: Completes the hash computation by performing final operations such as padding. Once engineDigest has been called, the engine should be reset (see reset). Resetting is the responsibility of the engine implementor.

Returns:
the array of bytes for the resulting hash value.
o engineReset 
 protected abstract void engineReset()
SPI: Resets the digest for further use.

o clone 
 public Object clone() throws CloneNotSupportedException
Returns a clone if the implementation is cloneable.

Returns:
a clone if the implementation is cloneable.
Throws: CloneNotSupportedException
if this is called on an implementation that does not support Cloneable.
Overrides:
clone in class Object 
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