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Class java.security.MessageDigest

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

public class MessageDigest

extends Object

Like other algorithm-based classes in JavaSecurity, the 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, digest 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.getMessageDigest("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 digest 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

MessageDigest(String)

Creates a message digest with the specified algorithm name.

clone()

Returns a clone if the implementation is cloneable.

digest()

Completes the hash computation by performing final operations such as padding.

digest(byte[])

Performs a full digest computation on the specified array of bytes, first calling update on the array, then calling digest().

engineDigest()

SPI:Completes the hash computation by performing final operations such as padding.

engineReset()

SPI:Resets the digest for further use.

engineUpdate(byte)

SPI:Updates the digest using the specified byte.

engineUpdate(byte[], int, int)

SPI:Updates the digest using the specified array of bytes, starting at the specified offset.

getAlgorithm()

Returns a string that identifies the algorithm, independent of implementation details.

getMessageDigest(String)

Returns a MessageDigest object using the specified digest algorithm from the default provider package.

getMessageDigest(String, String)

Returns a MessageDigest object using the specified digest algorithm, from the specified provider.

isEqual(byte[], byte[])

Compares two digests for equality.

reset()

Resets the digest for further use.

toString()

Returns a string representation of this message digest object.

update(byte)

Updates the digest using the specified byte.

update(byte[])

Updates the digest using the specified array of bytes.

update(byte[], int, int)

Updates the digest using the specified array of bytes, starting at the specified offset.

  protected MessageDigest(String algorithm)

Creates a message digest with the specified algorithm name.

Parameters:

algorithm - the name of the digest algorithm.

  public static MessageDigest getMessageDigest(String algorithm) throws NoSuchAlgorithmException

Returns a MessageDigest object using the specified digest algorithm from the default provider package. If the algorithm is not available in the default package, other packages are searched.

Parameters:

algorithm - the algorithm requested.

Returns:

a Message Digest object utilizing the specified algorithm.

Throws: NoSuchAlgorithmException

if the algorithm is not available in the caller's environment.

  public static MessageDigest getMessageDigest(String algorithm,
                                               String provider) throws NoSuchAlgorithmException, NoSuchProviderException

Returns a MessageDigest object using the specified digest algorithm, from the specified provider.

Parameters:

algorithm - the algorithm requested.

provider - the provider requested.

Returns:

a Message Digest object utilizing the specified algorithm.

Throws: NoSuchAlgorithmException

if the algorithm is not available in the package supplied by the requested provider.

Throws: NoSuchProviderException

if the specified provider is not available.

  public void update(byte input)

Updates the digest using the specified byte.

Parameters:

input - the byte with which to update the digest.

  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.

  public void update(byte input[])

Updates the digest using the specified array of bytes.

Parameters:

input - the array of bytes.

  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.

  public byte[] digest(byte input[])

Performs a full digest computation on the specified array of bytes, first calling update on the array, then calling digest().

Parameters:

input - the input to be updated before the digest is completed.

Returns:

the array of bytes for the resulting hash value.

  public String toString()

Returns a string representation of this message digest object.

Overrides:

toString in class Object

  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.

  public void reset()

Resets the digest for further use.

  public final String getAlgorithm()

Returns a string that identifies the algorithm, independent of implementation details. The name should be a standard JavaSecurity name. (i.e. SHA, MD5, etc). See algorithm names.

  protected abstract void engineUpdate(byte input)

SPI:Updates the digest using the specified byte.

Parameters:

input - the byte to update.

  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 update.

offset - the offset to start from in the array of bytes.

len - the number of bytes to use, starting at offset.

  protected abstract byte[] engineDigest()

SPI:Completes the hash computation by performing final operations such as padding. Once digest 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.

  protected abstract void engineReset()

SPI:Resets the digest for further use.

  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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