public abstract class

MessageDigest

extends MessageDigestSpi
java.lang.Object
   ↳ java.security.MessageDigestSpi
     ↳ java.security.MessageDigest

Class Overview

This MessageDigest class provides applications 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.

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. Client applications can test cloneability by attempting cloning and catching the CloneNotSupportedException: 

 MessageDigest md = MessageDigest.getInstance("SHA");

 try {
     md.update(toChapter1);
     MessageDigest tc1 = md.clone();
     byte[] toChapter1Digest = tc1.digest();
     md.update(toChapter2);
     ...etc.
 } catch (CloneNotSupportedException cnse) {
     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.

Note that this class is abstract and extends from MessageDigestSpi for historical reasons. Application developers should only take notice of the methods defined in this MessageDigest class; all the methods in the superclass are intended for cryptographic service providers who wish to supply their own implementations of message digest algorithms.

See Also

Summary

Protected Constructors
MessageDigest(String algorithm)
Creates a message digest with the specified algorithm name.
Public Methods
Object clone()
Returns a clone if the implementation is cloneable.
int digest(byte[] buf, int offset, int len)
Completes the hash computation by performing final operations such as padding.
byte[] digest()
Completes the hash computation by performing final operations such as padding.
byte[] digest(byte[] input)
Performs a final update on the digest using the specified array of bytes, then completes the digest computation.
final String getAlgorithm()
Returns a string that identifies the algorithm, independent of implementation details.
final int getDigestLength()
Returns the length of the digest in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.
static MessageDigest getInstance(String algorithm, String provider)
Returns a MessageDigest object that implements the specified digest algorithm.
static MessageDigest getInstance(String algorithm, Provider provider)
Returns a MessageDigest object that implements the specified digest algorithm.
static MessageDigest getInstance(String algorithm)
Returns a MessageDigest object that implements the specified digest algorithm.
final Provider getProvider()
Returns the provider of this message digest object.
static boolean isEqual(byte[] digesta, byte[] digestb)
Compares two digests for equality.
void reset()
Resets the digest for further use.
String toString()
Returns a string representation of this message digest object.
final void update(ByteBuffer input)
Update the digest using the specified ByteBuffer.
void update(byte[] input, int offset, int len)
Updates the digest using the specified array of bytes, starting at the specified offset.
void update(byte input)
Updates the digest using the specified byte.
void update(byte[] input)
Updates the digest using the specified array of bytes.
[Expand]
Inherited Methods
From class java.security.MessageDigestSpi
From class java.lang.Object

Protected Constructors

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.

Public Methods

public Object clone ()

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.

public int digest (byte[] buf, int offset, int len)

Completes the hash computation by performing final operations such as padding. The digest is reset after this call is made.

Parameters
buf output buffer for the computed digest
offset offset into the output buffer to begin storing the digest
len number of bytes within buf allotted for the digest
Returns
  • the number of bytes placed into buf
Throws
DigestException if an error occurs.

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 final update on the digest using the specified array of bytes, then completes the digest computation. That is, this method first calls update(input), passing the input array to the update method, 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.

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.

Returns
  • the name of the algorithm

public final int getDigestLength ()

Returns the length of the digest in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.

Returns
  • the digest length in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.

public static MessageDigest getInstance (String algorithm, String provider)

Returns a MessageDigest object that implements the specified digest algorithm.

A new MessageDigest object encapsulating the MessageDigestSpi implementation from the specified provider is returned. The specified provider must be registered in the security provider list.

Note that the list of registered providers may be retrieved via the Security.getProviders() method.

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 MessageDigest object that implements the specified algorithm.
Throws
NoSuchAlgorithmException if a MessageDigestSpi implementation for the specified algorithm is not available from the specified provider.
NoSuchProviderException if the specified provider is not registered in the security provider list.
IllegalArgumentException if the provider name is null or empty.
See Also

public static MessageDigest getInstance (String algorithm, Provider provider)

Returns a MessageDigest object that implements the specified digest algorithm.

A new MessageDigest object encapsulating the MessageDigestSpi implementation from the specified Provider object is returned. Note that the specified Provider object does not have to be registered in the provider list.

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 provider.
Returns
  • a MessageDigest object that implements the specified algorithm.
Throws
NoSuchAlgorithmException if a MessageDigestSpi implementation for the specified algorithm is not available from the specified Provider object.
IllegalArgumentException if the specified provider is null.
See Also

public static MessageDigest getInstance (String algorithm)

Returns a MessageDigest object that implements the specified digest algorithm.

This method traverses the list of registered security Providers, starting with the most preferred Provider. A new MessageDigest object encapsulating the MessageDigestSpi implementation from the first Provider that supports the specified algorithm is returned.

Note that the list of registered providers may be retrieved via the Security.getProviders() method.

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 that implements the specified algorithm.
Throws
NoSuchAlgorithmException if no Provider supports a MessageDigestSpi implementation for the specified algorithm.
See Also

public final Provider getProvider ()

Returns the provider of this message digest object.

Returns
  • the provider of this message digest 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 String toString ()

Returns a string representation of this message digest object.

Returns
  • a string representation of the object.

public final void update (ByteBuffer input)

Update the digest using the specified ByteBuffer. The digest is updated using the input.remaining() bytes starting at input.position(). Upon return, the buffer's position will be equal to its limit; its limit will not have changed.

Parameters
input the ByteBuffer

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 byte.

Parameters
input the byte with which to update the digest.

public void update (byte[] input)

Updates the digest using the specified array of bytes.

Parameters
input the array of bytes.