java.security

Class MessageDigest

java.lang.Object

java.security.MessageDigestSpi

java.security.MessageDigest

public abstract class MessageDigest
extends MessageDigestSpi

This MessageDigest class provides applications the functionality of a message digest algorithm, such as SHA-1 or SHA-256. 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.

Every implementation of the Java platform is required to support the following standard MessageDigest algorithms:

• MD5
• SHA-1
• SHA-256

These algorithms are described in the MessageDigest section of the Java Cryptography Architecture Standard Algorithm Name Documentation. Consult the release documentation for your implementation to see if any other algorithms are supported.

Author:

Benjamin Renaud

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	MessageDigest(String algorithm) Creates a message digest with the specified algorithm name.

Method Summary

Modifier and Type	Method and Description
Object	clone() Returns a clone if the implementation is cloneable.
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.
int	digest(byte[] buf, int offset, int len) Completes the hash computation by performing final operations such as padding.
String	getAlgorithm() Returns a string that identifies the algorithm, independent of implementation details.
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) Returns a MessageDigest object that implements the specified digest algorithm.
static MessageDigest	getInstance(String algorithm, String provider) Returns a MessageDigest object that implements the specified digest algorithm.
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.
void	update(byte input) Updates the digest using the specified byte.
void	update(byte[] input) Updates the digest using the specified array of bytes.
void	update(byte[] input, int offset, int len) Updates the digest using the specified array of bytes, starting at the specified offset.

Methods inherited from class java.security.MessageDigestSpi

engineDigest, engineDigest, engineGetDigestLength, engineReset, engineUpdate, engineUpdate

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

MessageDigest

protected MessageDigest(String algorithm)

Creates a message digest with the specified algorithm name.

Parameters:

algorithm - the standard name of the digest algorithm. See the MessageDigest section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.

Method Detail

clone

public Object clone()
             throws CloneNotSupportedException

Returns a clone if the implementation is cloneable.

Overrides:

clone in class MessageDigestSpi

Returns:

a clone if the implementation is cloneable.

Throws:

CloneNotSupportedException - if this is called on an implementation that does not support Cloneable.

See Also:

Cloneable

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.

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(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.

digest

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

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.

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 the MessageDigest section in the Java Cryptography Architecture Standard Algorithm Name Documentation for information about standard algorithm names.

Returns:

the name of the algorithm

getDigestLength

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.

Since:

1.2

getInstance

public static MessageDigest getInstance(String algorithm)
                                 throws NoSuchAlgorithmException

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.

Parameters:

algorithm - the name of the algorithm requested. See the MessageDigest section in the Java Cryptography Architecture Standard Algorithm Name Documentation 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.

getInstance

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

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.

Parameters:

algorithm - the name of the algorithm requested. See the MessageDigest section in the Java Cryptography Architecture Standard Algorithm Name Documentation 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.

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.

reset

public void reset()

Resets the digest for further use.

toString

public String toString()

Returns a string representation of this message digest object.

Overrides:

toString in class Object

Returns:

a string representation of the object.

update

public void update(byte input)

Updates the digest using the specified byte.

Parameters:

input - the byte with which to update the digest.

update

public void update(byte[] input)

Updates the digest using the specified array of bytes.

Parameters:

input - the array of bytes.

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.