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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-256");

 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:

  • SHA-1
  • SHA-256
These algorithms are described in the MessageDigest section of the Java Security Standard Algorithm Names Specification. Consult the release documentation for your implementation to see if any other algorithms are supported.
Since:
1.1
See Also:

Constructor Summary

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

Method Summary

Modifier and Type Method 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.
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)
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 MessageDigest getInstance(String algorithm, Provider provider)
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.
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.
final void update(ByteBuffer input)
Update the digest using the specified ByteBuffer.

Methods declared in class java.security.MessageDigestSpi

engineDigest, engineDigest, engineGetDigestLength, engineReset, engineUpdate, engineUpdate, engineUpdate

Methods declared in class java.lang.Object

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

Constructor Details

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 Security Standard Algorithm Names Specification for information about standard algorithm names.

Method Details

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.

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

Implementation Note:
The JDK Reference Implementation additionally uses the jdk.security.provider.preferred Security property to determine the preferred provider order for the specified algorithm. This may be different from the order of providers returned by Security.getProviders().
Parameters:
algorithm - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
Returns:
a MessageDigest object that implements the specified algorithm
Throws:
NoSuchAlgorithmException - if no Provider supports a MessageDigestSpi implementation for the specified algorithm
NullPointerException - if algorithm is null
See Also:

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.

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

Parameters:
algorithm - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
provider - the name of the provider.
Returns:
a MessageDigest object that implements the specified algorithm
Throws:
IllegalArgumentException - if the provider name is null or empty
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
NullPointerException - if algorithm is null
See Also:

getInstance

public static MessageDigest getInstance(String algorithm, Provider provider) throws NoSuchAlgorithmException
Returns a MessageDigest object that implements the specified digest algorithm.

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

Parameters:
algorithm - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
provider - the provider.
Returns:
a MessageDigest object that implements the specified algorithm
Throws:
IllegalArgumentException - if the specified provider is null
NoSuchAlgorithmException - if a MessageDigestSpi implementation for the specified algorithm is not available from the specified Provider object
NullPointerException - if algorithm is null
Since:
1.4
See Also:

getProvider

public final Provider getProvider()
Returns the provider of this message digest object.
Returns:
the provider of this message digest 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, 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.

update

public void update(byte[] input)
Updates the digest using the specified array of bytes.
Parameters:
input - the array of bytes.

update

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
Since:
1.5

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 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.

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.

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.

isEqual

public static boolean isEqual(byte[] digesta, byte[] digestb)
Compares two digests for equality. Two digests are equal if they have the same length and all bytes at corresponding positions are equal.
Implementation Note:
All bytes in digesta are examined to determine equality. The calculation time depends only on the length of digesta. It does not depend on the length of digestb or the contents of digesta and digestb.
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.

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-256"). See the MessageDigest section in the Java Security Standard Algorithm Names Specification 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

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:

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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