JavaTM 2 Platform
Std. Ed. v1.3.1

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 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:
DigestInputStream, DigestOutputStream

Constructor Summary
protected MessageDigest(String algorithm)
          Creates a message digest with the specified algorithm name.
 
Method Summary
 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)
          Generates a MessageDigest object that implements the specified digest algorithm.
static MessageDigest getInstance(String algorithm, String provider)
          Generates a MessageDigest object implementing the specified algorithm, as supplied from the specified provider, if such an algorithm is available from the provider.
 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.
 
Methods inherited from class java.security.MessageDigestSpi
engineDigest, engineDigest, engineGetDigestLength, engineReset, engineUpdate, engineUpdate
 
Methods inherited from class java.lang.Object
equals, finalize, 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 Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.
Method Detail

getInstance

public static MessageDigest getInstance(String algorithm)
                                 throws NoSuchAlgorithmException
Generates a MessageDigest object that implements the specified digest algorithm. If the default provider package provides an implementation of the requested digest algorithm, an instance of MessageDigest containing that implementation 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.

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.
NoSuchProviderException - if the provider is not available in the environment.
See Also:
Provider

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.

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
Following copied from class: java.lang.Object
Returns:
a string representation of the object.

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.

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.
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.

JavaTM 2 Platform
Std. Ed. v1.3.1

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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