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Class java.security.MessageDigest

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

public abstract class MessageDigest

extends MessageDigestSpi

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, and doing so will let client applications test cloneability using instanceof Cloneable before cloning:

 MessageDigest md = MessageDigest.getInstance("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 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, and ignore all the methods in the superclass.

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 final update on the digest using the specified array of bytes, then completes the digest computation.

digest(byte[], int, int)

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

getAlgorithm()

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

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.

getInstance(String)

Generates a MessageDigest object that implements the specified digest algorithm.

getInstance(String, String)

Generates a MessageDigest object implementing the specified algorithm, as supplied from the specified provider, if such an algorithm is available from the provider.

getProvider()

Returns the provider of this message digest object.

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 standard name of the digest algorithm. See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard algorithm names.

public static MessageDigest getInstance(String algorithm) throws NoSuchAlgorithmException

Generates a MessageDigest object that implements the specified digest algorithm. If the default provider package contains a MessageDigest subclass implementing the algorithm, an instance of that subclass 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.

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.

Throws: NoSuchProviderException

if the provider is not available in the environment.

See Also:

Provider

public final Provider getProvider()

Returns the provider of this message digest object.

Returns:

the provider of this message digest object

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 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 length of the digest

Throws: DigestException

if an error occurs.

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 on the array, 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 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 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.

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 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 MessageDigestSpi

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