Class java.security.MessageDigest





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Class java.security.MessageDigest


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



public abstract class MessageDigest
extends Object

This MessageDigest class provides 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.
Like other algorithm-based classes in Java Security,
MessageDigest has two major components:

Message Digest API (Application Program Interface)
This is the interface of methods called by applications needing
message digest services. The API consists of all public methods.
Message Digest SPI (Service Provider Interface)
This is the interface implemented by providers that supply
specific algorithms. It consists of all methods whose names
are prefixed by engine. Each such method is called by
a correspondingly-named public API method. For example,
the engineReset method is
called by the reset method.
The SPI methods are abstract; providers must supply a
concrete implementation.

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.


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.

engineDigest()
SPI: Completes the hash computation by performing final
operations such as padding.

engineReset()
SPI: Resets the digest for further use.

engineUpdate(byte)
SPI: Updates the digest using the specified byte.

engineUpdate(byte[], int, int)
SPI: Updates the digest using the specified array of bytes,
starting at the specified offset.

getAlgorithm()

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

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.

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.







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.







getInstance

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.



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



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 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.



toString

public String toString()


Returns a string representation of this message digest object.


Overrides:
toString in class 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.



engineUpdate

protected abstract void engineUpdate(byte input)


SPI: Updates the digest using the specified byte.


Parameters:
input - the byte to use for the update.



engineUpdate

protected abstract void engineUpdate(byte input[],
int offset,
int len)


SPI: Updates the digest using the specified array of bytes,
starting at the specified offset. This should be a no-op if
the digest has been finalized.


Parameters:
input - the array of bytes to use for the update.
offset - the offset to start from in the array of bytes.
len - the number of bytes to use, starting at
offset.



engineDigest

protected abstract byte[] engineDigest()


SPI: Completes the hash computation by performing final
operations such as padding. Once engineDigest has
been called, the engine should be reset (see reset). Resetting is the responsibility of the
engine implementor.


Returns:
the array of bytes for the resulting hash value.



engineReset

protected abstract void engineReset()


SPI: Resets the digest for further use.



clone

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 Object




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