Java in a Nutshell, 5th Edition [Electronic resources] نسخه متنی

This class defines an API for
computing a

message authentication
code
(MAC) that can check the
integrity of information transmitted between two parties that share a
secret key. A MAC is similar to a digital signature, except that it
is generated with a secret key rather than with a public/private key
pair. The Mac class is algorithm-independent and
provider-based. Obtain a Mac object by calling one
of the static getInstance( ) factory methods and
specifying the name of the desired MAC algorithm and, optionally, the
name of the provider of the desired implementation. In
Java 5.0 The
"SunJCE" provider implement MAC
algorithms with the following names:

After obtaining a Mac
object, initialize it by calling the init( )
method and specifying a SecretKey and, optionally,
a java.security.spec.AlgorithmParameterSpec
object. The "HmacMD5" and
"HmacSHA1" algorithms can use any
kind of SecretKey; they are not restricted to a
particular cryptographic algorithm. And neither algorithm requires an
AlgorithmParameterSpec object.

After
obtaining and initializing a Mac object, specify
the data for which the MAC is to be computed. If the data is
contained in a single byte array, simply pass it to doFinal(
). If the data is streaming or is stored in various
locations, you can supply the data in multiple calls to
update( ). In Java 5.0, you can pass a
ByteBuffer to update( ) which
facilities use with the java.nio New I/O API. End
the series of update( ) calls with a single call
to doFinal( ). Note that some versions of
doFinal( ) return the MAC data as the function
return value. Another version stores the MAC data in a byte array you
supply. If you use this version of doFinal( ), be
sure to call getMacLength( ) to instantiate an
array of the correct length.

A call to doFinal( )
resets the internal state of a Mac object. If you
want to compute a MAC for part of your data and then proceed to
compute the MAC for the full data, you should clone(
) the Mac object before calling
doFinal( ). Note, however, that
Mac implementations are not required to implement
Cloneable.

Figure 17-6. javax.crypto.Mac

public class

Mac implements Cloneable {
// Protected Constructors
protected

Mac (MacSpi

macSpi , java.security.Provider

provider , String

algorithm );
// Public Class Methods
public static final Mac

getInstance (String

algorithm )
throws java.security.NoSuchAlgorithmException;
public static final Mac

getInstance (String

algorithm , String

provider )
throws java.security.NoSuchAlgorithmException,
java.security.NoSuchProviderException;
public static final Mac

getInstance (String

algorithm , java.security.Provider

provider )
throws java.security.NoSuchAlgorithmException;
// Public Instance Methods
public final byte[ ]

doFinal ( ) throws IllegalStateException;
public final byte[ ]

doFinal (byte[ ]

input ) throws IllegalStateException;
public final void

doFinal (byte[ ]

output , int

outOffset )
throws ShortBufferException, IllegalStateException;
public final String

getAlgorithm ( );
public final int

getMacLength ( );
public final java.security.Provider

getProvider ( );
public final void

init (java.security.Key

key ) throws java.security.InvalidKeyException;
public final void

init (java.security.Key

key , java.security.spec
.AlgorithmParameterSpec

params ) throws java.security.InvalidKeyException,
java.security.InvalidAlgorithmParameterException;
public final void

reset ( );
public final void

update (byte

input ) throws IllegalStateException;

5.0 public final void

update (java.nio.ByteBuffer

input );
public final void

update (byte[ ]

input ) throws IllegalStateException;
public final void

update (byte[ ]

input , int

offset , int

len ) throws IllegalStateException;
// Public Methods Overriding Object
public final Object

clone ( ) throws CloneNotSupportedException;
}