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;  
}