https://openjdk.java.net/ 
/*
 * Copyright (c) 1998, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * questions.
 */

package javax.crypto;

import java.util.*;

import java.security.*;
import java.security.Provider.Service;
import java.security.spec.AlgorithmParameterSpec;

import java.nio.ByteBuffer;

import sun.security.util.Debug;
import sun.security.jca.*;
import sun.security.jca.GetInstance.Instance;


This class provides the functionality of a "Message Authentication Code"
(MAC) algorithm.

 A MAC provides a way to check
the integrity of information transmitted over or stored in an unreliable
medium, based on a secret key. Typically, message
authentication codes are used between two parties that share a secret
key in order to validate information transmitted between these
parties.

 A MAC mechanism that is based on cryptographic hash functions is
referred to as HMAC. HMAC can be used with any cryptographic hash function,
e.g., SHA256 or SHA384, in combination with a secret shared key. HMAC is
specified in RFC 2104.

 Every implementation of the Java platform is required to support the following standard Mac algorithms: 
    

  • HmacMD5
    • 

  • HmacSHA1
    • 

  • HmacSHA256
    • 


These algorithms are described in the

Mac section of the
Java Security Standard Algorithm Names Specification.
Consult the release documentation for your implementation to see if any
other algorithms are supported.

Author:Jan Luehe
Since:1.4
/**
 * This class provides the functionality of a "Message Authentication Code"
 * (MAC) algorithm.
 *
 * <p> A MAC provides a way to check
 * the integrity of information transmitted over or stored in an unreliable
 * medium, based on a secret key. Typically, message
 * authentication codes are used between two parties that share a secret
 * key in order to validate information transmitted between these
 * parties.
 *
 * <p> A MAC mechanism that is based on cryptographic hash functions is
 * referred to as HMAC. HMAC can be used with any cryptographic hash function,
 * e.g., SHA256 or SHA384, in combination with a secret shared key. HMAC is
 * specified in RFC 2104.
 *
 * <p> Every implementation of the Java platform is required to support
 * the following standard {@code Mac} algorithms:
 * <ul>
 * <li>{@code HmacMD5}</li>
 * <li>{@code HmacSHA1}</li>
 * <li>{@code HmacSHA256}</li>
 * </ul>
 * These algorithms are described in the
 * <a href="{@docRoot}/../specs/security/standard-names.html#mac-algorithms">
 * Mac section</a> of the
 * Java Security Standard Algorithm Names Specification.
 * Consult the release documentation for your implementation to see if any
 * other algorithms are supported.
 *
 * @author Jan Luehe
 *
 * @since 1.4
 */

public class Mac implements Cloneable {

    private static final Debug debug =
                        Debug.getInstance("jca", "Mac");

    private static final Debug pdebug =
                        Debug.getInstance("provider", "Provider");
    private static final boolean skipDebug =
        Debug.isOn("engine=") && !Debug.isOn("mac");

    // The provider
    private Provider provider;

    // The provider implementation (delegate)
    private MacSpi spi;

    // The name of the MAC algorithm.
    private final String algorithm;

    // Has this object been initialized?
    private boolean initialized = false;

    // next service to try in provider selection
    // null once provider is selected
    private Service firstService;

    // remaining services to try in provider selection
    // null once provider is selected
    private Iterator<Service> serviceIterator;

    private final Object lock;

    
Creates a MAC object.

Params:
  • macSpi – the delegate
  • provider – the provider
  • algorithm – the algorithm
/**
     * Creates a MAC object.
     *
     * @param macSpi the delegate
     * @param provider the provider
     * @param algorithm the algorithm
     */
    protected Mac(MacSpi macSpi, Provider provider, String algorithm) {
        this.spi = macSpi;
        this.provider = provider;
        this.algorithm = algorithm;
        serviceIterator = null;
        lock = null;
    }

    private Mac(Service s, Iterator<Service> t, String algorithm) {
        firstService = s;
        serviceIterator = t;
        this.algorithm = algorithm;
        lock = new Object();
    }

    
Returns the algorithm name of this Mac object. 
This is the same name that was specified in one of the getInstance calls that created this Mac object. 
Returns:the algorithm name of this Mac object.
/**
     * Returns the algorithm name of this {@code Mac} object.
     *
     * <p>This is the same name that was specified in one of the
     * {@code getInstance} calls that created this
     * {@code Mac} object.
     *
     * @return the algorithm name of this {@code Mac} object.
     */
    public final String getAlgorithm() {
        return this.algorithm;
    }

    
Returns a Mac object that implements the specified MAC algorithm. 
 This method traverses the list of registered security Providers,
starting with the most preferred Provider.
A new Mac object encapsulating the
MacSpi implementation from the first
Provider that supports the specified algorithm is returned.

 Note that the list of registered providers may be retrieved via the Security.getProviders() method. 
Params:
Throws:
See Also:
Implementation Note: The JDK Reference Implementation additionally uses the jdk.security.provider.preferred Security property to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned by Security.getProviders().
Returns:the new Mac object
/**
     * Returns a {@code Mac} object that implements the
     * specified MAC algorithm.
     *
     * <p> This method traverses the list of registered security Providers,
     * starting with the most preferred Provider.
     * A new Mac object encapsulating the
     * MacSpi implementation from the first
     * Provider that supports the specified algorithm is returned.
     *
     * <p> Note that the list of registered providers may be retrieved via
     * the {@link Security#getProviders() Security.getProviders()} method.
     *
     * @implNote
     * The JDK Reference Implementation additionally uses the
     * {@code jdk.security.provider.preferred}
     * {@link Security#getProperty(String) Security} property to determine
     * the preferred provider order for the specified algorithm. This
     * may be different than the order of providers returned by
     * {@link Security#getProviders() Security.getProviders()}.
     *
     * @param algorithm the standard name of the requested MAC algorithm.
     * See the Mac section in the <a href=
     *   "{@docRoot}/../specs/security/standard-names.html#mac-algorithms">
     * Java Security Standard Algorithm Names Specification</a>
     * for information about standard algorithm names.
     *
     * @return the new {@code Mac} object
     *
     * @throws NoSuchAlgorithmException if no {@code Provider} supports a
     *         {@code MacSpi} implementation for the specified algorithm
     *
     * @throws NullPointerException if {@code algorithm} is {@code null}
     *
     * @see java.security.Provider
     */
    public static final Mac getInstance(String algorithm)
            throws NoSuchAlgorithmException {
        Objects.requireNonNull(algorithm, "null algorithm name");
        List<Service> services = GetInstance.getServices("Mac", algorithm);
        // make sure there is at least one service from a signed provider
        Iterator<Service> t = services.iterator();
        while (t.hasNext()) {
            Service s = t.next();
            if (JceSecurity.canUseProvider(s.getProvider()) == false) {
                continue;
            }
            return new Mac(s, t, algorithm);
        }
        throw new NoSuchAlgorithmException
                                ("Algorithm " + algorithm + " not available");
    }

    
Returns a Mac object that implements the specified MAC algorithm. 
 A new Mac object encapsulating the
MacSpi implementation from the specified provider
is returned.  The specified provider must be registered
in the security provider list.

 Note that the list of registered providers may be retrieved via the Security.getProviders() method. 
Params:
Throws:
See Also:
Returns:the new Mac object
/**
     * Returns a {@code Mac} object that implements the
     * specified MAC algorithm.
     *
     * <p> A new Mac object encapsulating the
     * MacSpi implementation from the specified provider
     * is returned.  The specified provider must be registered
     * in the security provider list.
     *
     * <p> Note that the list of registered providers may be retrieved via
     * the {@link Security#getProviders() Security.getProviders()} method.
     *
     * @param algorithm the standard name of the requested MAC algorithm.
     * See the Mac section in the <a href=
     *   "{@docRoot}/../specs/security/standard-names.html#mac-algorithms">
     * Java Security Standard Algorithm Names Specification</a>
     * for information about standard algorithm names.
     *
     * @param provider the name of the provider.
     *
     * @return the new {@code Mac} object
     *
     * @throws IllegalArgumentException if the {@code provider}
     *         is {@code null} or empty
     *
     * @throws NoSuchAlgorithmException if a {@code MacSpi}
     *         implementation for the specified algorithm is not
     *         available from the specified provider
     *
     * @throws NoSuchProviderException if the specified provider is not
     *         registered in the security provider list
     *
     * @throws NullPointerException if {@code algorithm} is {@code null}
     *
     * @see java.security.Provider
     */
    public static final Mac getInstance(String algorithm, String provider)
            throws NoSuchAlgorithmException, NoSuchProviderException {
        Objects.requireNonNull(algorithm, "null algorithm name");
        Instance instance = JceSecurity.getInstance
                ("Mac", MacSpi.class, algorithm, provider);
        return new Mac((MacSpi)instance.impl, instance.provider, algorithm);
    }

    
Returns a Mac object that implements the specified MAC algorithm. 
 A new Mac object encapsulating the
MacSpi implementation from the specified Provider
object is returned.  Note that the specified Provider object
does not have to be registered in the provider list.

Params:
Throws:
See Also:
Returns:the new Mac object
/**
     * Returns a {@code Mac} object that implements the
     * specified MAC algorithm.
     *
     * <p> A new Mac object encapsulating the
     * MacSpi implementation from the specified Provider
     * object is returned.  Note that the specified Provider object
     * does not have to be registered in the provider list.
     *
     * @param algorithm the standard name of the requested MAC algorithm.
     * See the Mac section in the <a href=
     *   "{@docRoot}/../specs/security/standard-names.html#mac-algorithms">
     * Java Security Standard Algorithm Names Specification</a>
     * for information about standard algorithm names.
     *
     * @param provider the provider.
     *
     * @return the new {@code Mac} object
     *
     * @throws IllegalArgumentException if the {@code provider} is
     *         {@code null}
     *
     * @throws NoSuchAlgorithmException if a {@code MacSpi}
     *         implementation for the specified algorithm is not available
     *         from the specified {@code Provider} object
     *
     * @throws NullPointerException if {@code algorithm} is {@code null}
     *
     * @see java.security.Provider
     */
    public static final Mac getInstance(String algorithm, Provider provider)
            throws NoSuchAlgorithmException {
        Objects.requireNonNull(algorithm, "null algorithm name");
        Instance instance = JceSecurity.getInstance
                ("Mac", MacSpi.class, algorithm, provider);
        return new Mac((MacSpi)instance.impl, instance.provider, algorithm);
    }

    // max number of debug warnings to print from chooseFirstProvider()
    private static int warnCount = 10;

    
Choose the Spi from the first provider available. Used if
delayed provider selection is not possible because init()
is not the first method called.

/**
     * Choose the Spi from the first provider available. Used if
     * delayed provider selection is not possible because init()
     * is not the first method called.
     */
    void chooseFirstProvider() {
        if ((spi != null) || (serviceIterator == null)) {
            return;
        }
        synchronized (lock) {
            if (spi != null) {
                return;
            }
            if (debug != null) {
                int w = --warnCount;
                if (w >= 0) {
                    debug.println("Mac.init() not first method "
                        + "called, disabling delayed provider selection");
                    if (w == 0) {
                        debug.println("Further warnings of this type will "
                            + "be suppressed");
                    }
                    new Exception("Call trace").printStackTrace();
                }
            }
            Exception lastException = null;
            while ((firstService != null) || serviceIterator.hasNext()) {
                Service s;
                if (firstService != null) {
                    s = firstService;
                    firstService = null;
                } else {
                    s = serviceIterator.next();
                }
                if (JceSecurity.canUseProvider(s.getProvider()) == false) {
                    continue;
                }
                try {
                    Object obj = s.newInstance(null);
                    if (obj instanceof MacSpi == false) {
                        continue;
                    }
                    spi = (MacSpi)obj;
                    provider = s.getProvider();
                    // not needed any more
                    firstService = null;
                    serviceIterator = null;
                    return;
                } catch (NoSuchAlgorithmException e) {
                    lastException = e;
                }
            }
            ProviderException e = new ProviderException
                    ("Could not construct MacSpi instance");
            if (lastException != null) {
                e.initCause(lastException);
            }
            throw e;
        }
    }

    private void chooseProvider(Key key, AlgorithmParameterSpec params)
            throws InvalidKeyException, InvalidAlgorithmParameterException {
        synchronized (lock) {
            if (spi != null) {
                spi.engineInit(key, params);
                return;
            }
            Exception lastException = null;
            while ((firstService != null) || serviceIterator.hasNext()) {
                Service s;
                if (firstService != null) {
                    s = firstService;
                    firstService = null;
                } else {
                    s = serviceIterator.next();
                }
                // if provider says it does not support this key, ignore it
                if (s.supportsParameter(key) == false) {
                    continue;
                }
                if (JceSecurity.canUseProvider(s.getProvider()) == false) {
                    continue;
                }
                try {
                    MacSpi spi = (MacSpi)s.newInstance(null);
                    spi.engineInit(key, params);
                    provider = s.getProvider();
                    this.spi = spi;
                    firstService = null;
                    serviceIterator = null;
                    return;
                } catch (Exception e) {
                    // NoSuchAlgorithmException from newInstance()
                    // InvalidKeyException from init()
                    // RuntimeException (ProviderException) from init()
                    if (lastException == null) {
                        lastException = e;
                    }
                }
            }
            // no working provider found, fail
            if (lastException instanceof InvalidKeyException) {
                throw (InvalidKeyException)lastException;
            }
            if (lastException instanceof InvalidAlgorithmParameterException) {
                throw (InvalidAlgorithmParameterException)lastException;
            }
            if (lastException instanceof RuntimeException) {
                throw (RuntimeException)lastException;
            }
            String kName = (key != null) ? key.getClass().getName() : "(null)";
            throw new InvalidKeyException
                ("No installed provider supports this key: "
                + kName, lastException);
        }
    }

    
Returns the provider of this Mac object. 
Returns:the provider of this Mac object.
/**
     * Returns the provider of this {@code Mac} object.
     *
     * @return the provider of this {@code Mac} object.
     */
    public final Provider getProvider() {
        chooseFirstProvider();
        return this.provider;
    }

    
Returns the length of the MAC in bytes.

Returns:the MAC length in bytes.
/**
     * Returns the length of the MAC in bytes.
     *
     * @return the MAC length in bytes.
     */
    public final int getMacLength() {
        chooseFirstProvider();
        return spi.engineGetMacLength();
    }

    private String getProviderName() {
        return (provider == null) ? "(no provider)" : provider.getName();
    }

    
Initializes this Mac object with the given key. 
Params:
  • key – the key.
Throws:
/**
     * Initializes this {@code Mac} object with the given key.
     *
     * @param key the key.
     *
     * @exception InvalidKeyException if the given key is inappropriate for
     * initializing this MAC.
     */
    public final void init(Key key) throws InvalidKeyException {
        try {
            if (spi != null) {
                spi.engineInit(key, null);
            } else {
                chooseProvider(key, null);
            }
        } catch (InvalidAlgorithmParameterException e) {
            throw new InvalidKeyException("init() failed", e);
        }
        initialized = true;

        if (!skipDebug && pdebug != null) {
            pdebug.println("Mac." + algorithm + " algorithm from: " +
                getProviderName());
        }
    }

    
Initializes this Mac object with the given key and algorithm parameters. 
Params:
  • key – the key.
  • params – the algorithm parameters.
Throws:
/**
     * Initializes this {@code Mac} object with the given key and
     * algorithm parameters.
     *
     * @param key the key.
     * @param params the algorithm parameters.
     *
     * @exception InvalidKeyException if the given key is inappropriate for
     * initializing this MAC.
     * @exception InvalidAlgorithmParameterException if the given algorithm
     * parameters are inappropriate for this MAC.
     */
    public final void init(Key key, AlgorithmParameterSpec params)
            throws InvalidKeyException, InvalidAlgorithmParameterException {
        if (spi != null) {
            spi.engineInit(key, params);
        } else {
            chooseProvider(key, params);
        }
        initialized = true;

        if (!skipDebug && pdebug != null) {
            pdebug.println("Mac." + algorithm + " algorithm from: " +
                getProviderName());
        }
    }

    
Processes the given byte.

Params:
  • input – the input byte to be processed.
Throws:
/**
     * Processes the given byte.
     *
     * @param input the input byte to be processed.
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final void update(byte input) throws IllegalStateException {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        spi.engineUpdate(input);
    }

    
Processes the given array of bytes.

Params:
  • input – the array of bytes to be processed.
Throws:
/**
     * Processes the given array of bytes.
     *
     * @param input the array of bytes to be processed.
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final void update(byte[] input) throws IllegalStateException {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        if (input != null) {
            spi.engineUpdate(input, 0, input.length);
        }
    }

    
Processes the first len bytes in input, starting at offset inclusive. 
Params:
  • input – the input buffer.
  • offset – the offset in input where the input starts.
  • len – the number of bytes to process.
Throws:
/**
     * Processes the first {@code len} bytes in {@code input},
     * starting at {@code offset} inclusive.
     *
     * @param input the input buffer.
     * @param offset the offset in {@code input} where the input starts.
     * @param len the number of bytes to process.
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final void update(byte[] input, int offset, int len)
            throws IllegalStateException {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }

        if (input != null) {
            if ((offset < 0) || (len > (input.length - offset)) || (len < 0))
                throw new IllegalArgumentException("Bad arguments");
            spi.engineUpdate(input, offset, len);
        }
    }

    
Processes input.remaining() bytes in the ByteBuffer input, starting at input.position(). Upon return, the buffer's position will be equal to its limit; its limit will not have changed. 
Params:
  • input – the ByteBuffer
Throws:
Since:1.5
/**
     * Processes {@code input.remaining()} bytes in the ByteBuffer
     * {@code input}, starting at {@code input.position()}.
     * Upon return, the buffer's position will be equal to its limit;
     * its limit will not have changed.
     *
     * @param input the ByteBuffer
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     * @since 1.5
     */
    public final void update(ByteBuffer input) {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        if (input == null) {
            throw new IllegalArgumentException("Buffer must not be null");
        }
        spi.engineUpdate(input);
    }

    
Finishes the MAC operation.

A call to this method resets this Mac object to the state it was in when previously initialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). That is, the object is reset and available to generate another MAC from the same key, if desired, via new calls to update and doFinal. (In order to reuse this Mac object with a different key, it must be reinitialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). 
Throws:
Returns:the MAC result.
/**
     * Finishes the MAC operation.
     *
     * <p>A call to this method resets this {@code Mac} object to the
     * state it was in when previously initialized via a call to
     * {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     * That is, the object is reset and available to generate another MAC from
     * the same key, if desired, via new calls to {@code update} and
     * {@code doFinal}.
     * (In order to reuse this {@code Mac} object with a different key,
     * it must be reinitialized via a call to {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     *
     * @return the MAC result.
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final byte[] doFinal() throws IllegalStateException {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        byte[] mac = spi.engineDoFinal();
        spi.engineReset();
        return mac;
    }

    
Finishes the MAC operation.

A call to this method resets this Mac object to the state it was in when previously initialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). That is, the object is reset and available to generate another MAC from the same key, if desired, via new calls to update and doFinal. (In order to reuse this Mac object with a different key, it must be reinitialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). 
The MAC result is stored in output, starting at outOffset inclusive. 
Params:
  • output – the buffer where the MAC result is stored
  • outOffset – the offset in output where the MAC is stored
Throws:
/**
     * Finishes the MAC operation.
     *
     * <p>A call to this method resets this {@code Mac} object to the
     * state it was in when previously initialized via a call to
     * {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     * That is, the object is reset and available to generate another MAC from
     * the same key, if desired, via new calls to {@code update} and
     * {@code doFinal}.
     * (In order to reuse this {@code Mac} object with a different key,
     * it must be reinitialized via a call to {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     *
     * <p>The MAC result is stored in {@code output}, starting at
     * {@code outOffset} inclusive.
     *
     * @param output the buffer where the MAC result is stored
     * @param outOffset the offset in {@code output} where the MAC is
     * stored
     *
     * @exception ShortBufferException if the given output buffer is too small
     * to hold the result
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final void doFinal(byte[] output, int outOffset)
        throws ShortBufferException, IllegalStateException
    {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        int macLen = getMacLength();
        if (output == null || output.length-outOffset < macLen) {
            throw new ShortBufferException
                ("Cannot store MAC in output buffer");
        }
        byte[] mac = doFinal();
        System.arraycopy(mac, 0, output, outOffset, macLen);
        return;
    }

    
Processes the given array of bytes and finishes the MAC operation.

A call to this method resets this Mac object to the state it was in when previously initialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). That is, the object is reset and available to generate another MAC from the same key, if desired, via new calls to update and doFinal. (In order to reuse this Mac object with a different key, it must be reinitialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). 
Params:
  • input – data in bytes
Throws:
Returns:the MAC result.
/**
     * Processes the given array of bytes and finishes the MAC operation.
     *
     * <p>A call to this method resets this {@code Mac} object to the
     * state it was in when previously initialized via a call to
     * {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     * That is, the object is reset and available to generate another MAC from
     * the same key, if desired, via new calls to {@code update} and
     * {@code doFinal}.
     * (In order to reuse this {@code Mac} object with a different key,
     * it must be reinitialized via a call to {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     *
     * @param input data in bytes
     * @return the MAC result.
     *
     * @exception IllegalStateException if this {@code Mac} has not been
     * initialized.
     */
    public final byte[] doFinal(byte[] input) throws IllegalStateException
    {
        chooseFirstProvider();
        if (initialized == false) {
            throw new IllegalStateException("MAC not initialized");
        }
        update(input);
        return doFinal();
    }

    
Resets this Mac object. 
A call to this method resets this Mac object to the state it was in when previously initialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). That is, the object is reset and available to generate another MAC from the same key, if desired, via new calls to update and doFinal. (In order to reuse this Mac object with a different key, it must be reinitialized via a call to init(Key) or init(Key, AlgorithmParameterSpec). 
/**
     * Resets this {@code Mac} object.
     *
     * <p>A call to this method resets this {@code Mac} object to the
     * state it was in when previously initialized via a call to
     * {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     * That is, the object is reset and available to generate another MAC from
     * the same key, if desired, via new calls to {@code update} and
     * {@code doFinal}.
     * (In order to reuse this {@code Mac} object with a different key,
     * it must be reinitialized via a call to {@code init(Key)} or
     * {@code init(Key, AlgorithmParameterSpec)}.
     */
    public final void reset() {
        chooseFirstProvider();
        spi.engineReset();
    }

    
Returns a clone if the provider implementation is cloneable.

Throws:
Returns:a clone if the provider implementation is cloneable.
/**
     * Returns a clone if the provider implementation is cloneable.
     *
     * @return a clone if the provider implementation is cloneable.
     *
     * @exception CloneNotSupportedException if this is called on a
     * delegate that does not support {@code Cloneable}.
     */
    public final Object clone() throws CloneNotSupportedException {
        chooseFirstProvider();
        Mac that = (Mac)super.clone();
        that.spi = (MacSpi)this.spi.clone();
        return that;
    }
}