https://openjdk.java.net/ 
/*
 * Copyright (c) 1997, 2007, 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,
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 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package javax.crypto;

import java.util.*;

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

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


This class provides the functionality of a key agreement (or key
exchange) protocol.


The keys involved in establishing a shared secret are created by one of the
key generators (KeyPairGenerator or
KeyGenerator), a KeyFactory, or as a result from
an intermediate phase of the key agreement protocol.

 For each of the correspondents in the key exchange, doPhase
needs to be called. For example, if this key exchange is with one other
party, doPhase needs to be called once, with the
lastPhase flag set to true.
If this key exchange is
with two other parties, doPhase needs to be called twice,
the first time setting the lastPhase flag to
false, and the second time setting it to true.
There may be any number of parties involved in a key exchange.

Author:Jan Luehe
See Also:
Since:1.4
/**
 * This class provides the functionality of a key agreement (or key
 * exchange) protocol.
 * <p>
 * The keys involved in establishing a shared secret are created by one of the
 * key generators (<code>KeyPairGenerator</code> or
 * <code>KeyGenerator</code>), a <code>KeyFactory</code>, or as a result from
 * an intermediate phase of the key agreement protocol.
 *
 * <p> For each of the correspondents in the key exchange, <code>doPhase</code>
 * needs to be called. For example, if this key exchange is with one other
 * party, <code>doPhase</code> needs to be called once, with the
 * <code>lastPhase</code> flag set to <code>true</code>.
 * If this key exchange is
 * with two other parties, <code>doPhase</code> needs to be called twice,
 * the first time setting the <code>lastPhase</code> flag to
 * <code>false</code>, and the second time setting it to <code>true</code>.
 * There may be any number of parties involved in a key exchange.
 *
 * @author Jan Luehe
 *
 * @see KeyGenerator
 * @see SecretKey
 * @since 1.4
 */

public class KeyAgreement {

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

    // The provider
    private Provider provider;

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

    // The name of the key agreement algorithm.
    private final String algorithm;

    // 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 KeyAgreement object.

Params:
  • keyAgreeSpi – the delegate
  • provider – the provider
  • algorithm – the algorithm
/**
     * Creates a KeyAgreement object.
     *
     * @param keyAgreeSpi the delegate
     * @param provider the provider
     * @param algorithm the algorithm
     */
    protected KeyAgreement(KeyAgreementSpi keyAgreeSpi, Provider provider,
                           String algorithm) {
        this.spi = keyAgreeSpi;
        this.provider = provider;
        this.algorithm = algorithm;
        lock = null;
    }

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

    
Returns the algorithm name of this KeyAgreement object.

This is the same name that was specified in one of the
getInstance calls that created this
KeyAgreement object.

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

    
Returns a KeyAgreement object that implements the
specified key agreement algorithm.

 This method traverses the list of registered security Providers,
starting with the most preferred Provider.
A new KeyAgreement object encapsulating the
KeyAgreementSpi 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:
Returns:the new KeyAgreement object.
/**
     * Returns a <code>KeyAgreement</code> object that implements the
     * specified key agreement algorithm.
     *
     * <p> This method traverses the list of registered security Providers,
     * starting with the most preferred Provider.
     * A new KeyAgreement object encapsulating the
     * KeyAgreementSpi 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.
     *
     * @param algorithm the standard name of the requested key agreement
     * algorithm.
     * See Appendix A in the
     * <a href=
     *   "{@docRoot}/../technotes/guides/security/crypto/CryptoSpec.html#AppA">
     * Java Cryptography Architecture Reference Guide</a>
     * for information about standard algorithm names.
     *
     * @return the new <code>KeyAgreement</code> object.
     *
     * @exception NullPointerException if the specified algorithm
     *          is null.
     *
     * @exception NoSuchAlgorithmException if no Provider supports a
     *          KeyAgreementSpi implementation for the
     *          specified algorithm.
     *
     * @see java.security.Provider
     */
    public static final KeyAgreement getInstance(String algorithm)
            throws NoSuchAlgorithmException {
        List<Service> services =
                GetInstance.getServices("KeyAgreement", 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 KeyAgreement(s, t, algorithm);
        }
        throw new NoSuchAlgorithmException
                                ("Algorithm " + algorithm + " not available");
    }

    
Returns a KeyAgreement object that implements the
specified key agreement algorithm.

 A new KeyAgreement object encapsulating the
KeyAgreementSpi 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 KeyAgreement object.
/**
     * Returns a <code>KeyAgreement</code> object that implements the
     * specified key agreement algorithm.
     *
     * <p> A new KeyAgreement object encapsulating the
     * KeyAgreementSpi 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 key agreement
     * algorithm.
     * See Appendix A in the
     * <a href=
     *   "{@docRoot}/../technotes/guides/security/crypto/CryptoSpec.html#AppA">
     * Java Cryptography Architecture Reference Guide</a>
     * for information about standard algorithm names.
     *
     * @param provider the name of the provider.
     *
     * @return the new <code>KeyAgreement</code> object.
     *
     * @exception NullPointerException if the specified algorithm
     *          is null.
     *
     * @exception NoSuchAlgorithmException if a KeyAgreementSpi
     *          implementation for the specified algorithm is not
     *          available from the specified provider.
     *
     * @exception NoSuchProviderException if the specified provider is not
     *          registered in the security provider list.
     *
     * @exception IllegalArgumentException if the <code>provider</code>
     *          is null or empty.
     *
     * @see java.security.Provider
     */
    public static final KeyAgreement getInstance(String algorithm,
            String provider) throws NoSuchAlgorithmException,
            NoSuchProviderException {
        Instance instance = JceSecurity.getInstance
                ("KeyAgreement", KeyAgreementSpi.class, algorithm, provider);
        return new KeyAgreement((KeyAgreementSpi)instance.impl,
                instance.provider, algorithm);
    }

    
Returns a KeyAgreement object that implements the
specified key agreement algorithm.

 A new KeyAgreement object encapsulating the
KeyAgreementSpi 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 KeyAgreement object.
/**
     * Returns a <code>KeyAgreement</code> object that implements the
     * specified key agreement algorithm.
     *
     * <p> A new KeyAgreement object encapsulating the
     * KeyAgreementSpi 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 key agreement
     * algorithm.
     * See Appendix A in the
     * <a href=
     *   "{@docRoot}/../technotes/guides/security/crypto/CryptoSpec.html#AppA">
     * Java Cryptography Architecture Reference Guide</a>
     * for information about standard algorithm names.
     *
     * @param provider the provider.
     *
     * @return the new <code>KeyAgreement</code> object.
     *
     * @exception NullPointerException if the specified algorithm
     *          is null.
     *
     * @exception NoSuchAlgorithmException if a KeyAgreementSpi
     *          implementation for the specified algorithm is not available
     *          from the specified Provider object.
     *
     * @exception IllegalArgumentException if the <code>provider</code>
     *          is null.
     *
     * @see java.security.Provider
     */
    public static final KeyAgreement getInstance(String algorithm,
            Provider provider) throws NoSuchAlgorithmException {
        Instance instance = JceSecurity.getInstance
                ("KeyAgreement", KeyAgreementSpi.class, algorithm, provider);
        return new KeyAgreement((KeyAgreementSpi)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) {
            return;
        }
        synchronized (lock) {
            if (spi != null) {
                return;
            }
            if (debug != null) {
                int w = --warnCount;
                if (w >= 0) {
                    debug.println("KeyAgreement.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 KeyAgreementSpi == false) {
                        continue;
                    }
                    spi = (KeyAgreementSpi)obj;
                    provider = s.getProvider();
                    // not needed any more
                    firstService = null;
                    serviceIterator = null;
                    return;
                } catch (Exception e) {
                    lastException = e;
                }
            }
            ProviderException e = new ProviderException
                    ("Could not construct KeyAgreementSpi instance");
            if (lastException != null) {
                e.initCause(lastException);
            }
            throw e;
        }
    }

    private final static int I_NO_PARAMS = 1;
    private final static int I_PARAMS    = 2;

    private void implInit(KeyAgreementSpi spi, int type, Key key,
            AlgorithmParameterSpec params, SecureRandom random)
            throws InvalidKeyException, InvalidAlgorithmParameterException {
        if (type == I_NO_PARAMS) {
            spi.engineInit(key, random);
        } else { // I_PARAMS
            spi.engineInit(key, params, random);
        }
    }

    private void chooseProvider(int initType, Key key,
            AlgorithmParameterSpec params, SecureRandom random)
            throws InvalidKeyException, InvalidAlgorithmParameterException {
        synchronized (lock) {
            if (spi != null) {
                implInit(spi, initType, key, params, random);
                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 {
                    KeyAgreementSpi spi = (KeyAgreementSpi)s.newInstance(null);
                    implInit(spi, initType, key, params, random);
                    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 KeyAgreement object.

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

    
Initializes this key agreement with the given key, which is required to
contain all the algorithm parameters required for this key agreement.

 If this key agreement requires any random bytes, it will get them using the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness will be used.) 
Params:
  • key – the party's private information. For example, in the case
of the Diffie-Hellman key agreement, this would be the party's own
Diffie-Hellman private key.
Throws:
  • InvalidKeyException – if the given key is
inappropriate for this key agreement, e.g., is of the wrong type or
has an incompatible algorithm type.
/**
     * Initializes this key agreement with the given key, which is required to
     * contain all the algorithm parameters required for this key agreement.
     *
     * <p> If this key agreement requires any random bytes, it will get
     * them using the
     * {@link SecureRandom <code>SecureRandom</code>}
     * implementation of the highest-priority
     * installed provider as the source of randomness.
     * (If none of the installed providers supply an implementation of
     * SecureRandom, a system-provided source of randomness will be used.)
     *
     * @param key the party's private information. For example, in the case
     * of the Diffie-Hellman key agreement, this would be the party's own
     * Diffie-Hellman private key.
     *
     * @exception InvalidKeyException if the given key is
     * inappropriate for this key agreement, e.g., is of the wrong type or
     * has an incompatible algorithm type.
     */
    public final void init(Key key) throws InvalidKeyException {
        init(key, JceSecurity.RANDOM);
    }

    
Initializes this key agreement with the given key and source of
randomness. The given key is required to contain all the algorithm
parameters required for this key agreement.

 If the key agreement algorithm requires random bytes, it gets them
from the given source of randomness, random.
However, if the underlying
algorithm implementation does not require any random bytes,
random is ignored.

Params:
  • key – the party's private information. For example, in the case
of the Diffie-Hellman key agreement, this would be the party's own
Diffie-Hellman private key.
  • random – the source of randomness
Throws:
  • InvalidKeyException – if the given key is
inappropriate for this key agreement, e.g., is of the wrong type or
has an incompatible algorithm type.
/**
     * Initializes this key agreement with the given key and source of
     * randomness. The given key is required to contain all the algorithm
     * parameters required for this key agreement.
     *
     * <p> If the key agreement algorithm requires random bytes, it gets them
     * from the given source of randomness, <code>random</code>.
     * However, if the underlying
     * algorithm implementation does not require any random bytes,
     * <code>random</code> is ignored.
     *
     * @param key the party's private information. For example, in the case
     * of the Diffie-Hellman key agreement, this would be the party's own
     * Diffie-Hellman private key.
     * @param random the source of randomness
     *
     * @exception InvalidKeyException if the given key is
     * inappropriate for this key agreement, e.g., is of the wrong type or
     * has an incompatible algorithm type.
     */
    public final void init(Key key, SecureRandom random)
            throws InvalidKeyException {
        if (spi != null) {
            spi.engineInit(key, random);
        } else {
            try {
                chooseProvider(I_NO_PARAMS, key, null, random);
            } catch (InvalidAlgorithmParameterException e) {
                // should never occur
                throw new InvalidKeyException(e);
            }
        }
    }

    
Initializes this key agreement with the given key and set of
algorithm parameters.

 If this key agreement requires any random bytes, it will get them using the SecureRandom implementation of the highest-priority installed provider as the source of randomness. (If none of the installed providers supply an implementation of SecureRandom, a system-provided source of randomness will be used.) 
Params:
  • key – the party's private information. For example, in the case
of the Diffie-Hellman key agreement, this would be the party's own
Diffie-Hellman private key.
  • params – the key agreement parameters
Throws:
/**
     * Initializes this key agreement with the given key and set of
     * algorithm parameters.
     *
     * <p> If this key agreement requires any random bytes, it will get
     * them using the
     * {@link SecureRandom <code>SecureRandom</code>}
     * implementation of the highest-priority
     * installed provider as the source of randomness.
     * (If none of the installed providers supply an implementation of
     * SecureRandom, a system-provided source of randomness will be used.)
     *
     * @param key the party's private information. For example, in the case
     * of the Diffie-Hellman key agreement, this would be the party's own
     * Diffie-Hellman private key.
     * @param params the key agreement parameters
     *
     * @exception InvalidKeyException if the given key is
     * inappropriate for this key agreement, e.g., is of the wrong type or
     * has an incompatible algorithm type.
     * @exception InvalidAlgorithmParameterException if the given parameters
     * are inappropriate for this key agreement.
     */
    public final void init(Key key, AlgorithmParameterSpec params)
        throws InvalidKeyException, InvalidAlgorithmParameterException
    {
        init(key, params, JceSecurity.RANDOM);
    }

    
Initializes this key agreement with the given key, set of
algorithm parameters, and source of randomness.

Params:
  • key – the party's private information. For example, in the case
of the Diffie-Hellman key agreement, this would be the party's own
Diffie-Hellman private key.
  • params – the key agreement parameters
  • random – the source of randomness
Throws:
/**
     * Initializes this key agreement with the given key, set of
     * algorithm parameters, and source of randomness.
     *
     * @param key the party's private information. For example, in the case
     * of the Diffie-Hellman key agreement, this would be the party's own
     * Diffie-Hellman private key.
     * @param params the key agreement parameters
     * @param random the source of randomness
     *
     * @exception InvalidKeyException if the given key is
     * inappropriate for this key agreement, e.g., is of the wrong type or
     * has an incompatible algorithm type.
     * @exception InvalidAlgorithmParameterException if the given parameters
     * are inappropriate for this key agreement.
     */
    public final void init(Key key, AlgorithmParameterSpec params,
                           SecureRandom random)
        throws InvalidKeyException, InvalidAlgorithmParameterException
    {
        if (spi != null) {
            spi.engineInit(key, params, random);
        } else {
            chooseProvider(I_PARAMS, key, params, random);
        }
    }

    
Executes the next phase of this key agreement with the given
key that was received from one of the other parties involved in this key
agreement.

Params:
  • key – the key for this phase. For example, in the case of
Diffie-Hellman between 2 parties, this would be the other party's
Diffie-Hellman public key.
  • lastPhase – flag which indicates whether or not this is the last
phase of this key agreement.
Throws:
Returns:the (intermediate) key resulting from this phase, or null
if this phase does not yield a key
/**
     * Executes the next phase of this key agreement with the given
     * key that was received from one of the other parties involved in this key
     * agreement.
     *
     * @param key the key for this phase. For example, in the case of
     * Diffie-Hellman between 2 parties, this would be the other party's
     * Diffie-Hellman public key.
     * @param lastPhase flag which indicates whether or not this is the last
     * phase of this key agreement.
     *
     * @return the (intermediate) key resulting from this phase, or null
     * if this phase does not yield a key
     *
     * @exception InvalidKeyException if the given key is inappropriate for
     * this phase.
     * @exception IllegalStateException if this key agreement has not been
     * initialized.
     */
    public final Key doPhase(Key key, boolean lastPhase)
        throws InvalidKeyException, IllegalStateException
    {
        chooseFirstProvider();
        return spi.engineDoPhase(key, lastPhase);
    }

    
Generates the shared secret and returns it in a new buffer.

This method resets this KeyAgreement object, so that it
can be reused for further key agreements. Unless this key agreement is
reinitialized with one of the init methods, the same
private information and algorithm parameters will be used for
subsequent key agreements.

Throws:
Returns:the new buffer with the shared secret
/**
     * Generates the shared secret and returns it in a new buffer.
     *
     * <p>This method resets this <code>KeyAgreement</code> object, so that it
     * can be reused for further key agreements. Unless this key agreement is
     * reinitialized with one of the <code>init</code> methods, the same
     * private information and algorithm parameters will be used for
     * subsequent key agreements.
     *
     * @return the new buffer with the shared secret
     *
     * @exception IllegalStateException if this key agreement has not been
     * completed yet
     */
    public final byte[] generateSecret() throws IllegalStateException {
        chooseFirstProvider();
        return spi.engineGenerateSecret();
    }

    
Generates the shared secret, and places it into the buffer
sharedSecret, beginning at offset inclusive.

If the sharedSecret buffer is too small to hold the
result, a ShortBufferException is thrown.
In this case, this call should be repeated with a larger output buffer.

This method resets this KeyAgreement object, so that it
can be reused for further key agreements. Unless this key agreement is
reinitialized with one of the init methods, the same
private information and algorithm parameters will be used for
subsequent key agreements.

Params:
  • sharedSecret – the buffer for the shared secret
  • offset – the offset in sharedSecret where the
shared secret will be stored
Throws:
Returns:the number of bytes placed into sharedSecret
/**
     * Generates the shared secret, and places it into the buffer
     * <code>sharedSecret</code>, beginning at <code>offset</code> inclusive.
     *
     * <p>If the <code>sharedSecret</code> buffer is too small to hold the
     * result, a <code>ShortBufferException</code> is thrown.
     * In this case, this call should be repeated with a larger output buffer.
     *
     * <p>This method resets this <code>KeyAgreement</code> object, so that it
     * can be reused for further key agreements. Unless this key agreement is
     * reinitialized with one of the <code>init</code> methods, the same
     * private information and algorithm parameters will be used for
     * subsequent key agreements.
     *
     * @param sharedSecret the buffer for the shared secret
     * @param offset the offset in <code>sharedSecret</code> where the
     * shared secret will be stored
     *
     * @return the number of bytes placed into <code>sharedSecret</code>
     *
     * @exception IllegalStateException if this key agreement has not been
     * completed yet
     * @exception ShortBufferException if the given output buffer is too small
     * to hold the secret
     */
    public final int generateSecret(byte[] sharedSecret, int offset)
        throws IllegalStateException, ShortBufferException
    {
        chooseFirstProvider();
        return spi.engineGenerateSecret(sharedSecret, offset);
    }

    
Creates the shared secret and returns it as a SecretKey
object of the specified algorithm.

This method resets this KeyAgreement object, so that it
can be reused for further key agreements. Unless this key agreement is
reinitialized with one of the init methods, the same
private information and algorithm parameters will be used for
subsequent key agreements.

Params:
  • algorithm – the requested secret-key algorithm
Throws:
Returns:the shared secret key
/**
     * Creates the shared secret and returns it as a <code>SecretKey</code>
     * object of the specified algorithm.
     *
     * <p>This method resets this <code>KeyAgreement</code> object, so that it
     * can be reused for further key agreements. Unless this key agreement is
     * reinitialized with one of the <code>init</code> methods, the same
     * private information and algorithm parameters will be used for
     * subsequent key agreements.
     *
     * @param algorithm the requested secret-key algorithm
     *
     * @return the shared secret key
     *
     * @exception IllegalStateException if this key agreement has not been
     * completed yet
     * @exception NoSuchAlgorithmException if the specified secret-key
     * algorithm is not available
     * @exception InvalidKeyException if the shared secret-key material cannot
     * be used to generate a secret key of the specified algorithm (e.g.,
     * the key material is too short)
     */
    public final SecretKey generateSecret(String algorithm)
        throws IllegalStateException, NoSuchAlgorithmException,
            InvalidKeyException
    {
        chooseFirstProvider();
        return spi.engineGenerateSecret(algorithm);
    }
}