/*

 * Copyright 1996-2007 Sun Microsystems, Inc.  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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

package sun.security.ssl;

import java.math.BigInteger;

import java.security.*;

import javax.crypto.SecretKey;

import javax.crypto.KeyAgreement;

import javax.crypto.interfaces.DHPublicKey;

import javax.crypto.spec.*;

/**

 * This class implements the Diffie-Hellman key exchange algorithm.

 * D-H means combining your private key with your partners public key to

 * generate a number. The peer does the same with its private key and our

 * public key. Through the magic of Diffie-Hellman we both come up with the

 * same number. This number is secret (discounting MITM attacks) and hence

 * called the shared secret. It has the same length as the modulus, e.g. 512

 * or 1024 bit. Man-in-the-middle attacks are typically countered by an

 * independent authentication step using certificates (RSA, DSA, etc.).

 *

 * The thing to note is that the shared secret is constant for two partners

 * with constant private keys. This is often not what we want, which is why

 * it is generally a good idea to create a new private key for each session.

 * Generating a private key involves one modular exponentiation assuming

 * suitable D-H parameters are available.

 *

 * General usage of this class (TLS DHE case):

 *  . if we are server, call DHCrypt(keyLength,random). This generates

 *    an ephemeral keypair of the request length.

 *  . if we are client, call DHCrypt(modulus, base, random). This

 *    generates an ephemeral keypair using the parameters specified by the server.

 *  . send parameters and public value to remote peer

 *  . receive peers ephemeral public key

 *  . call getAgreedSecret() to calculate the shared secret

 *

 * In TLS the server chooses the parameter values itself, the client must use

 * those sent to it by the server.

 *

 * The use of ephemeral keys as described above also achieves what is called

 * "forward secrecy". This means that even if the authentication keys are

 * broken at a later date, the shared secret remains secure. The session is

 * compromised only if the authentication keys are already broken at the

 * time the key exchange takes place and an active MITM attack is used.

 * This is in contrast to straightforward encrypting RSA key exchanges.

 *

 * @author David Brownell

 */

final class DHCrypt {

    // group parameters (prime modulus and generator)

    private BigInteger modulus;                 // P (aka N)

    private BigInteger base;                    // G (aka alpha)

    // our private key (including private component x)

    private PrivateKey privateKey;

    // public component of our key, X = (g ^ x) mod p

    private BigInteger publicValue;             // X (aka y)

    /**

     * Generate a Diffie-Hellman keypair of the specified size.

     */

    DHCrypt(int keyLength, SecureRandom random) {

        try {

            KeyPairGenerator kpg = JsseJce.getKeyPairGenerator("DiffieHellman");

            kpg.initialize(keyLength, random);

            KeyPair kp = kpg.generateKeyPair();

            privateKey = kp.getPrivate();

            DHPublicKeySpec spec = getDHPublicKeySpec(kp.getPublic());

            publicValue = spec.getY();

            modulus = spec.getP();

            base = spec.getG();

        } catch (GeneralSecurityException e) {

            throw new RuntimeException("Could not generate DH keypair", e);

        }

    }

    /**

     * Generate a Diffie-Hellman keypair using the specified parameters.

     *

     * @param modulus the Diffie-Hellman modulus P

     * @param base the Diffie-Hellman base G

     */

    DHCrypt(BigInteger modulus, BigInteger base, SecureRandom random) {

        this.modulus = modulus;

        this.base = base;

        try {

            KeyPairGenerator kpg = JsseJce.getKeyPairGenerator("DiffieHellman");

            DHParameterSpec params = new DHParameterSpec(modulus, base);

            kpg.initialize(params, random);

            KeyPair kp = kpg.generateKeyPair();

            privateKey = kp.getPrivate();

            DHPublicKeySpec spec = getDHPublicKeySpec(kp.getPublic());

            publicValue = spec.getY();

        } catch (GeneralSecurityException e) {

            throw new RuntimeException("Could not generate DH keypair", e);

        }

    }

    static DHPublicKeySpec getDHPublicKeySpec(PublicKey key) {

        if (key instanceof DHPublicKey) {

            DHPublicKey dhKey = (DHPublicKey)key;

            DHParameterSpec params = dhKey.getParams();

            return new DHPublicKeySpec(dhKey.getY(), params.getP(), params.getG());

        }

        try {

            KeyFactory factory = JsseJce.getKeyFactory("DH");

            return (DHPublicKeySpec)factory.getKeySpec

                                            (key, DHPublicKeySpec.class);

        } catch (Exception e) {

            throw new RuntimeException(e);

        }

    }

    /** Returns the Diffie-Hellman modulus. */

    BigInteger getModulus() {

        return modulus;

    }

    /** Returns the Diffie-Hellman base (generator).  */

    BigInteger getBase() {

        return base;

    }

    /**

     * Gets the public key of this end of the key exchange.

     */

    BigInteger getPublicKey() {

        return publicValue;

    }

    /**

     * Get the secret data that has been agreed on through Diffie-Hellman

     * key agreement protocol.  Note that in the two party protocol, if

     * the peer keys are already known, no other data needs to be sent in

     * order to agree on a secret.  That is, a secured message may be

     * sent without any mandatory round-trip overheads.

     *

     * <P>It is illegal to call this member function if the private key

     * has not been set (or generated).

     *

     * @param peerPublicKey the peer's public key.

     * @returns the secret, which is an unsigned big-endian integer

     *  the same size as the Diffie-Hellman modulus.

     */

    SecretKey getAgreedSecret(BigInteger peerPublicValue) {

        try {

            KeyFactory kf = JsseJce.getKeyFactory("DiffieHellman");

            DHPublicKeySpec spec =

                        new DHPublicKeySpec(peerPublicValue, modulus, base);

            PublicKey publicKey = kf.generatePublic(spec);

            KeyAgreement ka = JsseJce.getKeyAgreement("DiffieHellman");

            ka.init(privateKey);

            ka.doPhase(publicKey, true);

            return ka.generateSecret("TlsPremasterSecret");

        } catch (GeneralSecurityException e) {

            throw new RuntimeException("Could not generate secret", e);

        }

    }

}