分析DH加密算法，一种适基于密钥一致协议的加密算法。

DH
Diffie-Hellman算法(D-H算法)，密钥一致协议。是由公开密钥密码体制的奠基人Diffie和Hellman所提出的一种思想。简单的说就是允许两名用户在公开媒体上交换信息以生成"一致"的、可以共享的密钥。换句话说，就是由甲方产出一对密钥（公钥、私钥），乙方依照甲方公钥产生乙方密钥对（公钥、私钥）。以此为基线，作为数据传输保密基础，同时双方使用同一种对称加密算法构建本地密钥（SecretKey）对数据加密。这样，在互通了本地密钥（SecretKey）算法后，甲乙双方公开自己的公钥，使用对方的公钥和刚才产生的私钥加密数据，同时可以使用对方的公钥和自己的私钥对数据解密。不单单是甲乙双方两方，可以扩展为多方共享数据通讯，这样就完成了网络交互数据的安全通讯！该算法源于中国的同余定理——中国馀数定理。 

流程分析：

1.甲方构建密钥对儿，将公钥公布给乙方，将私钥保留；双方约定数据加密算法；乙方通过甲方公钥构建密钥对儿，将公钥公布给甲方，将私钥保留。
2.甲方使用私钥、乙方公钥、约定数据加密算法构建本地密钥，然后通过本地密钥加密数据，发送给乙方加密后的数据；乙方使用私钥、甲方公钥、约定数据加密算法构建本地密钥，然后通过本地密钥对数据解密。
3.乙方使用私钥、甲方公钥、约定数据加密算法构建本地密钥，然后通过本地密钥加密数据，发送给甲方加密后的数据；甲方使用私钥、乙方公钥、约定数据加密算法构建本地密钥，然后通过本地密钥对数据解密。

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通过java代码实现如下：Coder类见 Java加密技术（一）

import java.security.Key;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.HashMap;
import java.util.Map;

import javax.crypto.Cipher;
import javax.crypto.KeyAgreement;
import javax.crypto.SecretKey;
import javax.crypto.interfaces.DHPrivateKey;
import javax.crypto.interfaces.DHPublicKey;
import javax.crypto.spec.DHParameterSpec;

/**
 * DH安全编码组件
 *
 * @author 梁栋
 * @version 1.0
 * @since 1.0
 */
public abstract class DHCoder extends Coder {
    public static final String ALGORITHM = "DH";

    /**
     * 默认密钥字节数
     *
     * <pre>
     * DH
     * Default Keysize 1024
     * Keysize must be a multiple of 64, ranging from 512 to 1024 (inclusive).
     * </pre>
     */
    private static final int KEY_SIZE = 1024;

    /**
     * DH加密下需要一种对称加密算法对数据加密，这里我们使用DES，也可以使用其他对称加密算法。
     */
    public static final String SECRET_ALGORITHM = "DES";
    private static final String PUBLIC_KEY = "DHPublicKey";
    private static final String PRIVATE_KEY = "DHPrivateKey";

    /**
     * 初始化甲方密钥
     *
     * @return
     * @throws Exception
     */
    public static Map<String, Object> initKey() throws Exception {
        KeyPairGenerator keyPairGenerator = KeyPairGenerator
                .getInstance(ALGORITHM);
        keyPairGenerator.initialize(KEY_SIZE);

        KeyPair keyPair = keyPairGenerator.generateKeyPair();

        // 甲方公钥
        DHPublicKey publicKey = (DHPublicKey) keyPair.getPublic();

        // 甲方私钥
        DHPrivateKey privateKey = (DHPrivateKey) keyPair.getPrivate();

        Map<String, Object> keyMap = new HashMap<String, Object>(2);

        keyMap.put(PUBLIC_KEY, publicKey);
        keyMap.put(PRIVATE_KEY, privateKey);
        return keyMap;
    }

    /**
     * 初始化乙方密钥
     *
     * @param key
     *            甲方公钥
     * @return
     * @throws Exception
     */
    public static Map<String, Object> initKey(String key) throws Exception {
        // 解析甲方公钥
        byte[] keyBytes = decryptBASE64(key);
        X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(ALGORITHM);
        PublicKey pubKey = keyFactory.generatePublic(x509KeySpec);

        // 由甲方公钥构建乙方密钥
        DHParameterSpec dhParamSpec = ((DHPublicKey) pubKey).getParams();

        KeyPairGenerator keyPairGenerator = KeyPairGenerator
                .getInstance(keyFactory.getAlgorithm());
        keyPairGenerator.initialize(dhParamSpec);

        KeyPair keyPair = keyPairGenerator.generateKeyPair();

        // 乙方公钥
        DHPublicKey publicKey = (DHPublicKey) keyPair.getPublic();

        // 乙方私钥
        DHPrivateKey privateKey = (DHPrivateKey) keyPair.getPrivate();

        Map<String, Object> keyMap = new HashMap<String, Object>(2);

        keyMap.put(PUBLIC_KEY, publicKey);
        keyMap.put(PRIVATE_KEY, privateKey);

        return keyMap;
    }

    /**
     * 加密<br>
     *
     * @param data
     *            待加密数据
     * @param publicKey
     *            甲方公钥
     * @param privateKey
     *            乙方私钥
     * @return
     * @throws Exception
     */
    public static byte[] encrypt(byte[] data, String publicKey,
            String privateKey) throws Exception {

        // 生成本地密钥
        SecretKey secretKey = getSecretKey(publicKey, privateKey);

        // 数据加密
        Cipher cipher = Cipher.getInstance(secretKey.getAlgorithm());
        cipher.init(Cipher.ENCRYPT_MODE, secretKey);

        return cipher.doFinal(data);
    }

    /**
     * 解密<br>
     *
     * @param data
     *            待解密数据
     * @param publicKey
     *            乙方公钥
     * @param privateKey
     *            乙方私钥
     * @return
     * @throws Exception
     */
    public static byte[] decrypt(byte[] data, String publicKey,
            String privateKey) throws Exception {

        // 生成本地密钥
        SecretKey secretKey = getSecretKey(publicKey, privateKey);
        // 数据解密
        Cipher cipher = Cipher.getInstance(secretKey.getAlgorithm());
        cipher.init(Cipher.DECRYPT_MODE, secretKey);

        return cipher.doFinal(data);
    }

    /**
     * 构建密钥
     *
     * @param publicKey
     *            公钥
     * @param privateKey
     *            私钥
     * @return
     * @throws Exception
     */
    private static SecretKey getSecretKey(String publicKey, String privateKey)
            throws Exception {
        // 初始化公钥
        byte[] pubKeyBytes = decryptBASE64(publicKey);

        KeyFactory keyFactory = KeyFactory.getInstance(ALGORITHM);
        X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(pubKeyBytes);
        PublicKey pubKey = keyFactory.generatePublic(x509KeySpec);

        // 初始化私钥
        byte[] priKeyBytes = decryptBASE64(privateKey);

        PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(priKeyBytes);
        Key priKey = keyFactory.generatePrivate(pkcs8KeySpec);

        KeyAgreement keyAgree = KeyAgreement.getInstance(keyFactory
                .getAlgorithm());
        keyAgree.init(priKey);
        keyAgree.doPhase(pubKey, true);

        // 生成本地密钥
        SecretKey secretKey = keyAgree.generateSecret(SECRET_ALGORITHM);

        return secretKey;
    }

    /**
     * 取得私钥
     *
     * @param keyMap
     * @return
     * @throws Exception
     */
    public static String getPrivateKey(Map<String, Object> keyMap)
            throws Exception {
        Key key = (Key) keyMap.get(PRIVATE_KEY);

        return encryptBASE64(key.getEncoded());
    }

    /**
     * 取得公钥
     *
     * @param keyMap
     * @return
     * @throws Exception
     */
    public static String getPublicKey(Map<String, Object> keyMap)
            throws Exception {
        Key key = (Key) keyMap.get(PUBLIC_KEY);

        return encryptBASE64(key.getEncoded());
    }
}

再给出一个测试类：

import static org.junit.Assert.*;

import java.util.Map;

import org.junit.Test;

/**
 *
 * @author 梁栋
 * @version 1.0
 * @since 1.0
 */
public class DHCoderTest {

    @Test
    public void test() throws Exception {
        // 生成甲方密钥对儿
        Map<String, Object> aKeyMap = DHCoder.initKey();
        String aPublicKey = DHCoder.getPublicKey(aKeyMap);
        String aPrivateKey = DHCoder.getPrivateKey(aKeyMap);

        System.err.println("甲方公钥:
" + aPublicKey);
        System.err.println("甲方私钥:
" + aPrivateKey);

        // 由甲方公钥产生本地密钥对儿
        Map<String, Object> bKeyMap = DHCoder.initKey(aPublicKey);
        String bPublicKey = DHCoder.getPublicKey(bKeyMap);
        String bPrivateKey = DHCoder.getPrivateKey(bKeyMap);

        System.err.println("乙方公钥:
" + bPublicKey);
        System.err.println("乙方私钥:
" + bPrivateKey);

        String aInput = "abc ";
        System.err.println("原文: " + aInput);

        // 由甲方公钥，乙方私钥构建密文
        byte[] aCode = DHCoder.encrypt(aInput.getBytes(), aPublicKey,
                bPrivateKey);

        // 由乙方公钥，甲方私钥解密
        byte[] aDecode = DHCoder.decrypt(aCode, bPublicKey, aPrivateKey);
        String aOutput = (new String(aDecode));

        System.err.println("解密: " + aOutput);

        assertEquals(aInput, aOutput);

        System.err.println(" ===============反过来加密解密================== ");
        String bInput = "def ";
        System.err.println("原文: " + bInput);

        // 由乙方公钥，甲方私钥构建密文
        byte[] bCode = DHCoder.encrypt(bInput.getBytes(), bPublicKey,
                aPrivateKey);

        // 由甲方公钥，乙方私钥解密
        byte[] bDecode = DHCoder.decrypt(bCode, aPublicKey, bPrivateKey);
        String bOutput = (new String(bDecode));

        System.err.println("解密: " + bOutput);

        assertEquals(bInput, bOutput);
    }

}

控制台输出：

甲方公钥:
MIHfMIGXBgkqhkiG9w0BAwEwgYkCQQD8poLOjhLKuibvzPcRDlJtsHiwXt7LzR60ogjzrhYXrgHz
W5Gkfm32NBPF4S7QiZvNEyrNUNmRUb3EPuc3WS4XAkBnhHGyepz0TukaScUUfbGpqvJE8FpDTWSG
kx0tFCcbnjUDC3H9c9oXkGmzLik1Yw4cIGI1TQ2iCmxBblC+eUykAgIBgANDAAJAdAWBVmIzqcko
Ej6qFjLDL2+Y3FPq1iRbnOyOpDj71yKaK1K+FhTv04B0zy4DKcvAASV7/Gv0W+bgqdmffRkqrQ==

甲方私钥:
MIHRAgEAMIGXBgkqhkiG9w0BAwEwgYkCQQD8poLOjhLKuibvzPcRDlJtsHiwXt7LzR60ogjzrhYX
rgHzW5Gkfm32NBPF4S7QiZvNEyrNUNmRUb3EPuc3WS4XAkBnhHGyepz0TukaScUUfbGpqvJE8FpD
TWSGkx0tFCcbnjUDC3H9c9oXkGmzLik1Yw4cIGI1TQ2iCmxBblC+eUykAgIBgAQyAjACJRfy1LyR
eHyD+4Hfb+xR0uoIGR1oL9i9Nk6g2AAuaDPgEVWHn+QXID13yL/uDos=

乙方公钥:
MIHfMIGXBgkqhkiG9w0BAwEwgYkCQQD8poLOjhLKuibvzPcRDlJtsHiwXt7LzR60ogjzrhYXrgHz
W5Gkfm32NBPF4S7QiZvNEyrNUNmRUb3EPuc3WS4XAkBnhHGyepz0TukaScUUfbGpqvJE8FpDTWSG
kx0tFCcbnjUDC3H9c9oXkGmzLik1Yw4cIGI1TQ2iCmxBblC+eUykAgIBgANDAAJAVEYSfBA+I9nr
dWw3OBv475C+eBrWBBYqt0m6/eu4ptuDQHwV4MmUtKAC2wc2nNrdb1wmBhY1X8RnWkJ1XmdDbQ==

乙方私钥:
MIHSAgEAMIGXBgkqhkiG9w0BAwEwgYkCQQD8poLOjhLKuibvzPcRDlJtsHiwXt7LzR60ogjzrhYX
rgHzW5Gkfm32NBPF4S7QiZvNEyrNUNmRUb3EPuc3WS4XAkBnhHGyepz0TukaScUUfbGpqvJE8FpD
TWSGkx0tFCcbnjUDC3H9c9oXkGmzLik1Yw4cIGI1TQ2iCmxBblC+eUykAgIBgAQzAjEAqaZiCdXp
2iNpdBlHRaO9ir70wo2n32xNlIzIX19VLSPCDdeUWkgRv4CEj/8k+/yd

原文: abc
解密: abc
 ===============反过来加密解密==================
原文: def
解密: def

如我所言，甲乙双方在获得对方公钥后可以对发送给对方的数据加密，同时也能对接收到的数据解密，达到了数据安全通信的目的！