1.       什么是对称式加密和非对称式加密

对称式加密：解密方式是加密方式的逆运算，即加密和解密使用的是同一个密钥（初等加密算法）。

非对称式加密：加密和解密需要两个密钥进行，这两个密钥是公钥和私钥（高等加密算法）。

2.       对称式加密和非对称式加密的主要实现方式

对称式加密主要实现方式：

1）  DES（Data Encryption Standard）：安全性较差，一般不建议使用。

2）  3DES：三重DES，针对DES密钥长度较短的问题，3DES可以实现最少112位、最高168位密钥长度。

3）  AES：相比于3DES拥有更好的安全性。

4）  PBE（Password Based Encryption）：基于口令的加密，结合了DES和AES的优点。

非对称式加密主要实现方式：

1）  DH（Diffie-Hellman）：密钥交换算法。

2）  RSA：基于因子分解。

3）  ElGamal：基于离散对数。

4）  ECC（Elliptical Curve Cryptography）：椭圆曲线加密。

3.       对称式加密和非对称式加密的对比

非对称加密与对称加密相比，其安全性更好：对称加密的通信双方使用相同的秘钥，如果一方的秘钥遭泄露，那么整个通信就会被破解。而非对称加密使用一对秘钥，一个用来加密，一个用来解密，而且公钥是公开的，秘钥是自己保存的，不需要像对称加密那样在通信之前要先同步密钥。

4.       对称式加密在JAVA中的实现（PBE加密）：

public class PBE {
    private static String src="security with PBE";
    public static void main(String[] args){
        jdkPBE();
    }

    public static void jdkPBE(){
        try {
            //初始化盐（扰码）
            SecureRandom random=new SecureRandom();
            byte[] salt=random.generateSeed(8);

            //口令与密钥
            String password="CSDN";
            PBEKeySpec pbeKeySpec=new PBEKeySpec(password.toCharArray());
            SecretKeyFactory factory=SecretKeyFactory.getInstance("PBEWITHMD5andEDS");
            Key key=factory.generateSecret(pbeKeySpec);

            //加密
            PBEParameterSpec pbeParameterSpec=new PBEParameterSpec(salt,100);//参数1.盐，参数2.迭代次数
            Cipher cipher=Cipher.getInstance("PBEWITHMD5andEDS");
            cipher.init(Cipher.ENCRYPT_MODE,key,pbeParameterSpec);
            byte[] result=cipher.doFinal(src.getBytes());
            System.out.println("jdk pbe encrypt:" + Base64.encodeToString(result,1));

            //解密
            cipher.init(Cipher.DECRYPT_MODE,key,pbeParameterSpec);
            result=cipher.doFinal(result);
            System.out.println("jdk pbe decrypt:"+new String(result));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

5.      非对称式加密在JAVA中的实现（DH加密）：

public class DH {
    private static String src="security with DH";
    public static void main(String[] args){
        jdkDH();
    }

    public static void jdkDH(){
        try {
            //初始化发送方密钥
            KeyPairGenerator senderKeyPairGenerator=KeyPairGenerator.getInstance("DH");
            senderKeyPairGenerator.initialize(512);
            KeyPair senderKeyPair=senderKeyPairGenerator.generateKeyPair();
            byte[] senderPublicKeyEnc=senderKeyPair.getPublic().getEncoded();

            //初始化接收方密钥
            KeyFactory receiverKeyFactory=KeyFactory.getInstance("DH");
            X509EncodedKeySpec x509EncodedKeySpec=new X509EncodedKeySpec(senderPublicKeyEnc);
            PublicKey receiverPublicKey=receiverKeyFactory.generatePublic(x509EncodedKeySpec);
            DHParameterSpec dhParameterSpec=((DHPublicKey)receiverPublicKey).getParams();
            KeyPairGenerator receiverKeyPairGenerator=KeyPairGenerator.getInstance("DH");
            receiverKeyPairGenerator.initialize(dhParameterSpec);
            KeyPair receiverKeyPair=receiverKeyPairGenerator.generateKeyPair();
            PrivateKey receiverPrivateKey=receiverKeyPair.getPrivate();
            byte[] receiverPublicKeyEnc =receiverKeyPair.getPublic().getEncoded();

            //密钥构建
            KeyAgreement receiverKeyAgreement = KeyAgreement.getInstance("DH");
            receiverKeyAgreement.init(receiverPrivateKey);
            receiverKeyAgreement.doPhase(receiverPublicKey, true);
            SecretKey receiverDesKey=receiverKeyAgreement.generateSecret("DES");

            KeyFactory senderKeyFactory=KeyFactory.getInstance("DH");
            x509EncodedKeySpec=new X509EncodedKeySpec(receiverPublicKeyEnc);
            PublicKey senderPublicKey=senderKeyFactory.generatePublic(x509EncodedKeySpec);
            KeyAgreement senderKeyAgreement=KeyAgreement.getInstance("DH");
            senderKeyAgreement.init(senderKeyPair.getPrivate());
            senderKeyAgreement.doPhase(senderPublicKey, true);
            SecretKey senderDesKey=senderKeyAgreement.generateSecret("DES");
            if(Objects.equals(receiverDesKey,senderDesKey)){
                System.out.println("双方密钥相同");
            }

            //加密
            Cipher cipher=Cipher.getInstance("DES");
            cipher.init(Cipher.ENCRYPT_MODE,senderDesKey);
            byte[] result=cipher.doFinal(src.getBytes());
            System.out.println("jdk dh encrypt:"+Base64.encodeToString(result,1));

            //解密
            cipher=Cipher.getInstance("DES");
            cipher.init(Cipher.DECRYPT_MODE,receiverDesKey);
            result=cipher.doFinal(result);
            System.out.println("jdk dh encrypt:"+new String(result));

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

6.      非对称式加密在JAVA中的实现（RSA加密）：

public class RSA {
    private static String src="security with rsa";
    public static void main(String[] args){
        jdkRSA();
    }

    public static void jdkRSA(){
        try {
            //初始化密钥
            KeyPairGenerator keyPairGenerator=KeyPairGenerator.getInstance("RSA");
            keyPairGenerator.initialize(512);//64的整倍数，512~65536
            KeyPair keyPair=keyPairGenerator.generateKeyPair();
            RSAPrivateKey rsaPrivateKey= (RSAPrivateKey) keyPair.getPrivate();
            RSAPublicKey rsaPublicKey= (RSAPublicKey) keyPair.getPublic();
            System.out.println("Public key:"+ Base64.encodeToString(rsaPrivateKey.getEncoded(),1));
            System.out.println("Private key:"+ Base64.encodeToString(rsaPrivateKey.getEncoded(),1));

            //私钥加密，公钥解密——加密
            PKCS8EncodedKeySpec pkcs8EncodedKeySpec=new PKCS8EncodedKeySpec(rsaPrivateKey.getEncoded());
            KeyFactory keyFactory=KeyFactory.getInstance("RSA");
            PrivateKey privateKey=keyFactory.generatePrivate(pkcs8EncodedKeySpec);
            Cipher cipher=Cipher.getInstance("RSA");
            cipher.init(Cipher.ENCRYPT_MODE, privateKey);
            byte[] result=cipher.doFinal(src.getBytes());
            System.out.println("私钥加密，公钥解密——加密:"+Base64.encodeToString(result,1));

            //私钥加密，公钥解密——解密
            X509EncodedKeySpec x509EncodedKeySpec=new X509EncodedKeySpec(rsaPublicKey.getEncoded());
            keyFactory=KeyFactory.getInstance("RSA");
            PublicKey publicKey=keyFactory.generatePublic(x509EncodedKeySpec);
            cipher=Cipher.getInstance("RSA");
            cipher.init(Cipher.DECRYPT_MODE,publicKey);
            result=cipher.doFinal(result);
            System.out.println(new String(result));

            //公钥加密，私钥解密——加密
            x509EncodedKeySpec=new X509EncodedKeySpec(rsaPublicKey.getEncoded());
            keyFactory=KeyFactory.getInstance("RSA");
            publicKey=keyFactory.generatePublic(x509EncodedKeySpec);
            cipher=Cipher.getInstance("RSA");
            cipher.init(Cipher.ENCRYPT_MODE,publicKey);
            result=cipher.doFinal(src.getBytes());
            System.out.println("钥加密，私钥解密——加密:"+Base64.encodeToString(result,1));

            //公钥加密，私钥解密——解密
            pkcs8EncodedKeySpec=new PKCS8EncodedKeySpec(rsaPrivateKey.getEncoded());
            keyFactory=KeyFactory.getInstance("RSA");
            privateKey=keyFactory.generatePrivate(pkcs8EncodedKeySpec);
            cipher=Cipher.getInstance("RSA");
            cipher.init(Cipher.DECRYPT_MODE,privateKey);
            cipher.doFinal(result);
            System.out.println("公钥加密，私钥解密——解密:" + Base64.encodeToString(result, 1));

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}