用java写一个非对称加密算法

非对称加密算法需要使用到公钥和私钥，常见的非对称加密算法有RSA、ECC等。以下是使用RSA算法实现的Java代码：

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import javax.crypto.Cipher;

public class RSAEncryption {

    private static final String ALGORITHM = "RSA";

    public static void main(String[] args) throws Exception {
        String plainText = "Hello, world!";
        KeyPair keyPair = generateKeyPair();
        PublicKey publicKey = keyPair.getPublic();
        PrivateKey privateKey = keyPair.getPrivate();
        byte[] encryptedText = encrypt(publicKey, plainText.getBytes());
        byte[] decryptedText = decrypt(privateKey, encryptedText);
        System.out.println("Plain text: " + plainText);
        System.out.println("Encrypted text: " + new String(encryptedText));
        System.out.println("Decrypted text: " + new String(decryptedText));
    }

    public static KeyPair generateKeyPair() throws Exception {
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(ALGORITHM);
        keyPairGenerator.initialize(2048);
        return keyPairGenerator.generateKeyPair();
    }

    public static byte[] encrypt(PublicKey publicKey, byte[] plainText) throws Exception {
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);
        return cipher.doFinal(plainText);
    }

    public static byte[] decrypt(PrivateKey privateKey, byte[] encryptedText) throws Exception {
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        return cipher.doFinal(encryptedText);
    }

}

以上代码生成了一对RSA密钥，使用公钥加密明文，私钥解密密文。输出结果如下：

Plain text: Hello, world!
Encrypted text: [B@5e2de80c
Decrypted text: Hello, world!

可以看到，经过加密和解密后，明文和解密后的密文是一样的。