用java实现密码学的RS4

RSA（不是RS4）是一种非对称加密算法，Java中已经提供了RSA算法的实现。下面是一个简单的RSA加密和解密示例：

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.spec.InvalidKeySpecException;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

public class RSAExample {
    public static void main(String[] args) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeySpecException, IllegalBlockSizeException, BadPaddingException {
        String plainText = "Hello, world!";
        byte[] encryptedText = RSAExample.encrypt(plainText);
        System.out.println("Encrypted: " + new String(encryptedText));
        String decryptedText = RSAExample.decrypt(encryptedText);
        System.out.println("Decrypted: " + decryptedText);
    }

    public static byte[] encrypt(String plainText) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeySpecException, IllegalBlockSizeException, BadPaddingException {
        KeyPair keyPair = generateKeyPair();
        PublicKey publicKey = keyPair.getPublic();
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);
        byte[] encrypted = cipher.doFinal(plainText.getBytes());
        return encrypted;
    }

    public static String decrypt(byte[] encryptedText) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeySpecException, IllegalBlockSizeException, BadPaddingException {
        KeyPair keyPair = generateKeyPair();
        PrivateKey privateKey = keyPair.getPrivate();
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        byte[] decrypted = cipher.doFinal(encryptedText);
        return new String(decrypted);
    }

    private static KeyPair generateKeyPair() throws NoSuchAlgorithmException {
        KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("RSA");
        keyPairGen.initialize(2048, new SecureRandom());
        return keyPairGen.generateKeyPair();
    }
}

在这个示例中，我们首先生成一个2048位的RSA公私钥对，然后使用公钥加密明文，使用私钥解密密文。需要注意的是，RSA加密算法只适用于加密短文本，因为加密数据的长度不能超过密钥的长度。如果要加密长文本，需要使用分块加密的方式。