import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.security.Key;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.util.Scanner;

import javax.crypto.Cipher;

import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;
/**
 * 
 * @author leon
 *
 */
class RSAUtils {
  //預設公鑰的持久化檔案存放位置
	private static String PUBLIC_KEY_FILE = "PublicKey";
	//預設私鑰的持久化檔案存放位置
	private static String PRIVATE_KEY_FILE = "PrivateKey";

	//設定公私鑰持久化檔案的存放位置
	public static void setKeyPath(String path) {
		if (PUBLIC_KEY_FILE.equals("PublicKey")) {
			PUBLIC_KEY_FILE = path + (path.endsWith("//")?"PublicKey":"/PublicKey");
			PRIVATE_KEY_FILE = path + (path.endsWith("//")?"PrivateKey":"/PrivateKey");
		}
	}

	//建立公私鑰對
	private static void createKeyPair() throws Exception {
		//使用RSA演算法獲得金鑰對生成器物件keyPairGenerator
		KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
		//設定金鑰長度為1024
		keyPairGenerator.initialize(1024);
		//生成金鑰對
		KeyPair keyPair = keyPairGenerator.generateKeyPair();
		//獲取公鑰
		Key publicKey = keyPair.getPublic();
		//獲取私鑰
		Key privateKey = keyPair.getPrivate();
		//儲存公鑰物件和私鑰物件為持久化檔案
		ObjectOutputStream oos1 = null;
		ObjectOutputStream oos2 = null;
		try {
			oos1 = new ObjectOutputStream(new FileOutputStream(PUBLIC_KEY_FILE));
			oos2 = new ObjectOutputStream(
					new FileOutputStream(PRIVATE_KEY_FILE));
			oos1.writeObject(publicKey);
			oos2.writeObject(privateKey);
		} catch (IOException e) {
			throw new RuntimeException(e);
		} finally {
			oos1.close();
			oos2.close();
		}
	}
    
	//RSA加密
	public static String encryptWithRSA(String str) throws Exception {
		createKeyPair();
		Key publicKey = null;
		//讀取持久化的公鑰物件
		ObjectInputStream ois = null;
		try {
			ois = new ObjectInputStream(new FileInputStream(PUBLIC_KEY_FILE));
			publicKey = (Key) ois.readObject();
		} catch (IOException e) {
			throw new RuntimeException(e);
		} finally {
			ois.close();
		}

		//獲取一個加密演算法為RSA的加解密器物件cipher。
		Cipher cipher = Cipher.getInstance("RSA");
		//設定為加密模式,並將公鑰給cipher。
		cipher.init(Cipher.ENCRYPT_MODE, publicKey);
		//獲得密文
		byte[] secret = cipher.doFinal(str.getBytes());
		//進行Base64編碼
		return new BASE64Encoder().encode(secret);
	}

	//RSA解密
	public static String decryptWithRSA(String secret) throws Exception {
		Key privateKey;
		ObjectInputStream ois = null;
		try {
			ois = new ObjectInputStream(new FileInputStream(PRIVATE_KEY_FILE));
			privateKey = (Key) ois.readObject();
		} catch (IOException e) {
			throw new RuntimeException(e);
		} finally {
			ois.close();
		}
		Cipher cipher = Cipher.getInstance("RSA");
		//傳遞私鑰，設定為解密模式。
		cipher.init(Cipher.DECRYPT_MODE, privateKey);
		//解密器解密由Base64解碼後的密文,獲得明文位元組陣列
		byte[] b = cipher.doFinal(new BASE64Decoder().decodeBuffer(secret));
		//轉換成字串
		return new String(b);

	}
}
public class RSA_Demo {
  public static void main(String[] args) throws Exception {
		//設定公私鑰對存放路徑，可選，預設是工程目錄
		//RSAUtils.setKeyPath(str);
		System.out.println("請輸入明文：");
		Scanner sca = new Scanner(System.in);
		String str =sca.nextLine();
		System.out.println("============================");
		String secret = RSAUtils.encryptWithRSA(str);
		System.out.println("經過RSA加密後的密文為：");
		System.out.println(secret);
		System.out.println("============================");
		String original = RSAUtils.decryptWithRSA(secret);
		System.out.println("經過RSA解密後的原文為：");
		System.out.println(original);
	}
}