package security;
import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;
import javax.crypto.spec.*;
import javax.crypto.interfaces.*;
import java.io.*;
import java.math.*;
public class RSADemo {
	public RSADemo() {
	}
	public static void generateKey() {
		try {
			KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
			kpg.initialize(1024);
			KeyPair kp = kpg.genKeyPair();
			PublicKey pbkey = kp.getPublic();
			PrivateKey prkey = kp.getPrivate();
			// 保存公鑰
			FileOutputStream f1 = new FileOutputStream("pubkey.dat");
			ObjectOutputStream b1 = new ObjectOutputStream(f1);
			b1.writeObject(pbkey);
			// 保存私鑰
			FileOutputStream f2 = new FileOutputStream("privatekey.dat");
			ObjectOutputStream b2 = new ObjectOutputStream(f2);
			b2.writeObject(prkey);
		} catch (Exception e) {
		}
	}
	public static void encrypt() throws Exception {
		String s = "Hello World!";
		// 獲取公鑰及參數(shù)e,n
		FileInputStream f = new FileInputStream("pubkey.dat");
		ObjectInputStream b = new ObjectInputStream(f);
		RSAPublicKey pbk = (RSAPublicKey) b.readObject();
		BigInteger e = pbk.getPublicExponent();
		BigInteger n = pbk.getModulus();
		System.out.println("e= " + e);
		System.out.println("n= " + n);
		// 獲取明文m
		byte ptext[] = s.getBytes("UTF-8");
		BigInteger m = new BigInteger(ptext);
		// 計算密文c
		BigInteger c = m.modPow(e, n);
		System.out.println("c= " + c);
		// 保存密文
		String cs = c.toString();
		BufferedWriter out =
			new BufferedWriter(
				new OutputStreamWriter(new FileOutputStream("encrypt.dat")));
		out.write(cs, 0, cs.length());
		out.close();
	}
	public static void decrypt() throws Exception {
		// 讀取密文
		BufferedReader in =
			new BufferedReader(
				new InputStreamReader(new FileInputStream("encrypt.dat")));
		String ctext = in.readLine();
		BigInteger c = new BigInteger(ctext);
		// 讀取私鑰
		FileInputStream f = new FileInputStream("privatekey.dat");
		ObjectInputStream b = new ObjectInputStream(f);
		RSAPrivateKey prk = (RSAPrivateKey) b.readObject();
		BigInteger d = prk.getPrivateExponent();
		// 獲取私鑰參數(shù)及解密
		BigInteger n = prk.getModulus();
		System.out.println("d= " + d);
		System.out.println("n= " + n);
		BigInteger m = c.modPow(d, n);
		// 顯示解密結(jié)果
		System.out.println("m= " + m);
		byte[] mt = m.toByteArray();
		System.out.println("PlainText is ");
		for (int i = 0; i < mt.length; i++) {
			System.out.print((char) mt[i]);
		}
	}
	public static void main(String args[]) {
		try {
			generateKey();
			encrypt();
			decrypt();
		} catch (Exception e) {
			System.out.println(e.toString());
		}
	}
}