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();
   // 保存公钥
   FileOutputStream f1 = new FileOutputStream("pubkey.dat");
   ObjectOutputStream b1 = new ObjectOutputStream(f1);
   b1.writeObject(pbkey);
   // 保存私钥
   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!";
  // 获取公钥及参数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);
  // 获取明文m
  byte ptext[] = s.getBytes("UTF-8");
  BigInteger m = new BigInteger(ptext);
  // 计算密文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 {
  // 读取密文
  BufferedReader in =
   new BufferedReader(
    new InputStreamReader(new FileInputStream("encrypt.dat")));
  String ctext = in.readLine();
  BigInteger c = new BigInteger(ctext);
  // 读取私钥
  FileInputStream f = new FileInputStream("privatekey.dat");
  ObjectInputStream b = new ObjectInputStream(f);
  RSAPrivateKey prk = (RSAPrivateKey) b.readObject();
  BigInteger d = prk.getPrivateExponent();
  // 获取私钥参数及解密
  BigInteger n = prk.getModulus();
  System.out.println("d= " + d);
  System.out.println("n= " + n);
  BigInteger m = c.modPow(d, n);
  // 显示解密结果
  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());
  }
 }
}