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());		}	}}