RSA.java

package javaapplication1;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.math.BigInteger;
import java.security.Key;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.KeySpec;
import java.security.spec.RSAPrivateKeySpec;
import java.security.spec.RSAPublicKeySpec;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
public class RSA {

   RSA() throws Exception{
       //public static void main(String[] args) throws Exception {
        generateKeys();
        rsaEncrypt("D:\\Address.txt", "D:\\encrypted.txt");
        rsaDecrypt("D:\\encrypted.txt", "D:\\decrypted.txt");
    }

    public static void generateKeys() throws Exception {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(2048);
        KeyPair kp = kpg.genKeyPair();
        PublicKey publicKey = kp.getPublic();
        PrivateKey privateKey = kp.getPrivate();

        System.out.println("keys created");

        KeyFactory fact = KeyFactory.getInstance("RSA");
        RSAPublicKeySpec pub = fact.getKeySpec(publicKey,
                RSAPublicKeySpec.class);
        RSAPrivateKeySpec priv = fact.getKeySpec(privateKey,
                RSAPrivateKeySpec.class);

        saveToFile("public.key", pub.getModulus(), pub.getPublicExponent());
        saveToFile("private.key", priv.getModulus(), priv.getPrivateExponent());

        System.out.println("keys saved");
    }

    public static void saveToFile(String fileName, BigInteger mod,
            BigInteger exp) throws IOException {
        ObjectOutputStream fileOut = new ObjectOutputStream(
                new BufferedOutputStream(new FileOutputStream(fileName)));
        try {
            fileOut.writeObject(mod);
            fileOut.writeObject(exp);
        } catch (Exception e) {
            throw new IOException("Unexpected error");
        } finally {
            fileOut.close();
            System.out.println("Closed writing file.");
        }
    }

    // Return the saved key
    static Key readKeyFromFile(String keyFileName) throws IOException {
        InputStream in = new FileInputStream(keyFileName);
        ObjectInputStream oin = new ObjectInputStream(new BufferedInputStream(
                in));
        try {
            BigInteger m = (BigInteger) oin.readObject();
            BigInteger e = (BigInteger) oin.readObject();
            KeyFactory fact = KeyFactory.getInstance("RSA");
            if (keyFileName.startsWith("public"))
                return fact.generatePublic(new RSAPublicKeySpec(m, e));
            else
                return fact.generatePrivate(new RSAPrivateKeySpec(m, e));
        } catch (Exception e) {
            throw new RuntimeException("Spurious serialisation error", e);
        } finally {
            oin.close();
            System.out.println("Closed reading file.");
        }
    }

    // Use this PublicKey object to initialize a Cipher and encrypt some data
    public static void rsaEncrypt(String file_loc, String file_des)
            throws Exception {
        byte[] data = new byte[32];
        int i;

        System.out.println("start encyption");

        Key pubKey = readKeyFromFile("public.key");
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, pubKey);

        FileInputStream fileIn = new FileInputStream(file_loc);
        FileOutputStream fileOut = new FileOutputStream(file_des);
        CipherOutputStream cipherOut = new CipherOutputStream(fileOut, cipher);

        // Read in the data from the file and encrypt it
        while ((i = fileIn.read(data)) != -1) {
            cipherOut.write(data, 0, i);
        }

        // Close the encrypted file
        cipherOut.close();
        fileIn.close();

        System.out.println("encrypted file created");
    }

    // Use this PublicKey object to initialize a Cipher and decrypt some data
    public static void rsaDecrypt(String file_loc, String file_des)
            throws Exception {
        byte[] data = new byte[32];
        int i;

        System.out.println("start decyption");

        Key priKey = readKeyFromFile("private.key");
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, priKey);

        FileInputStream fileIn = new FileInputStream(file_loc);
        CipherInputStream cipherIn = new CipherInputStream(fileIn, cipher);
        FileOutputStream fileOut = new FileOutputStream(file_des);

        // Write data to new file
        while ((i = cipherIn.read()) != -1) {
            fileOut.write(i);
        }

        // Close the file
        fileIn.close();
        cipherIn.close();
        fileOut.close();

        System.out.println("decrypted file created");

    }
}