RSA Example...

       

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;

import javax.crypto.Cipher;


public class EncryptionUtil {

  /**
   * String to hold name of the encryption algorithm.
   */
  public static final String ALGORITHM = "RSA";
  private static final int MAX_LENGTH = 16384;

  /**
   * String to hold the name of the private key file.
   */
  public static final String PRIVATE_KEY_FILE = "C:/keys/private.key";

  /**
   * String to hold name of the public key file.
   */
  public static final String PUBLIC_KEY_FILE = "C:/keys/public.key";

  /**
   * Generate key which contains a pair of private and public key using 16384
   * bytes. Store the set of keys in Prvate.key and Public.key files.
   * 
   * @throws NoSuchAlgorithmException
   * @throws IOException
   * @throws FileNotFoundException
   */
  public static void generateKey() {
    try {
      final KeyPairGenerator keyGen = KeyPairGenerator.getInstance(ALGORITHM);
      //MAXIMUM KEY LENGTH
      keyGen.initialize(MAX_LEGTH);
      final KeyPair key = keyGen.generateKeyPair();

      File privateKeyFile = new File(PRIVATE_KEY_FILE);
      File publicKeyFile = new File(PUBLIC_KEY_FILE);

      // Create files to store public and private key
      if (privateKeyFile.getParentFile() != null) {
        privateKeyFile.getParentFile().mkdirs();
      }
      privateKeyFile.createNewFile();

      if (publicKeyFile.getParentFile() != null) {
        publicKeyFile.getParentFile().mkdirs();
      }
      publicKeyFile.createNewFile();

      // Saving the Public key in a file
      ObjectOutputStream publicKeyOS = new ObjectOutputStream(
          new FileOutputStream(publicKeyFile));
      publicKeyOS.writeObject(key.getPublic());
      publicKeyOS.close();

      // Saving the Private key in a file
      ObjectOutputStream privateKeyOS = new ObjectOutputStream(
          new FileOutputStream(privateKeyFile));
      privateKeyOS.writeObject(key.getPrivate());
      privateKeyOS.close();
    } catch (Exception e) {
      e.printStackTrace();
    }

  }

  /**
   * The method checks if the pair of public and private key has been generated.
   * 
   * @return flag indicating if the pair of keys were generated.
   */
  public static boolean areKeysPresent() {

    File privateKey = new File(PRIVATE_KEY_FILE);
    File publicKey = new File(PUBLIC_KEY_FILE);

    if (privateKey.exists() && publicKey.exists()) {
      return true;
    }
    return false;
  }

  /**
   * Encrypt the plain text using public key.
   * 
   * @param text
   *          : original plain text
   * @param key
   *          :The public key
   * @return Encrypted text
   * @throws java.lang.Exception
   */
  public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
      // get an RSA cipher object and print the provider
      final Cipher cipher = Cipher.getInstance(ALGORITHM);
      // encrypt the plain text using the public key
      cipher.init(Cipher.ENCRYPT_MODE, key);
      cipherText = cipher.doFinal(text.getBytes());
    } catch (Exception e) {
      e.printStackTrace();
    }
    return cipherText;
  }

  /**
   * Decrypt text using private key.
   * 
   * @param text
   *          :encrypted text
   * @param key
   *          :The private key
   * @return plain text
   * @throws java.lang.Exception
   */
  public static String decrypt(byte[] text, PrivateKey key) {
    byte[] dectyptedText = null;
    try {
      // get an RSA cipher object and print the provider
      final Cipher cipher = Cipher.getInstance(ALGORITHM);

      // decrypt the text using the private key
      cipher.init(Cipher.DECRYPT_MODE, key);
      dectyptedText = cipher.doFinal(text);

    } catch (Exception ex) {
      ex.printStackTrace();
    }

    return new String(dectyptedText);
  }

}



    try {

      // Check if the pair of keys are present else generate those.
      if (!EncryptionUtil.areKeysPresent()) {
        // Method generates a pair of keys using the RSA algorithm and stores it
        // in their respective files
       EncryptionUtil.generateKey();
      }

      final String originalText = "In practice, we need to store the public and private keys somewhere. Typically, the private key will be placed on our server, and the public key distributed to clients. To store the key, we simply need to pull out the modulus and the public and private exponents, then write these numbers to some file (or put in whatever convenient place).";
      ObjectInputStream inputStream = null;

      // Encrypt the string using the public key
      inputStream = new ObjectInputStream(new FileInputStream(EncryptionUtil.PUBLIC_KEY_FILE));
      final PublicKey publicKey = (PublicKey) inputStream.readObject();
      final byte[] cipherText = EncryptionUtil.encrypt(originalText, publicKey);

      // Decrypt the cipher text using the private key.
      inputStream = new ObjectInputStream(new FileInputStream(EncryptionUtil.PRIVATE_KEY_FILE));
      final PrivateKey privateKey = (PrivateKey) inputStream.readObject();
      final String plainText = EncryptionUtil.decrypt(cipherText, privateKey);

      // Printing the Original, Encrypted and Decrypted Text
      System.out.println("Original: " + originalText);
      System.out.println("Encrypted: " +cipherText.toString());
      System.out.println("Decrypted: " + plainText);

    } catch (Exception e) {
      e.printStackTrace();
    }