RSA Encryption Example

Encryption is a one of the ways to achieve data security. Encryption is converting the data in the plain text into an unreadable text called the cipher text. Decryption is converting the cipher text back to plain text.

This encryption/decryption of data is part of cryptography. Encryption and decryption generally require the use of some secret information, referred to as a key, which is used in converting plain text to cipher text and vice versa.

Symmetric key cryptography refers to encryption methods in which both the sender and receiver share the same key.

Asymmetric key cryptography (also known as public key cryptography) refers to a system that requires two separate keys, one of which is secret and one of which is public. Although different, the two parts of the key pair are mathematically linked. One key encrypts the plain text, and the other decrypts the cipher text. Neither key can perform both functions. The public key, or the key used to encrypt information can be freely distributed.

 RSA is one of the algorithm for public-key cryptography that is based on factoring large integers. RSA stands for Ron Rivest, Adi Shamir and Leonard Adleman, who first publicly described it.

Following example shows how to encrypt/decrypt information using RSA algorithm in Java.

The KeyPairGenerator class instance is used to generate the pair of public and private key for RSA algorithm and are saved into the files.

The Cipher class instance is used encrypt/decrypt information using the pair of keys generated above.

package in.javadigest.encryption;

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;

/**
 * @author JavaDigest
 * 
 */
public class EncryptionUtil {

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

  /**
   * 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 1024
   * 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);
      keyGen.initialize(1024);
      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);
  }

  /**
   * Test the EncryptionUtil
   */
  public static void main(String[] args) {

    try {

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

      final String originalText = "Text to be encrypted ";
      ObjectInputStream inputStream = null;

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

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

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

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