import java.io.*;
import java.math.BigInteger;

import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;

import javax.crypto.*;
import javax.crypto.spec.*;
import javax.crypto.interfaces.*;

/**
 * Decryption of a message which is stored in file <code>ciphertext</code>
 * with the private key stored in file <code>privatekey</code>. Use RSAKeyGen
 * to generate the <code>publickey</code> and <code>privatekey</code> files.
 *
 * @see RSAKeyGen
 * @see RSAEncrypt
 *
 * @version $Revision: 1.1 $
 */
public class RSADecrypt
{
   /**
    * Reads the encrypted message in file <code>ciphertext</code> and the
    * private key in file <code>privatekey</code>, decrypts the message with
    * private key and shows the plaintext message on standard output.
    *
    * @param filename the name of the file containing the ciphertext.
    */
   public RSADecrypt(String inFileName, String outFileName) {
      try {
         /* Get the encrypted message from file. */
         FileInputStream cipherfile = new FileInputStream(inFileName);
         byte[] ciphertext = new byte[cipherfile.available()];
         cipherfile.read(ciphertext);
         cipherfile.close();

         /* Get the private key from file. */
         PrivateKey privatekey = readPrivateKey("privatekey");

         /* Create cipher for decryption. */
         Cipher decrypt_cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
         decrypt_cipher.init(Cipher.DECRYPT_MODE, privatekey);

         /* Reconstruct the plaintext message. */
         byte[] plaintext = decrypt_cipher.doFinal(ciphertext);
         FileOutputStream plainfile = new FileOutputStream(outFileName);
         plainfile.write(plaintext);
         plainfile.close();
      } catch (Exception e) {
         e.printStackTrace();
      }
   }

   /**
    * Reads the PKCS#8 standard encoded RSA private key in
    * <code>filename</code>.
    *
    * @param filename The name of the file with the private key.
    *
    * @return The private key in <code>filename</code>.
    */
   public static PrivateKey readPrivateKey(String filename) throws Exception {
      FileInputStream file = new FileInputStream(filename);
      byte[] bytes = new byte[file.available()];
      file.read(bytes);
      file.close();
      PKCS8EncodedKeySpec privspec = new PKCS8EncodedKeySpec(bytes);
      KeyFactory factory = KeyFactory.getInstance("RSA");
      PrivateKey privkey = factory.generatePrivate(privspec);
      return privkey;
   }

   /**
    * The main method just calls the constructor.
    *
    * @param arg The command line arguments.
    */
   public static void main(String[] arg) {
      if (arg.length != 2) {
         System.err.println("Usage:  java RSADecrypt <src file> <dest file>");
      } else {
         new RSADecrypt(arg[0],arg[1]);
      }
   }
}