import java.io.File;

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.OutputStream;

import javax.crypto.Cipher;

import javax.crypto.SecretKey;

import javax.crypto.SecretKeyFactory;

import javax.crypto.spec.DESKeySpec;

public class MyDecrypt {

/**

* This program accepts an encrypted binary file (created by MyEncrypt)

* and the agreed upon string password. Then it decrypts the file

* and saves it to a file named decrypt.bin in the current directory.

*

* Data Encryption Standard (DES) is used, options:

* DES/ECB/PKCS5Padding.

*

* @param args arguments to the main method

*

* Usage: java MyDecrypt input.txt mypassword

*/

public static void main(String[] args) {

if (args.length != 2) {

System.err.println("usage: java MyDecrypt <input file> <password>");

return;

}

/** make sure the input file exists */

String fname = args[0]; //full path name

File f = new File(fname);

if(!f.exists()){

System.err.println("error: input file does not exist");

return;

}

/** make sure password is at least eight characters */

String password = args[1];

if(password == null || password.length() < 8){

System.err.println("error: password must be at least eight characters");

return;

}

String sOutputFile = "decrypt.bin";

try{

/** create the DES key */

byte[] desKeyData = password.getBytes(); //same password as sender

DESKeySpec desKeySpec = new DESKeySpec(desKeyData);

SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");

SecretKey myDesKey = keyFactory.generateSecret(desKeySpec);

/** create and initialize the cipher */

Cipher desCipher;

desCipher = Cipher.getInstance("DES/ECB/PKCS5Padding"); //same options as sender

desCipher.init(Cipher.DECRYPT_MODE, myDesKey);

/** read in the encrypted file */

byte[] textEncrypted = readFile(fname);

/** decrypt the data */

byte[] textDecrypted = desCipher.doFinal(textEncrypted);

/** output the decrypted data to a binary file */

outputBinaryFile(sOutputFile, textDecrypted);

} catch (Exception e) {

/** just catch all exceptions and display a generic error */

System.err.println("error: unable to decrypt the file");

}

}

public static byte[] readFile(String fullPathName) {

FileInputStream fileInputStream;

File file = new File(fullPathName);

byte[] bytes = new byte[(int) file.length()];

try {

//convert file into array of bytes

fileInputStream = new FileInputStream(file);

fileInputStream.read(bytes);

fileInputStream.close();

}catch(Exception e){

e.printStackTrace();

}

return bytes;

}

public static void outputBinaryFile(String fname, byte[] arr) {

OutputStream out = null;

try {

out = new FileOutputStream(fname);

out.write(arr);

out.close();

System.out.println("file written: " + fname);

} catch (IOException e) {

e.printStackTrace();

}

}

}