Bad Padding Exception while using AES encryption

Question

I am working on AES encryption for my project. I decided to do File Encryption When i execute encryption and decryption in same function ,it runs without any flaw.But when i do it separately, decryption part experience some problem.

I used the same key for both functions but it does not work on decryption alone.

Here is my Encryption code:

 public void enc(File a) throws FileNotFoundException {


                try {
                    c = new Scanner(a).useDelimiter("\\Z").next();

                    String key = "Bar12345Sar12346"; // 128 bit key
                    Key aesKey = new SecretKeySpec(key.getBytes(), "AES");
                    Cipher cipher = Cipher.getInstance("AES");

                    cipher.init(Cipher.ENCRYPT_MODE, aesKey);
                    byte[] encrypted = cipher.doFinal(c.getBytes());

                    str1 = new String(encrypted);
                    textField_1.setText(str1);

                    empty(a); \\ To clear the text file

                    PrintWriter writer = new PrintWriter(a);
                    writer.println(str1);                       
                    writer.close();

                } catch (InvalidKeyException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (NoSuchAlgorithmException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (NoSuchPaddingException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IllegalBlockSizeException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (BadPaddingException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

}

Here is my decryption code:

public void dec(File b) throws FileNotFoundException
{
    try {
        c1 = new Scanner(b).useDelimiter("\\Z").next();
        byte[] by=c1.getBytes();

        String key = "Bar12345Sar12345"; // 128 bit key
        Key aesKey = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");

        cipher.init(Cipher.DECRYPT_MODE, aesKey);
        String decrypted = new String(cipher.doFinal(by));

        str2 = new String(decrypted);
        System.out.println(str2);
        System.out.println("3");

        textField_2.setText(str2);

        empty(b);

        PrintWriter writer = new PrintWriter(b);
        writer.println(str2);                       
        writer.close();

    } catch (InvalidKeyException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (NoSuchAlgorithmException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (NoSuchPaddingException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalBlockSizeException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (BadPaddingException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }


}

This is the error message when i execute decryption separately.

javax.crypto.BadPaddingException: Given final block not properly padded
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:975)
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:833)
at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:446)
at javax.crypto.Cipher.doFinal(Cipher.java:2165)
at Main.dec(Main.java:117)
at Main$4.actionPerformed(Main.java:228)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

You have two mistakes:

Use the same key in encryption and decryption:

Bar12345Sar12346 vs Bar12345Sar12345

Encrypted data is not a String:

You can not use this code to transform byte[] to String because the encrypted data is not representable as characters.

str1 = new String(encrypted);

You have to use a binary file, not a text file.For example, using Java 7 Files

//Write to file
Files.write(file.toPath(), encryptedData);

//Read from file
byte encryptedData[] = Files.readAllBytes(file.toPath());