why AES java decryption return extra characters?

Question

Excuse me for bad English. i use mcrypt which i get it from here MCrypt for php and java. in my android application i need php and java communicate securely so i get above mentioned AES. the problem is when php sends encrypted data, java can decrypt it but some extra characters are included.

JAVA Code

    import java.security.NoSuchAlgorithmException;

    import javax.crypto.Cipher;
    import javax.crypto.NoSuchPaddingException;
    import javax.crypto.spec.IvParameterSpec;
    import javax.crypto.spec.SecretKeySpec;

    public class MCrypt {

            private String iv = "fedcba9876543210";//Dummy iv (CHANGE IT!)
            private IvParameterSpec ivspec;
            private SecretKeySpec keyspec;
            private Cipher cipher;

            private String SecretKey = "0123456789abcdef";//Dummy secretKey (CHANGE IT!)

            public MCrypt()
            {
                    ivspec = new IvParameterSpec(iv.getBytes());

                    keyspec = new SecretKeySpec(SecretKey.getBytes(), "AES");

                    try {
                            cipher = Cipher.getInstance("AES/CBC/NoPadding");
                    } catch (NoSuchAlgorithmException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                    } catch (NoSuchPaddingException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                    }
            }

            public byte[] encrypt(String text) throws Exception
            {
                    if(text == null || text.length() == 0)
                            throw new Exception("Empty string");

                    byte[] encrypted = null;

                    try {
                            cipher.init(Cipher.ENCRYPT_MODE, keyspec, ivspec);

                            encrypted = cipher.doFinal(padString(text).getBytes());
                    } catch (Exception e)
                    {                       
                            throw new Exception("[encrypt] " + e.getMessage());
                    }

                    return encrypted;
            }

            public byte[] decrypt(String code) throws Exception
            {
                    if(code == null || code.length() == 0)
                            throw new Exception("Empty string");

                    byte[] decrypted = null;

                    try {
                            cipher.init(Cipher.DECRYPT_MODE, keyspec, ivspec);

                            decrypted = cipher.doFinal(hexToBytes(code));
                    } catch (Exception e)
                    {
                            throw new Exception("[decrypt] " + e.getMessage());
                    }
                    return decrypted;
            }



            public static String bytesToHex(byte[] data)
            {
                    if (data==null)
                    {
                            return null;
                    }

                    int len = data.length;
                    String str = "";
                    for (int i=0; i<len; i++) {
                            if ((data[i]&0xFF)<16)
                                    str = str + "0" + java.lang.Integer.toHexString(data[i]&0xFF);
                            else
                                    str = str + java.lang.Integer.toHexString(data[i]&0xFF);
                    }
                    return str;
            }


            public static byte[] hexToBytes(String str) {
                    if (str==null) {
                            return null;
                    } else if (str.length() < 2) {
                            return null;
                    } else {
                            int len = str.length() / 2;
                            byte[] buffer = new byte[len];
                            for (int i=0; i<len; i++) {
                                    buffer[i] = (byte) Integer.parseInt(str.substring(i*2,i*2+2),16);
                            }
                            return buffer;
                    }
            }



            private static String padString(String source)
            {
              char paddingChar = ' ';
              int size = 16;
              int x = source.length() % size;
              int padLength = size - x;

              for (int i = 0; i < padLength; i++)
              {
                      source += paddingChar;
              }

              return source;
            }
    }

the PHP code

 <?php 

    class MCrypt
    {
            private $iv = 'fedcba9876543210'; #Same as in JAVA
            private $key = '0123456789abcdef'; #Same as in JAVA


            function __construct()
            {
            }

            function encrypt($str) {

              //$key = $this->hex2bin($key);    
              $iv = $this->iv;

              $td = mcrypt_module_open('rijndael-128', '', 'cbc', $iv);

              mcrypt_generic_init($td, $this->key, $iv);
              $encrypted = mcrypt_generic($td, $str);

              mcrypt_generic_deinit($td);
              mcrypt_module_close($td);

              return bin2hex($encrypted);
            }

            function decrypt($code) {
              //$key = $this->hex2bin($key);
              $code = $this->hex2bin($code);
              $iv = $this->iv;

              $td = mcrypt_module_open('rijndael-128', '', 'cbc', $iv);

              mcrypt_generic_init($td, $this->key, $iv);
              $decrypted = mdecrypt_generic($td, $code);

              mcrypt_generic_deinit($td);
              mcrypt_module_close($td);

              return utf8_encode(trim($decrypted));
            }

            protected function hex2bin($hexdata) {
              $bindata = '';

              for ($i = 0; $i < strlen($hexdata); $i += 2) {
                    $bindata .= chr(hexdec(substr($hexdata, $i, 2)));
              }

              return $bindata;
            }

    }

so the scenario is java send data (simple text) in JSON format to PHP , php extract the data , encrypt it and finally echo it in JSON format

PHP call:

<?php

$data =json_decode(file_get_contents('php://input'), true);

$data=$data["request"];
require_once "encryption.php";
$etool=new MCrypt();
$data=$etool->encrypt($data);
$array=array('data'=>$data);
echo json_encode($array);

JAVA code:

 //sb is StringBuilder
JSONObject j=new JSONObject(sb.toString());
encryption etool=new encryption();
result=j.get("data").toString();
result= new String(etool.decrypt( result ));
Log.d("success remote ",result );

the result is:

example������

and if i use Farsi/Arabic word it get worse

like this-> درود����������������

in addition, i checked other questions but i could not get the answer.

AES encryption, got extra trash characters in decrypted file and
PHP MCRYPT encrypt/decrypt returns invisible strange characters?

thank you in advance!

Answer 1

AES encrypt in blocks of 16 bytes. If your input ain't a multiple of 16 bytes, a padding scheme is needed. As you have not specified any padding option for Mcrypt, it uses "zero padding".

In your Java code you specify "NoPadding" when you instantiate your Cipher:

 cipher = Cipher.getInstance("AES/CBC/NoPadding");

So Java considers the padding done by php to be part of the encrypted data.

You just need to ensure that your php and Java code uses the same padding scheme.