PHP decryption function fail

Question

I'm trying to send data from my Android application to my Apache2 server running PHP 5.5 while having AES-128 encryption/decryption on both sides.

The weird thing is when I run the java code on Eclipse to encrypt the data (as a test) and take the encrypted result to decrypt it using PHP on Netbeans, it works just fine. Transferring the code to Android also gives the same encrypted results, but the decryption function on the server doesn't give back any results, it just gives a null, I'm using the exact same code I used on Netbeans which worked.

Here's the code

   if(isset($_POST['param']))
  {
    $param = $_POST['param'];

    $param=decrypt($param, "57238004e784498bbc2f8bf984565090");
}
else
    echo "No post Request Received";


function decrypt($encrypted, $key) {
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, hex2bin($key),hex2bin($encrypted), MCRYPT_MODE_ECB);
        echo $decrypted;
    $padSize = ord(substr($decrypted, -1));
    return substr($decrypted, 0, $padSize*-1);
 }

echo $decrypted gives a null and the return as well.

Example: Cipher: 269B3F5A2208C533AACB51243CFB9CFB Decrypted to: 28

Anybody know what the problem might be?

basic debugging: var_dump($decrypted). if it's boolean false, then it didn't decrypt at all. — Marc B
– Marc B, Commented Dec 11, 2015 at 17:26
1. You've posted your encryption key. You should probably use a different one now. 2. Can you post an example encrypted string and what it should decrypt to? 3. ECB sucks. Use CBC. — Sammitch
– Sammitch, Commented Dec 11, 2015 at 17:38
Aside from the security concerns (ECB mode? No MAC?), we can't diagnose this unless you include a sample ciphertext that you're passing to the application. — Scott Arciszewski
– Scott Arciszewski, Commented Dec 11, 2015 at 17:40

Sammitch · Accepted Answer · 2015-12-11 19:27:09Z

After reading up on padding methods I can no longer take issue with the method used to pad the input, PKCS5. However, the method used to strip the padding from the output still leaves a definite possibility for introducing the error you're encountering.

This is because a 16-byte string ending in z [ord('z') == 122] will cause the current method to do substr($data, 0, -122); which returns false.

In order to be certain that the final bytes are padding you need to read the last byte, validate that the last $padSize bytes in the output are repetitions of that byte, and only trim the string in that case.

<?php
function decrypt($encrypted, $key) {
    $decrypted = mcrypt_decrypt(
        MCRYPT_RIJNDAEL_128,
        hex2bin($key),
        hex2bin($encrypted),
        MCRYPT_MODE_ECB);
    return unpad($decrypted);
}

function unpad($data) {
    $padSize    = ord(substr($data, -1));
    $padStr     = substr($data, strlen($data) - $padSize);
    $padCheck   = str_pad('', $padSize, chr($padSize));
    if( strcmp($padStr, $padCheck) === 0 ) {
        return substr($data, 0, $padSize*-1);
    } else {
        return $data;
    }
}

$key    = '57238004e784498bbc2f8bf984565090';
$data   = '269B3F5A2208C533AACB51243CFB9CFB';

var_dump(decrypt($data, $key));
// Output: string(2) "28"

Additionally, the ECB mode is pretty much the weakest you could possibly choose. See the images at the end of the ECB section in this wikipedia article for an excellent example.

Use CBC.

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