Decrypted string is sometimes not same as encrypted source

Question

class Auth extends MySQLi {
public function aes_enc($encrypt, $mc_key, $iv) {
    $passcrypt = trim(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, substr($mc_key, 0, 32), trim($encrypt), MCRYPT_MODE_CBC, $iv));
    return $passcrypt;

}

public function aes_dec($decrypt, $mc_key, $iv) {

    $decrypted = trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128, substr($mc_key, 0, 32), trim($decrypt), MCRYPT_MODE_CBC, $iv));
    return $decrypted;

}

public function salt() {
return str_shuffle('abcdefghijklmnoprsquvzyx0123456789-.,;:_<>');
}

public function iv() {
return mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC), MCRYPT_RAND);
}
}

And on test.php, following code:

<?
require('Auth.php');
$Auth = new Auth;

$str = "verygudlongpassword";

for ($i = 0; $i < 1000; $i++) {
    $salt = sha1($Auth->salt());
    $iv = $Auth->iv();

    $enc = $Auth->aes_enc($str, $salt, $iv);
    $dec = $Auth->aes_dec($enc, $salt, $iv);

    if ($str != $dec) {
        echo $salt . "<br>\n";
    }
}
?>

Sometimes, $dec != $str. Why is this happening? I am not even saving anything into DB atm, so it's not that. Thanks for help.

i dont really have anything more to say, but site isnt letting me post. (nvm that part)

Answer 1

After reviewing your code and playing with it locally. It would appear that your decryption leaves some whitespace on the decrypted text. I removed the trim() function from all locations except the return value from aes_dec() and the code now encrypts/decrypts your string successfully 1000 times.

So it would seem trimming was the problem and the solution.

class Auth extends MySQLi {
    public function aes_enc($encrypt, $mc_key, $iv)
    {
        $passcrypt = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, substr($mc_key, 0, 32), $encrypt, MCRYPT_MODE_CBC, $iv);
        return $passcrypt;
    }

    public function aes_dec($decrypt, $mc_key, $iv)
    {
        $decrypted = trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128, substr($mc_key, 0, 32), $decrypt, MCRYPT_MODE_CBC, $iv));
        return $decrypted;

    }

    public function salt()
    {
        return str_shuffle('abcdefghijklmnoprsquvzyx0123456789-.,;:_<>');
    }

    public function iv()
    {
        return mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC), MCRYPT_RAND);
    }
}

$Auth = new Auth;

$str = "verygudlongpassword";

for ($i = 0; $i < 1000; $i++) {
    $salt = sha1($Auth->salt());
    $iv = $Auth->iv();

    $enc = $Auth->aes_enc($str, $salt, $iv);
    $dec = $Auth->aes_dec($enc, $salt, $iv);

    if ($str != $dec) {
        echo "Decryption failed!<br>\n";
    } else {
        echo "Decryption success! String: $dec<br>\n";
    }
}