PHP MCrypt decryption fails unless using AES-256

Question

I have written a class to encrypt and decrypt strings, but I can't use any cipher except MCRYPT_RIJNDAEL_256. The program always report that the message is corrupted. How can I fix it?

The ciphers I have tested which fails are MCRYPT_RIJNDAEL_128 MCRYPT_RIJNDAEL_192 MCRYPT_BLOWFISH MCRYPT_SERPENT and MCRYPT_TWOFISH.

Here's my code:

class Crypt {
    private $masterKey;
    private $subKey;
    private $cipher = MCRYPT_RIJNDAEL_256 ;
    private $cipherMode = MCRYPT_MODE_CFB;
    //private $hashAlog = 'sha256';

    public function __construct($masterKey) {
        $this->masterKey = $masterKey;
    }

    public function setKey($masterKey) {
        $this->__construct($masterKey);
    }

    public function encrypt($message) {
        $iv = mcrypt_create_iv($this->getIVSize());
        $hmac = $this->signMsg($message);
        $this->genSubKey($iv);
        $cipherText = mcrypt_encrypt($this->cipher, $this->subKey, $message, $this->cipherMode, $iv);
        $cipherText = $iv . $hmac . $cipherText;
        return base64_encode($cipherText);
    }

    public function decrypt($enc_message) {
        $mixedMsg = base64_decode($enc_message);
        $iv = substr($mixedMsg, 0, $this->getIVSize());
        $this->genSubKey($iv);
        $hmac = substr($mixedMsg, $this->getIVSize(), strlen($this->signMsg(null)));
        $cipherText = substr($mixedMsg, $this->getIVSize() + strlen($this->signMsg(null)));
        $message = mcrypt_decrypt($this->cipher, $this->subKey, $cipherText, $this->cipherMode, $iv);
        if(!$message)
            die("Decrypt Error!");
        if($hmac != $this->signMsg($message))
            die("Message Corrupted");
        return $message;
    }

    private function genSubKey($iv) {
        $this->subKey = hash_pbkdf2("sha256", $this->masterKey, $iv, 50000, $this->getKeySize());
    }

    private function getKeySize() {
        return mcrypt_get_key_size($this->cipher, $this->cipherMode);
    }

    private function getIVSize() {
        return mcrypt_get_iv_size($this->cipher, $this->cipherMode);
    }

    private function signMsg($message) {
        return hash_hmac("sha512", $message, $this->masterKey, true);
    }
}

Answer 1

The problem is caused by the hash_pbkdf2 function in the genSubKey function. Since hash_pbkdf2 will output hex encoded strings, it will be twice as long as the key size. To solve this, we need to pass true as an additional parameter to it, let it output raw bytes and fit the key size.

Here's the corrected code:

private function genSubKey($iv) {
    $this->subKey = hash_pbkdf2("sha256", $this->masterKey, $iv, 50000, $this->getKeySize(), true);
}