Enc/Dec AES 256 CBC, with data consistency validation

aes256cbc.php

private static function iv()
	{
		$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
		return mcrypt_create_iv($iv_size, MCRYPT_RAND);
	}

	static function encrypt($str, $key32)
	{
		# Prepend 4-chars data hash to the data itself for validation after decryption
		$str = substr(md5($str), 0, 4).$str;

		# Prepend $iv to decrypted data
		$iv = self::iv();
		$enc = $iv.mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key32, $str, MCRYPT_MODE_CBC, $iv);
		return base64_encode($enc);
	}

	static function decrypt($str, $key32)
	{
		$str = base64_decode($str);
		if ($str === false || strlen($str) < 32)
			return null;
		$iv = substr($str, 0, 32);
		$encrypted = substr($str, 32);
		$decrypted = rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key32, $encrypted, MCRYPT_MODE_CBC, $iv), "\0");

		if ($decrypted === false || is_null($decrypted) || strlen($decrypted) < 4)
			return null;
		$dataHash = substr($decrypted, 0, 4);
		$data = substr($decrypted, 4);
		if (substr(md5($data), 0, 4) !== $dataHash)
			return null;

		return $data;
	}