public static function verifyMessageSignature($secret, $body = null, $signature = null) {

		// Compute hash of message using shared secret:
		$body = is_null($body) ? file_get_contents('php://input') : $body;
		$hash = base64_encode(hash_hmac('sha256', $body, $secret, true));

		// Compare hash to the signature:
		$signature = is_null($signature) ? $_SERVER['HTTP_X_ACUITY_SIGNATURE'] : $signature;
		if ($hash !== $signature) {
			throw new Exception('This message was forged!');
		}
	}