private function signatureHeader()
    {
        if ($signature = $this->fetchHeader('Signature')) {
            return $signature;
        } elseif ($authorization = $this->fetchHeader('Authorization')) {
            return substr($authorization, strlen('Signature '));
        } else {
            throw new Exception('HTTP message has no Signature or Authorization header');
        }
    }
private function signatureHeader()
    {
        if ($signature = $this->fetchHeader('Signature')) {
            return $signature;
        } elseif ($authorization = $this->fetchHeader('Authorization')) {
            return substr($authorization, strlen('Signature '));
        } else {
            throw new Exception('HTTP message has no Signature or Authorization header');
        }
    }
@return string

@throws Exception