public function setSharedKey($key)
    {
        // If the public key is a file name then convert it to
        // the contents of the file (which should be an RSA public
        // key in PEM format)
        if (file_exists($key)) {
            $this->shared_key = file_get_contents($key);
        } elseif (! empty($key)) {
            $this->shared_key = $key;
        }

        return $this;
    }