The oik-edd plugin provides an API key handler for use by premium OIK plugins sold using the Easy Digital Downloads plugin. How it works:
Customer purchases a plugin from your site. An API key may or may not be generated automatically, but an order is taken. When the order is satisfied the API key can be requested. The customer copies and pastes the API key into the oik options Plugins page.
Whenever a plugin update or download for a premium plugin is requested the API key has to match.
Note: If you prefer to deliver your oik plugins using WooCommerce then you should use the oik-woo plugin.