A software conditional access system (CAS) for media services provided to a Media Distribution Device, such as a set top box (STB), is disclosed. The STB may be connected to a Media Delivery Service Provider via a broadband connection. Over the broadband connection, a CAS application may be downloaded to the STB, maintained and dynamically updated. Because the CAS is implemented by software on the STB, it is inexpensive to install, maintain and update. When a user desires to purchase media services through the STB, a request, with identifying information, is sent to the Media Delivery Service Provider via the broadband connection. The service provider cross-references the identifying information with a user's account to determine whether to allow the user to view the media service requested. If the service provider determines to allow the user to view the media service, the user's account (typically maintained by the service provider) is charged and a key is downloaded via the broadband connection to the STB. The key allows the user to view the requested media service to be presented for user viewing, hearing or other use. After the requested media service ends, after a predetermined amount of time, etc., the CAS application may delete the key.