I've got custom developed sales system (in PHP, let's say it's domain.com) and I would like to add informational section to it. Instead of developing it from the ground up I thought that it would be better to publish information by Wordpress (for example published on subdomain infos.domain.com).
The problem is I would like to make it accessible only for users logged in to domain.com system. So when somebody enters infos.domain.com he sees some text like "Please login to domain.com". When somebody is logged in to domain.com he should be able to read all the pages in infos.domain.com. Commenting them is not necessary (read-only is enough).
As I know Wordpress rather from administrator's perspective I tried to find some plugin to solve that, but I failed. Maybe you can advice some ready-made (evan paid) solution? If it does not exist please, advise how it could be solved.
Login to domain.com is based on POST form and cookies, passwords in database are hashed with salt. domain.com system can be extended to provide some API.