I have a client who has a separate web interface with public profiles(not using C5), and they would like to link to our C5 website, and auto login. However they do not want the user to login twice, so they are after some sort of token/query string/api key login system, where a user can click view questionnaires on the original website and be automatically logged into our C5 site.

Does anyone have any ideas on how this could be achieved?