It is just that each site is customized for a certain section of the population, based on demographics. A user would be most comfortable in his or her home site. Even though the database for all the sites is the same, a user would mostly be browsing the sub set of data applicable to him or her. To keep things simple, we decided to restrict the user's access to the data through only one site.

The idea is to restrict the user to his or her home site. I am not very familiar with openID, so I am not sure whether that will work

I will take a look at Crypt::CBC - I do appreciate your time and advice.