I need to implement a kind of single sign-on on my portal to remove the hassle of a customer who otherwise needs to login multiple times for data order. The data products may come from different vendors online directly. Each vendor has its own data web services, which I do not want to touch. I request each vendor to implement a special web service to accept an order, and directly deliver products to customers after that customer login only once at my portal. My question is what is the best way and possible ways to handle this. I read somewhat on SSO, Federated SSO, and FID. But still not clear where to go.