The DevForce services, EntityService.svc and EntityServer.svc, build up a CustomBinding, using the protocol scheme from the URI to determine whether SSL is used.
Basically what you're doing, but with the added check of the uri.Scheme, something like this:
Uri uri = new Uri(Application.Current.Host.Source, "../Services/Authentication.svc");
if (uri.Scheme == "http")
BasicHttpBinding binding = new BasicHttpBinding();
else if (uri.Scheme == "https")
BasicHttpBinding binding = new BasicHttpBinding(BasicHttpSecurityMode.Transport);
EndpointAddress ep = new EndpointAddress(uri);
service = new ServiceAuthentication.AuthenticationClient(binding, ep);