I'd created an MVC project using boilerplate OWIN authentication which was all working well.
I needed real-time signalling and setup a SignalR hub which was fully functioning, just without utilising the authenticated user.
When I added the [Authorize] attribute to the hub class and loaded the page that connects to the hub the Console in the F12 Developer Tools reported a 401 error with 'DENIED - The requested resource requires user authentication'.
If I didn't use the [Authorize] attribute the hub Context.User was null every time, so OnConnected and any methods I called didn't have Context.User set. This meant I couldn't work out which user was communicating.
All the other questions I'd found on StackOverflow had answers along the lines of "the Context.User will be set with the IPrincipal Identity details from the web page" which didn't help me as that wasn't happening for me.
The User was however set for all MVC controllers.