Url.Action not including port number

You could include it like this:

@Url.Action(
    "authenticate", 
    "auth", 
    null, 
    "https", 
    Request.Url.Host + ":" + Request.Url.Port
)

Of course this means that your local web server must support SSL which is not the case with Cassini. You could use IIS Express for this to work.