I agree that `Pair<String> usernameAndPassword` is a comparable non-abstraction over `String username, String password`.  I just think that it's less intuitive; it's a little distracting, in fact, when I read it.

In this case, you're not hurting from too many method parameters (3 is plenty safe), so I don't think an Authentication class is strictly necessary.  But I wouldn't be opposed to it.