Cockpit locking systems revisited

[Gauges and Dials]

(This thread is not for discussion of the specifics of the Germanwings crash, nor for any discussions of the details of any given system on any existing aircraft.)

From first principles, what do we want a cockpit locking system to accomplish? What are the relative risks we are trying to mitigate?

For the purposes of a design exercise, we need to assume that the bad guy or bad guys have obtained knowledge of any and all codes or passwords, and/or have obtained physical possession of any and all keys or access tokens.

There are really only three fundamental design choices; the system must be one of the following:

1. Someone inside the cockpit can deny access, no matter what anybody outside the cockpit is doing.
2. Someone outside the cockpit can gain access, no matter what anyone inside the cockpit is doing.
3. Sometimes #1 above; sometimes #2, depending upon other factors (for example, depressurization?)

What do we want, and why?