North Korea Wants Joint Probe into Sony Hack

The Sony hack left the corporation in shambles, the President of the US weighs in on not releasing the movie, the FBI declared North Korea is behind the hacks, and now North Korea wants to team up with the US to find the real culprit behind the hack…..or else. So where does that leave the US in options?
Moreover, Obama talked about a "proportional" response. What would that be in this case?