The US and North Korea are exchanging threats and counter-threats in the aftermath of the hacking of Sony Pictures. The FBI has not released all its evidence, but it's blamed North Korea for the hacking with an unusual degree of certainty. North Korea denies the vastly expensive attack -- but says whoever was responsible did a "righteous" thing. President Obama and Republicans disagree over whether to call it "cyber-warfare" or "cyber-vandalism" and what the response should be. Some security experts question the FBI's conclusion that North Korea was responsible in the first place.