North Korea doesn't always brag about its provocations. Consider the sinking of the South Korean warship Cheonan, which was sunk by a mysterious explosion that was later assessed to have been a torpedo, while off a South Korean island within spitting distance of North Korean waters. North Korea denied any involvement (http://en.wikipedia.org/wiki/ROKS_Cheonan_sinking). Also, while hacker and hacktivist groups tend to be quite open about claiming credit, nation-state hackers tend to be very quiet about it (I don't think any country anywhere has officially claimed credit for hacking, offhand). Given what I've read on various sites, I certainly think it's a credible theory that North Korea was behind it (Motive, Means, and it matches their style), though I certainly don't think I'd rule out other possibilities completely.