I'm in the camp it was North Korea 100%. China's statement even confirmed it en passing. Regarding their "denial", well doh.
Regarding the DDoSing them it's bloody efficient and just an awesome attack, whoever did it. You might say "well but they don't have the internet so who cares?" But that's why it's so ****ing good. First, it doesn't reflect on the citizens, because rightly they don't have it anyway, so they are not suffering from the lack of it. If they had internet, that would be a terrible measure, for it would turn their citizens blind against their goverment and pissed against westerners. No such hazzard. Right now, the only people who in NK have access are the really well off, friends of the government and hackers. Not many, perhaps a few thousand. And that means it's basically a "luxury item" that someone *wink wink* has denied them. And that reflects poorly on Kim's real constituency, not the people, but his high rank supporters.
Thus it's mostly a real nuisance for the weirdo psychopaths at the top of the chain, and who knows, if they can keep it closed down, they might even kill little Kim because they can't access youporn anymore.