If this hack was to be assigned to a foreign government, Iran would be a much better prospect.
They are still burned about STUXNET, and a chance to throw a scare into the Great Satan would be very tempting.
And they have a number of connections already with North Korea (Axis of Evil, does anybody remember?).
China. China would be a much better prospect. They can make it look like the entire hack came from one of the six total computers in NK.
NK's internet access is through China.
NK gets its daily sustinence from China.
NK gets its marching orders from China.
If this wasn't China, then it was NK hackers trained and hosted in China.
There's no way NK did this on their own. They don't have the facilities, hell they don't even have electricity for lights in the damn' country. If anything, China was behind this.
I'd bet the farm on that.
Don’t forget that Iran hacked in the Sands casino company because the owner made anti-Iran and pro-Isreal statements. It could have been a blueprint for the DPRK.