This is the engineering estimate of NK progress done a few months ago, extremely accurate.

www.space.com/35681-north-korea-intercontinental-ballistic-missiles-threat.html

The North Korean missile program got its start with the importation of Soviet Scuds, which made their way into the nation in the 1970s. North Korea reworked Scud technology into a number of variants over the years, apparently with the help of Soviet engineers (many of whom fled the USSR after its 1991 collapse).