Actually, at best the nuclear deal prevents Iran from developing nukes with resources inside Iran. That's long been a weak spot in dealing with Iran.
The deal says nothing about Iran buying nukes from North Korea.
As for the actual deal, I think a recent Washington Post review slamming a book on how bad the nuclear deal is demonstrates why Iran can and so will cheat:
But the point isn't really about Solomon's reporting, or his sources, why, for instance, he spoke with this official and not another. Rather, as the conclusion to Sciolino's review makes plain, the main issue is to defend the nuclear deal—and not just against ideological opponents, columnists, and pundits. No, the much more dangerous threat to President Obama's signature foreign policy initiative and its supporters comes from those who have no ideological axe to grind at all—the wide array of experts and journalists who are simply relaying facts. They have to be silenced because there is too much at stake: money, prestige, and from the most paranoid corners of the pro-JCPOA community, world peace.
With America's Left so committed to defending the deal in Manichean terms--either the deal or war--the terrible enforcement terms of the deal will never be tested.
And you don't still believe in all that sanctions "snap back" nonsense, do you? How cute.
That is, no indications of cheating by Iran will ever be clear enough, or bad enough if true, or supported by a smoking gun, or worth the consequences (which backers see as war) to crack down on Iran for breaking terms of the deal.
Heck, the "deal" isn't even legally binding on Iran, truth be told. It was a gentlemen's agreement and we're short at least one gentleman on this.
Besides, these people believe, in a decade Iran will be a responsible regional partner because of the Smart Diplomacy that gave us the deal!
With Western liberals running interference for Iran in the belief that the Iran deal is all that stands between the demise of world peace (as if that exists!), Iran doesn't have to hide their cheating--just deny wrongdoing and cloak their cheating enough to create sufficient doubt for Western liberals to justify inaction.
Have a super sparkly day.