I bet he does have it, and keeps it right beside the President's Book of Secrets and the Ark of the Convenant.I guarantee you it won't take years to break the code, and I doubt very seriously anything in that file will change the world. Despite leaks, wars, etc., governments still execute their business in the same way they did 3000 years ago, so there's no reason to believe it will change over one file. Sounds like a conveniently timed myth to keep the media interested.
"Hack-proof" is a pipe dream. That's like saying doctors should have made the world disease-free by now. Hackers find holes and exploit them; security experts close the holes and develop new ways to track the hackers; hackers find new ways to evade detection and in turn find new holes. It's an arms race fought in cyberspace. Nothing will ever be hack-proof.
Come on Mom, why can't I play football in the street? No one's hit me so far!
Consider this scenario: North Korea knows it has enemies in the West, and the overwhelming disapproval of the international community for it's nuclear program. Until now they've felt safe in China's shadow, assured that any attack by South Korea or the US would be repulsed not only by their Army, but also by the overwhelming air power of the Chinese Air Force.
Then Wikileaks releases diplomatic messages indicating that the Chinese might turn their back on North Korea in a conflict, and may already be working out a plan with the US on how to handle the aftermath. Where does that leave the North Korean government? Unable to rely on a superpower's protection but unwilling to back down from their conflict with the South, they have to choose one of two directions: (1) back down on all fronts to avoid a conflict (unlikely), or (2) become more belligerent and aim to do the most civilian damage possible since the situation is hopeless anyway.
It's easy to cast all governments and politicians as inherently evil and naively believe that truth conquers all. But the reality is that governments need to be able to discuss matters in a private context to achieve any honest dialogue and to ensure proper context behind any official release. For example, the quotes from Chinese diplomats regarding North Korea may very well be the ramblings of a disenchanted public servant, rather than the feeling of the Chinese government as a whole. But without that context North Korea might assume those opinions to be official (but disavowed) Chinese policy and execute option #2 above.
If Assange wants to leak stuff, let him leak stuff going on now in 10 years. By then it's less likely to cause a conflict, but still recent enough to generate the change he purports to advocate. Does anyone really believe that a file capable of "changing the world" would be irrelevant in 10 years?