I'm unwilling to declare the publishing leaked information should be a crime, because it is so often vital that people feel free to publish information that is in the public interest.
On the other hand, I do believe that you have to strike a balance between the public's interest in knowing what their government is doing (which is real) and the public's interest in having their government negotiate on their behalf to prevent wars, disarm enemies, or even just build allies under tense circumstances.
Where I instinctively tend to come down is on the side of giving the benefit of the doubt to peacemakers and am inherently sceptical of warmongers. Diplomats are often responsible for conducting tricky, private negotiations to try to prevent nations from going to war with each other. And if they are successful, you may never hear about it. So I have an inclination to say there are a lot of useful things that they do that can't be done in secret. A friend and colleague challenged me today whether there really are things that governments need to do that they should be allowed to do in secret in a Democratic society. My answer is a cautious and carefully managed yes - I don't think they should have a blank check to conceal all their actions from the people who put them into power, but if I have to balance the chance of deescalating a conflict with Iran or risking nuclear war (for instance) I'm willing to be kept in the dark for a period of time.
What I struggle with, and what I honestly don't know, is what the ultimate effects of a world of perfect leakability would be. I can imagine a number of scenarios. I suspect the most likely is that government officials simply adapt their working practices, as TSA officials have, to elevate their privacy concerns to the next level. You can easily envision a rule that diplomats are only allowed to brief their superiors in person or by secure telephone and may not put anything in writing.
The end result? No paper trail is created, making it impossible for anyone to leak but also for future historians to ever know what "really" happened. Also, potentially, this would lead to spectacularly biased and wrong-headed foreign policy as the top leaders could never see a horses mouth analysis or summary.
Or, if the world adapted to a situation where it was understood that all communications take place in the open, then we would have to treat every delicate discussion with crazy regimes like North Korea as if they might be listening. This means that either we can't frankly assess their situation (For instance, you couldn't say, "The Dear Leader is very old and may die soon, after which the nation may be plummeted into disaster. Meanwhile, he's desperately trying to aquire nuclear weapons before the End of Days.") or that we can never in fact engage in diplomacy with such regimes.
I may be wrong - I honestly don't know. How do you envision this playing out?