Is the "leaking" of documents a purely modern phenomenon, or were there politically significant leaks in earlier periods of history?

In the United States it has been a practice since the Early Republic. Joanne B. Freeman writes about this in her 2001 book Affairs of Honor.

A lot of times what would happen would be a member of a party would intercept private letters between members/leaders of the opposing party and then publish them in newspapers. This is very similar to the sort of weaponization of documents that we see today.

So suppose that a Republican or Anti-federalist, depending on the year, intercepted a letter from a Federalist Party memeber that hinted at ideals of elitism or monarchism. The Republican would then send that to a Republican based newspaper like the National Gazette and say, "Look at this! The Federalists believe that the common man is not capable of governing himself. They're elitists and monarchists!"

Now the main difference in what I'm talking about and what we see today is that I make no reference to any internal party leaks. The type I've talked about are related to oppositional party interception of documents. I would be very interested though if anyone had any sort of example of an internal party leak.

Hope this is informational/interesting. I'll be happy to fix anything or provide more detailed citation once I'm back at my computer.

Edit: Confirmed source/fixed wording