Surely the answer is watermarking
You know one of the first things that was developed with a noddy neural network and some spare time ?
A mechanism for subtly refactoring English into several semantically identical, but subtly different texts. Think of a precis but instead of summarising, you simply reword, with appropriate punctuation.
The idea being that you feed in your "memo" and then issue the unique outputs to your staff. If one gets leaked, you damn well know the point of origin.
It also had a happy side effect of alerting the sender to any unauthorised collaborations, should a recipient be thick enough (and they were) to comment that they appeared to have a different copy.
It was taken far enough to validate it worked, and then it was made *very* clear that it was not at all suitable for modern politics.