“Remember when you applied to work at the Census Bureau?”
I’ve never actually applied to work at the Census Bureau, but that phrase would mean something to past-me that I know nobody else but me knows about because it’s never been documented outside of my own head.
Of course, now that I’ve typed that here, I’m going to have to use a different shibboleth when this actually comes up since now THEY have their hands on that sentence.
It doesn’t matter, because they don’t know how I felt about it. They don’t know how I felt about anything unless I wrote it down.
Easy. Over the years I’ve had a steady stream of ideas for stories, novels, poems, songs, etc., many just a first sentence or an image or line of dialogue to build from. Lots of them have stuck in my memory without me ever committing them to paper, disc, or other storage medium or speaking of them to anyone else.
On September 17, 1980, Jasper Hodgenquintz spent the afternoon at the airport parking lot writing down license plate numbers in a small spiral notebook.
Ain’t nobody going to write that to me but me. Of course, I’ve burned that one now, but I have hundreds of others should I need them.
There are several women with whom I’ve worked over the years, on whom I’ve had crushes. I never told anyone about the crushes, and certainly never acted on them. So, a message to past me, which specifically features those names, would be pretty convincing.
I missed that bit. But even then I had my specific thought processes and concerns about those cases that were kept to myself.
That would be super easy. I’ve got a ton of private thoughts I’ve never shared with anyone.
I have many thoughts and memories that I have always kept entirely to myself. In particular, I’ve never shared many of the vivid, bizarre dreams I’ve had since childhood, nor have I shared many negative (and sometimes positive) thoughts I’ve had toward particular individuals over the years. I’d have no trouble convincing myself that the person writing to me was me if any of those thoughts and memories were recounted in the message.
In fact, I’d just need 2 words in the message header: “headless ostrich” to know that it was unambiguously a message from me.
…of course, now that I’ve written those 2 words online, I can’t use them anymore to verify my identity. Damn you! 
Sure. I have stories in my head that haven’t been written down, and that I haven’t told to anybody. Some of them have been running for a lot longer than ten years. I wouldn’t need all thousand words.
Yes, sure. I have always had a rich inner life, and I consider questions such as the OP’s and plan accordingly.
My very first memory was startling enough that I know I would know it. No one has ever been told it.
I do relay a first memory when asked about it. But it’s my next memories that I tell about. They seem more palatable to others.
So I’m safe.
I probably couldn’t. But I also wouldn’t believe the fake, either. I just would need something really strong to prove it, and I can’t think of anything.
There are so many conditionals in this theoretical thought experiment, I don’t think I could get past them. There’s nothing I can definitely be sure are unique to me that can’t be determined or guessed at by a careful observer.
Strange that this organisation knows everyting you ever typed or wrote, every person you spoke to, every dollar you spent and so on but does not have access to the one message you have to send and simply copy it too. And I see an inconsistency here:
If past me knows present me will have this task to solve today past me would think of something easy and short and remember it until today, then present me would use it.
But OK, 1,000 characters? About seven X-crements (née Twits)? What would I write to myself? Kill Donald Trump/Vladimir Putin… before it is too late? Buy Tesla/Apple/Amazon… shares? Sounds trite to me.
And if former me knew that present me would have to send the message would he (me!) know why it is so important? Odds are he (me) would then already know the message anyway. I would remember. And the Organisation would know, as they know everything I have read in the past.
I don’t know why, but time travel, even time travel of ideas only, always feels like a lame solution to the problems at any given time. As Deux ex Machina go, it is on the level of “then came the aliens in their flying saucers and saved the day.” I could tell myself to do the most crazy, high risk stuff imaginable as I will be alive and well ten years later, but present me would remember that that didn’t happen.
But as I remember the message I got ten years ago, the one I am going to write now for you all to see, I might as well send it again. It goes (will have went) like this:
Someone must have been telling lies about Josef K., he knew he had done
nothing wrong but, one morning, he was arrested. Every day at eight in
the morning he was brought his breakfast by Mrs. Grubach’s cook–Mrs.
Grubach was his landlady–but today she didn’t come. That had never
happened before. K. waited a little while, looked from his pillow at the
old woman who lived opposite and who was watching him with an
inquisitiveness quite unusual for her, and finally, both hungry and
disconcerted, rang the bell. There was immediately a knock at the door
and a man entered. He had never seen the man in this house before. He
was slim but firmly built, his clothes were black and close-fitting,
with many folds and pockets, buckles and buttons and a belt, all of
which gave the impression of being very practical but without making it
very clear what they were actually for. “Who are you?” asked K., sitting
half upright in his bed. The man, however, ignored the question as if
his arrival simply had to be accepted, and merely replied, “You rang?”
“Anna should have brought me my breakfast,” said K. He tried to work
out who the man actually was, first in silence, just through
observation and by thinking about it, but the man didn’t stay still to
be looked at for very long. Instead he went over to the door, opened it
slightly, and said to someone who was clearly standing immediately
behind it, “He wants Anna to bring him his breakfast.” There was a little laughter in the neighbouring room, it was not clear from the sound of it whether there were several people laughing.
Yes. I like creative writing.
I don’t write to share my stories. I do it because it’s therapeutic (it helps with my anxiety).
There are a TON of short stories only I know about.
I think this would be incredibly easy for most people.
For example, I wrote a song many years ago that has never seen the light of day. It remains in my noggin.
I could come up with dozens of other examples.
mmm
Sent by itself I’m sure Shadow Organization could convince me that it’s me via “fly on the wall” stuff I never told anyone.
If I was to get two messages and have to determine which was authentic, I’m confident that I would know my own buttons and unspoken thoughts well enough to have the edge.
In fact, I’d guess that once I got them both the “Remember when you broke the cookie jar but glued it back together, eh? eh? And kissed Sally Smith behind the mulberry bush? Huh?” one would seem positively quaint in its attempts.
Im on the fence as to whether this is for a novel OP is writing or whether OP is from a shadowy nefarious organization who is collecting info on us to use in a future shadowy nefarious plot to trick out past selves.
I could not do it. My past self from 10 years ago could not be convinced that time travel to the past was possible. My past self would attribute the purported message from the future to be the result of an elaborate ruse, mental illness, or hallucinations.
Yep! 
There are loads of stories, poems, songs, I’ve written that are based, often very remotely, on incidents from my life that I’ve never had occasion to reveal, or discuss, with anyone because who cares? In a moment, I could revisit almost anything I’ve ever written and trace it back to a source, an observation, an incident that I, and only I, know about.