Painting seed words on the wall of a cave. Only cave dude knows the hidden cave he painted them on, and no one else is going to be checking every cave with torches.

And if he puts it in a story, like: "together human hunt deer autumn snack beef pen horse feed pig shiver", anyone finding it would be like, "oh yeah, we do that but why write down on wall"

Even historians thousands of years later would still not be able to steal cave dude's bitcoin, because they will look at it and read all kinds of extraneous depth out of it.