That sounds similar to the plot for The Day the Earth Stood Still (in particular the original 1951 version), except that it wasn't a secret message. I seem to

That sounds similar to the plot for The Day the Earth Stood Still (in particular the original 1951 version), except that it wasn't a secret message. I seem to remember reading a story that was more similar to what you were describing, but can't remember the name or much about it.