As you pore through an old book on famous composers, you find an interesting passage.
It wasn't until the American Revolution that this note was found locked in a drawer. It had been taken from the composer's manuscripts and smuggled across the sea to the then-colonies. American intelligence tried to figure out what the message said, but could not. One man was confident that the message had crucial information to their winning the Revolution in mere days but could not break the code.
What did the code say?
The composer in question is Johann Sebastian Bach...
... Or at least he would have been fifty years ago.
This code is based on a particular piece of music.
That piece of music is popularly misattributed to Bach.