The characters in Cincinnatus encounter lots of secret messages. While it makes for good fiction, the secret form of communication used throughout Cincinnatus actually exists.
Steganography is used to insert secret messages into digital images. You can only retrieve the message if you know where to look and have the key to decipher it.
The modern use of steganography with digital technology began around 1985. Steganographers can insert secret messages into videos, websites, pictures and sound files.
It sounds complicated, but you don’t have to be a spy or a computer genius to become a steganographer.