Sending secret messages using codes has been a practice since the ancient time. In the Roman Empire, military officers would let messengers cut their hair, tattoo a secret message on their head, and send them to another place to relay the message once the hair had grown.
In Sparta, a device called scytale consisting of a cylindrical tool with a strip of parchment was used to encode and decode messages. Julius Caesar was known to perform letter substitutions to messages to make them unreadable. The method is now known as the Caesar cipher. » Read more