* The oldest means of sending secret messages is to simply conceal them by one trick or another. The ancient Greek historian Herodotus wrote that when the Persian Emperor Xerxes moved to attack Greece in 480 BCE, the Greeks were warned by an Greek named Demaratus who was living in exile in Persia. In those days, wooden tablets covered with wax were used for writing. Demaratus wrote a message on the wooden tablet itself and then covered it with wax, allowing the vital information to be smuggled out of the country.
The science of sending concealed messages is known as “steganography”, Greek for “concealed writing”. Other classical techniques for smuggling a message included tattooing it on the scalp of a messenger, letting his hair grow back, and then sending him on a journey. At the other end, the recipient shaved the messenger’s hair off and read the message.
Steganography has a long history, leading to inventions such as invisible ink and “microdots”, or highly miniaturized microfilm images that could be hidden almost anywhere. Microdots are a common feature in old spy movies and TV shows. However, steganography is not very secure by itself. If someone finds the hidden message, all its secrets are revealed. That led to the idea of obscuring the message so that it could not be read even if it were intercepted, and the result was “cryptography”, Greek for “hidden writing”. The result was the development of “codes”, or secret languages, and “ciphers”, or scrambled messages.
* The distinction between codes and ciphers is commonly misunderstood. A “code” is essentially a secret language invented to conceal the meaning of a message. The simplest form of a code is the “jargon code”, in which a particular arbitrary phrase, for an arbitrary example:
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