Introduction To Codes, Ciphers, & Codebreaking
by Greg Goebel

[1.1] BASIC CONCEPTS
* 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 BC, 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 tatooing 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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The nightingale sings at dawn.

-- corresponds to a particular predefined message that may not, in fact shouldn't have, anything to do with the jargon code phrase. The actual meaning of this might be:
The supply drop will take place at 0100 hours tomorrow.

Jargon codes have been used for a long time, most significantly in World War II, when they were used to send commands to resistance fighters. However, from a cryptographic point of view they're not very interesting. A proper code would run something like this:
BOXER SEVEN SEEK TIGER5 AT RED CORAL

This uses "codewords" to report that a friendly military force codenamed BOXER SEVEN is now hunting an enemy force codenamed TIGER5 at a location codenamed RED CORAL. This particular code is weak in that the "SEEK" and "AT" words provide information to a codebreaker on the structure of the message. In practice, military codes are often defined as "codenumbers" rather than codewords, using a codebook that provides a dictionary of code numbers and their equivalent words. For example, this message might be coded as:
85772 24799 10090 59980 12487

-- where "85772" maps to BOXER SEVEN, "12487" maps to "RED CORAL", and so on. Codewords and codenumbers are referred to collectively as "codegroups". The words they represent are referred to as "plaintext" or, more infrequently, "cleartext", "plaincode", or "placode".

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