Code talkers are usually associated with United States military personnel who were tasked with the transmission of secret tactical messages during the world wars. They used their knowledge of Native American languages to fluently encrypt and decrypt messages over military telephone or radio communications.
The Navajo language (Diné bizaad) was especially interesting for building a code as it has a unique complex grammar, numerous dialects, and was an unwritten language at the time. This made it unintelligible to anyone without extensive exposure and training.
In 1942, Navajo men proved the idea of a Navajo code under staged simulated combat conditions by transmitting and decoding messages in 20 seconds instead of 30 minutes it took the machines of the time.
The Navajo code was formally modeled on the Navy phonetic alphabet, a set of words used to spell out letters in oral communication. As spelling out all words letter by letter was determined to be too time-consuming, some terms, concepts, tactics, and machines received distinct codes.
The Navajo code remained classified until 1968. It is the only oral military code that has never been broken.
The base alphabet of the Navajo code assigns Latin letters to Navajo code words. To strengthen the code against frequency analysis (i.e. guessing the underlying letters based on how often code words appear in messages) some letters were assigned to multiple code words between which code talkers switched randomly.
The full Navajo Code dictionary, including code words for common military terms, was published on the website of the U.S. Navy.