What made the Navajo language so perfect for the basis of a code?

Navajo made a good basis for a code for several reasons. Any unknown language is very hard to figure out if it is not related to a known one. There were very few non Navajos who spoke the language at the time. Some estimates put that number at 30-40 people. Dictionaries and grammars had not been published internationally. Although it is very similar to Apache it is not understandable to others (in Canada and Alaska) in the larger language family, the Na-Dene. It was very hard for mainly non native speakers because the consonants are difficult, the vowels can be tonal and nasalized and the grammar is complex. It the guy whose idea it was, Philip Johntson, had grown up on the Navajo reservation, the son of a missionary and spoke the language and had been in the Army in WWI. There were also a large enough group of young Navajos who were fluent in Navajo. This was not the case for many other native languages. This was because Navajo are one of the largest tribes, and it was very remote area in the 1940's so most still spoke Navajo. many young men in their 20's had learned English in boarding schools.

The man who taught a Navajo class that I took in Kayenta, AZ had been punished for speaking Navajo and not knowing English when he entered boarding school in Chinle at age 7.

The code was not just spoken Navajo so to native speakers it would sound like a strange string of Words. Basically it was a alphabet substitution code. So if you wanted to spell an english Word that started with c, you would have cat stand for "c" and use the navajo Word for cat, "mósí". Sometimes to mix it up you could use the would cow, Navajo for cow is "béégashii"

Note: in these examples the marks over the vowels means those vowels are high tone, those without marks are low tone.