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If nothing else, adding encryption on top of the Navajo language makes known-plaintext much more difficult.

Navajo has no written language.

Not so. There has been a written form for Navajo for the last few decades (at least). It uses the Roman alphabet but the mapping of sound-to-character is not the same as for English. Vowels aren't even a, e, i, o, and u, which certainly doesn't help speakers of English trying to form syllables. To my eye, it more closely resembles modem noise on an ASCII terminal than anything else, but it does exist. (no, I can't read it) Navajo code talkers spoke with Navajo words, but not with Navajo meanings. Parts of speech that should be nouns became adjectives, etc. Navajo Elders who heard the code talkers commented "It sounds like Navaho, but it doesn't make any _sense_!". -Ya-ha'-tey! Bill Yerazunis