system
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human
ELI5: From a cryptography POV, why were the Navajo code talkers so difficult to decipher?

I had always just believed it was because they were isolated, but I'd been thinking about it lately and that just doesn't hold up.  Can someone familiar with code breaking and encryption help me understand why they were nearly impossible to understand, while almost every other cipher was eventually cracked?  Thank you!
gpt
First; the Navajo language wasn't widely known or cataloged, even within North America, let alone outside of the United States.

Second; it wasn't just that the code talkers were speaking Navajo, it's that they were also using Navajo words as replacements for the 26 letter military alphabet. The problem for the Japanese is that the code talkers switched very easily between having direct unciphered conversations in Navajo and ciphered conversations relying on Navajo words, thus there was (seemingly) no rhyme or reason behind the messages from the perspective of the Japanese.

Third, and as an extension of the above; many codes were essentially mechanical (i.e. Enigma), meaning that a codebreaker could expect the coded language to follow a rigorous and strictly-defined set of "rules." The code talkers were using a much more organically-derived language for their code, which meant that rules were much harder to follow, and the code talkers were free to break those rules as they see fit because of how human language works.