The National Archives, in its documentation of the program, confirms that Japanese intelligence never succeeded in breaking the Navajo code as used by the Marine Corps.

Japanese signals personnel who intercepted Code Talker transmissions reportedly described the language as sounding like nothing they had ever encountered — an assessment that, in the context of the code’s phonological complexity and the specialized vocabulary layered on top of it, was not surprising.

Speed was the other dimension of the code’s advantage.

A trained Code Talker could encode, transmit, and receive a message in the time it took a mechanical encryption system to process the beginning of the same message.

In a 1945 test conducted after the war, a Navajo Code Talker transmitted and decoded a short message in twenty seconds.

The same message, processed through a mechanical encryption system, took thirty minutes.