For a communication system to be secure, the key to the encryption system must be limited to only those that need it.
According to Auguste Kerckhoffs' Law of Cryptography (also called Kerckhoffs' assumption or Kerckhoffs' principle):
"the majority of civilian cryptography makes use of publicly-known algorithms. By contrast, ciphers used to protect classified government or military information are often kept secret" (Absolute Astronomy website)
meaning that in the case of the Navajo, the encryption key the Navajo dialect was virtually 'unknown' or secret in that only a handful of non-Indians understood the language. With code talkers, the language would be the 'secret' key, much like the 'twins speak' - the secret language that twins develop between themselves that is unknown except to them.
Works Cited/Consulted
Its syntax and tonal qualities, not to mention dialects, make it unintelligible to anyone without extensive exposure and training. It has no alphabet or symbols, and is spoken only on the Navajo lands of the American Southwest. One estimate indicates that less than 30 non-Navajos, none of them Japanese, could understand the language at the outbreak of World War II. (U.S. Navy Historical Achieves)
Given that only Native Americans of the Navajo Nation would know the language, in addition to a relative handful of non-Navajo being able to understand and speak the language, the Navajo language was practically the ultimate code.
During the three and a half years that the code was in use in the Pacific, the code was never broken.
There was one known instance of a native Navajo being forced to decipher the code.
Sgt. Joe Kieyoomia was a Navajo in the U.S. Army in the Philippine Islands at the beginning of World War II and was captured after the fall of Corregidor in 1942. During his 43 months in Japanese prisoner-of-war camps, he was tortured to decipher intercepted Navajo messages:
"One day two Japanese women visited me," he says. "They wrote Navajo words in English and asked what they meant. So, I told them: "This means bird, this means turtle, this means water." (How Effective Was Navajo Code?)
What prevented the Japanese from breaking the Navajo code was the second level of encryption that was used. Since Kieyoomia was not trained to be a code talker he was un-able to make any relationship between the code words and military terms.
While the Navajo code talkers were communicating in their own language, then didn't just say 'tanks are coming up the road', they 'codified' the phrases. Instead of calling a tank a tank it was called 'tortoise', a 'rocket' was called a 'sand boil', a 'flare' was called a 'light streak', and 'comma' was called a 'tail drop'.
For words that did not directly translate into Navajo, the code talkers would use a phonetic alphabet, but with a twist. Instead of using the Navajo equivalent of 'alpha' to mean the letter 'a', there were several variations: 'a' could be called 'ant', 'apple', or 'axe', 'b' could called 'badger', 'bear', or 'barrell'. With the exceptions of the letters 'w', 'x', 'y', and 'z', the rest of the Anglo alphabet had two to three different variants.
The use of two different code systems together made the Navajo code talkers a most valuable asset during World War II
In order to protect the code, the frontline code talkers were assigned a bodyguard. A code talker from the 3rd Marine Division, Bill Toledo, said his bodyguard was with him at all times. The role of the bodyguard was to protect the code, meaning if they were certain the code talker will be captured they have to kill a fellow marine. However not all code talkers were assigned a bodyguard, it depended on the the location and the probability of capture.
Fortunately none of the code talkers were captured and the Japanese were left in the dark.