1942: Navajo Code Talkers

During World War II, coded radio transmission was the fastest way to deliver commands to units oversea. Cryptographers on both sides became adept at intercepting and decoding their opponents' transmissions. In 1942, the Marine Corps found a new way to keep their communications secure with the Navajo Code Talkers. 
 
Marines from the Navajo tribe began to send secure voice transmissions based on their native language. Since only a small group of Americans spoke Navajo, it was impossible for the enemy to gain intelligence from any intercepted messages. Additionally, the Navajo Code Talkers proved faster and more accurate than Morse Code or any machine.  
 
The unique Navajo language gave the Marines a strategic advantage during the Battle of Iwo Jima and countless other World War II battles. The program was highly classified for 25 years and, to this day, there's no indication any intercepted Navajo code was successfully deciphered.