In World War II, navy code books were bound with lead covers to make it easy to safely jettison the books overboard if necessary. Such code books where a bit l

In World War II, navy code books were bound with lead covers to make it easy to safely jettison the books overboard if necessary. Such code books where a bit like dictionaries containing random alphanumeric sequences for each useful plaintext word. This was then subjected to an additional layer of encipherment. Such a code was generally beyond the ability of foes to break. Many years past the battle of Midway, it was finally revealed that the US had broken the Japanese Navy code known as “Purple”, explaining the United States lucky outcome in that important battle. I believe it was through a combination of obtaining one of the code books and cryptanalysis.