Japanese naval codes

The abbreviations used herein for the various cryptographic systems are those provided by the cryptographic Western organizations.


JN-25 is the name given to the main security code used by the Imperial Japanese Navy to encrypt their communications before the Second World War and during it, the number 25 is derived from the fact that it was the 25th anniversary of the Navy code if Japan (Japanese Navy) to be identified by the forces of the Allies.

During the period of its use, the code JN-25 was often revised and amended by introducing new codes and new vocabularies, sometimes simultaneously, to force opponents to a new job for each new version of cryptanalysis. In particular, the code was modified significantly immediately prior to December 7, 1941, the date on which it happened ‘s attack on Pearl Harbor.

This latest version of the code JN-25 was the one decrypted during May 1942, in sufficient detail to provide a notice to the Americans that led to the American victory at the Battle of Midway.

British, Australian, Dutch and U.S. cooperated on attacks on JN-25 well before the attack on Pearl Harbor. The Japanese Navy was not engaged in a real battle until the end of 1941, so there was little traffic available to work on. Orders and discussions of the Japanese Imperial Navy could usually travel on safer routes the encrypted transmissions, such as couriers or delivery directly by its own vessel. The public reporting differ, but the most credible agree that the version of JN-25 in use before December 1941 had not been deciphered by more than 10% at the time of the attack. The JN-25 traffic increased immensely with the outbreak of naval warfare at the end of 1941, and provided the “depth” of cryptographic required to decipher substantially the existing version of JN-25 and the following.

The U.S. effort was directed from Washington, DC, as part of a structure called the U.S. Navy’s OP-20-G. It was centered on the Combat Intelligence Unit (Station HYPO), commanded by Joseph Rochefort. With the assistance of Station CAST in the Philippines and later the British in Hong Kong and later in Singapore, and punch card tabulators IBM (when available), was brought to a successful attack edition of the JN-25, which entered into force on 1 December 1941. This teamwork led to the significant progress in early 1942. Were used in the attacks with known plaintext to decipher the common Japanese formalities contained in the messages, as in “I have the honor to inform Your Excellency” and the use of formal titles stylized.

Note that the Purple code (also known as AN-1), used by the Japanese foreign ministry as his system more secure, he had no connections with any of the cryptographic versions of the JN-25, or with any other encryption systems used by ‘ Japanese military before and during the war. The Purple traffic was diplomatic, not military, and in the period before the attack on Pearl Harbor, the Japanese military, which controlled the politics of their country, they did not trust enough for the Foreign Ministry to reveal much. The traffic JN-25, on the other hand, was limited to military matters, especially the operations of the Japanese Imperial Navy, from which one could infer tactical or strategic information. Nevertheless, the traffic Purple was very valuable, especially later in the war, and was generally referred to as “Magic”.

The encryption algorithm

This code consisted of a vocabulary of about 33000 sentences, words and letters to each of which was associated with a five-digit number. The words do not exist in, they were broken down into syllables of kana (the Japanese syllabary) each of which was associated with, again, a five-digit number. From communication to encrypt was obtained and then an encrypted message composed by a set of five-digit numbers.

At this point it was produced another cipher of equal length, obtained from a codebook of random numbers (but always five digits) organized in “pages”, “rows” and “columns”: a hat-trick “page-row-column” was the encryption key.

The final encrypted message to be transmitted was finally obtained by simply subtracting the second from the first encrypted message, the result was still a set of five-digit numbers, from which it was possible to reconstruct the original message, in theory, just being in possession of the cipher numbers random and knowing the encryption key used.

An example

Suppose you want to send the message: Domani Ricognizione Sottomarina Urgente in Australia browsing the vocabulary we obtain the following correspondences word-number: Australia = 45261Domani = 38659Ricognizione = 29640Sottomarino = 97850Urgente = 24713then the encrypted message will be:38659 29640 97850 24713 45261Now if, according to the encryption key selected, the cipher furnish the following numbers:2250810989426321984724983…the encrypted message to be transmitted would be38659 29640 97850 24713 45261 Message cipher to encrypt22508 10989 42632 19847 24983 Message crittografante—————————–16151 19761 55228 15976 21388 message encrypted


JN-40 replaced the JN-25 code as the primary super-encrypted. In September, 1942, a mistake by the Japanese gave some clues to the British code breakers stationed in Mombasa, Kenya, which, starting from next November, they were able to decrypt all encrypted communications with this code, allowing Allied submarines track and attack several enemy ships.



The Fleet Auxiliary System, derived from the code of cargo shipping JN-40.


Code based on the simple transposition and substitution, used to launch Navigation Alarms.


Code used in commercial shipping.

Cryptographic hardware

World War II

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