Type B Cipher Machine

[1] The cipher codenamed "Purple" replaced the Type A Red machine previously used by the Japanese Foreign Office.

The Navy believed the IJN's Purple machine was sufficiently difficult to break that it did not attempt to revise it to improve security.

Japanese diplomatic communications at negotiations for the Washington Naval Treaty were broken by the American Black Chamber in 1922, and when this became publicly known, there was considerable pressure to improve their security.

The development of the machine was the responsibility of the Japanese Navy Institute of Technology, Electric Research Department, Section 6.

In 1928, the chief designer Kazuo Tanabe and Navy Commander Genichiro Kakimoto developed a prototype of Red, "Roman-typewriter cipher machine".

The prototype used the same principle as the Kryha cipher machine, having a plugboard, and was used by the Japanese Navy and Ministry of Foreign Affairs at negotiations for the London Naval Treaty in 1930.

The 91-shiki injiki Roman-letter model was also used by the Ministry of Foreign Affairs as "Type A Cipher Machine", codenamed "Red" by United States cryptanalysts.

A significant weak point was that it enciphered vowels (AEIOUY) and consonants separately, perhaps to reduce telegram costs.

The keyboard on the second typewriter becomes the input and the twenties letters pass through the stepping switch stages in the opposite order.

The rotors on each layer are attached to a single shaft that advances from one stator contact to the next whenever an electromagnet connected to a ratchet is pulsed.

Both the Japanese version and the early American analog constructed each stage from several smaller stepping switches of the type used in telephone central offices.

The fragment of a Type 97 Japanese machine on display at the National Cryptologic Museum, the largest piece known in existence, has three 7-layer stepping switches (see photo).

An additional layer was used in the improved analog to automatically set each switch bank to the initial position specified in the key.

The improved analog organized the wiring more neatly with three matrices of soldering terminals visible above each stepping switch in the photograph.

The permutations used in the twenties cipher were "brilliantly" chosen, according to Friedman, and it became clear that periodicities would be unlikely to be discovered by waiting for enough traffic encrypted on a single indicator, since the plugboard alphabets changed daily.

This provided enough traffic based on the identical settings (6 messages with indicator 59173) to have a chance of finding some periodicity that would reveal the inner workings of the twenties cipher.

Around 2 pm on 20 September 1940 Genevieve Grotjan carried a set of work sheets up to a group of men engrossed in conversation and politely attempted to get Frank Rowlett's attention.

[7]: 99  A pair of other messages using indicator 59173 were decrypted by 27 September, coincidentally the date that the Tripartite Agreement between Nazi Germany, Fascist Italy, and Imperial Japan was announced.

In April 1941, Hans Thomsen, a diplomat at the German embassy in Washington, D.C., sent a message to Joachim von Ribbentrop, the German foreign minister, informing him that "an absolutely reliable source" had told Thomsen that the Americans had broken the Japanese diplomatic cipher (that is, Purple).

A third was sent to England in January 1941 on HMS King George V, which had brought Ambassador Halifax to the U.S. That Purple analog was accompanied by a team of four American cryptologists, two Army, two Navy, who received information on British successes against German ciphers in exchange.

[4]: p.23  The Purple intercepts proved important in the European theater due to the detailed reports on German plans sent in that cipher by the Japanese ambassador in Berlin.

The United States obtained portions of a Purple machine from the Japanese Embassy in Germany following Germany's defeat in 1945 (see image above) and discovered that the Japanese had used a stepping switch almost identical in its construction to the one Leo Rosen of SIS had chosen when building a duplicate (or Purple analog machine) in Washington in 1939 and 1940.

The stepping switch was a uniselector; a standard component used in large quantities in automatic telephone exchanges in countries like America, Britain, Canada, Germany and Japan, with extensive dial-telephone systems.

[11] A complete Jade cipher machine, built on similar principles but without the sixes and twenties separation, was captured and is on display at NSA's National Cryptologic Museum.

Decryption and typing difficulties at the embassy, coupled with ignorance of the importance of it being on time, were major reasons for the "Nomura Note" to be delivered late.

During World War II, the Japanese ambassador to Nazi Germany, General Hiroshi Oshima, was well-informed on German military affairs.

[12] Since those messages were being read by the Allies, they provided valuable intelligence about German military preparations against the forthcoming invasion of Western Europe.

"[13] The decrypted Purple traffic and Japanese messages generally were the subject of acrimonious hearings in Congress after World War II in connection with an attempt to decide who, if anyone, had allowed the attack at Pearl Harbor to happen and so should be blamed.

[citation needed] (See the Pearl Harbor advance-knowledge conspiracy theory article for additional detail on the controversy and the investigations.)

Analog of the Japanese Type B Cipher Machine (codenamed Purple) built by the U.S. Army Signal Intelligence Service
Purple analog equipment in use
Japanese Type A (RED) cipher machine
Fragment of a Type 97 "Purple" cipher machine recovered from the Japanese embassy in Berlin at the end of World War II. Purple code was reverse engineered by the United States Army's Signal Intelligence Service in 1940.
Side view of recovered Type 97 fragment. The three stepping switches implemented one stage of the "twenties" cipher, with 25 permutations of 20 letters encoded by the mass of wires underneath.
Close up of fragment's stepping switches showing seven contact layers
An equivalent analog to the Purple machine reconstructed by the US Signals Intelligence Service. A hand-operated Red analog is also visible
Six level telephone stepping switch similar to the type used by SIS to build its first Purple. analog. On display at The Telephone Museum .
Schematic diagram of SIS Purple analog machine
Front panel of the March 1944 improved U.S. Army PURPLE analog. The three rows of indicator lights show the position of the rotor in each stage. a removable plugboard in the center selects the alphabet. A six-position switch selects the stepping order and buttons on the box at top center are used to set the initial position of each rotor. Exhibit at the National Cryptologic Museum
Internal wiring of the improved U.S. Army PURPLE analog. All three stepping motor stages for the "twenties letters" are shown. The three large rectangular panels with many wires implement the substitution matrices for each stage. Each twenties stepping switch is located beneath its wiring panel. The stepping switches for the sixes letters are near the middle.
Close-up of the six-position switch that selects the stepping order in the improved U.S. analog
PURPLE ciphertext of the first part of the 14-part message which was delivered by the Japanese to the U.S. Government on 7 December 1941 . Note the hand-written calculations at the upper right which deduce the initial positions of the rotors and the stepping order from the message indicator.