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Enigma T (Tirpitz)
The Japanese Enigma - wanted item

The Enigma T, codenamed Tirpitz, was an Enigma cipher machine developed during WWII by the Germans especially for use by the Japanese Army. It was based on the commercial Enigma K, but had differently wired wheels (and ETW) and multiple turnovers on each wheel. Furthermore, it had an Eintrittswalze (ETW) that was wired differently from all other Enigma machines.

The machine was intented for communication between the German and Japanese Navy. The agreement for this was signed on 11 September 1942 by German Vice Admiral Erhard Maertens and Japanese Admiral Tadao Yokoi. At the time, Maertens was Director of the German Naval Communication Service and Yokoi was the Japanese Naval Attaché in Berlin. This followed the earlier Japanese-German Military Agreement of 18 January 1942 [2].

All German-Japanese communication would be encrypted with a machine that was referred to as T-Enigma or Enigma Model T. It was called TIRPITZ by the Germans, and the Japanese called it TIRUPITSU. The US Navy referred to the machine as OPAL and the traffic was named JN-18 [2]. The official name for the machine system was Japanese-German Joint Use Code No. 3.
Enigma T-244. Photo by John Alexander, courtesy Bletchley Park.

The system consisted of an operational procedure, named TIRPITZ, and a key list with the name GARTENZAUN (garden fence). The operational procedures were effective from 1 August 1943 until the end of the war. In 1999, in an article in Cryptologia, Frode Weierud gave a detailed account of the TIRPITZ and its historical context. It is available for download below [2]. Much of the information on this page is based on that article.

It is not exactly known how many T-Enigmas were actually built. The Japanese ordered 800 machines, but for various reasons this amount was never delivered. There were delays in design and manufacture, and it was increasingly difficult to get sufficient supply of materials. Furthermore, the Germans started to doubt the security of the machine. In the meantime, the Japanese used two manual systems: Sumatra (later Sumatra 2) and TOGO (later TOGO 2).

The Enigma T is based on the design of the Commercial Enigma (K) and not on the more common Steckered Enigma (I) that was used by the German Armed Forces. As the machine was to be used for communication between the Navies of Germany and Japan, they needed a strong cipher, but didn't want to 'give away' their crown jewels (i.e. the Service Enigma with Steckerbrett). Instead they took a standard Enigma K and improved it in several ways.

First of all, the entry wheel (ETW) was wired in a random fashion, different from all other machines. The machine was supplied with 8 coding wheels (3 in the machine). The most important difference however, was the presence of 5 turnover notches on each of the 8 wheels.

This caused far more frequent wheel turnovers and extended the cipher period (as 5 is a relative prime of 26). In some cases, the operational procedure instructed the user to advance the settable UKW manually by one position after each group of 5 letters, adding extra complexity.
Tirpitz wheel 3 courtesy GCHQ

The first order for Enigma T machines was for 400 units, but this order was never fully delivered. As there were shortages in material, only small batches of machines were supplied to the Japanese. Some shipments got losts whenever the carrying U-boat was sunk and one machine was compromised in Guadalcanal on 15 February 1943. The Germans experimented with the machine and started doubting its security, especially when using it for a great volume of traffic.

In June 2009 we had a chance to make some close-up photographs of an Enigma-T wheel. This wheel belongs to the Tirpitz Enigma with serial number T47 that is part of the private collection of the British Intelligence Service GCQH. More images below. Shortly afterwards, GCHQ would unexpectedly donate an Enigma-T with serial number T244 to the Bletchley Park museum.

Left side of wheel 3 Right side of wheel 3 Side of wheel 3 Left side of wheel 3 Right side of wheel 3
1 / 5
Left side of wheel 3
2 / 5
Right side of wheel 3
3 / 5
Side of wheel 3
4 / 5
Left side of wheel 3
5 / 5
Right side of wheel 3

A new Enigma
Apparently, the Japanese had acquired the manufacturing rights for the Enigma T, as on 18 May 1943 they are cancelling that order. Around the same time, the Germans propose a new machine for joint communication: the 02562-A-Enigma. According to a report, this is a Steckered machine, most likely based on the standard Service Enigma. The Germans propose to replace the remainder of the order (another 400 machines) with this new type.

Part of the initial batch of 400 machines had already been delivered however, and some machine were under construction. After the Japanese Naval Attaché expressed his concern about this, the Germans guaranteed that the new machine would be interoperable with the old one. Using the new machine without plugs (Steckern) would make the machine compatible with the Enigma T.

If this was the case, the machine would cryptographically be considerably stronger than the Service Enigma. It would combine the strength of the plug board, the more frequent turnovers and the increased cipher period. Had it been possible to deploy this machine widely, it might have defeated the American and British codebreakers. However, at this point during the war, the material shortages made it impossible to build large quantities of any machines.

Captured machines
From then on, small batches of the new machine were delivered to Japan, whilst the initial order for Enigma T machines was also (partly) fulfilled. In August 1944, a large batch of Enigma T machines was captured by the Allied Forces in a warehouse in the vicinity of Lorient. Although there are conflicting accounts, it is likely that some 70 machines were captured here.

T-244 for Bletchley Park
When Bletchley Park was first opened as a museum, around 2000, they had an Enigma on display that could be touched by the public. It was part of the so-called Crypto Trail that allowed visitors to follow the flow of an Enigma message. Researchers discovered however, that the machine on public display was an extremely rare Enigma M3: the standard machine used by the Kriegsmarine (German Navy), of which only a handful has survived.

The machine was removed from the hands-on exhibition in 2007 and was locked away in BP's safe. Since 2011 it is back on public display again, as part of their Enigma showcase, but this time it is protected by bullet-proof glass.

As they wanted another machine for the hands-on experience, BP director Simons Greenish asked GCHQ in mid-2007 for a spare Enigma machine of which they had more than one available. He was assuming to get a standard 3-wheel Steckered Enigma (Service) machine, but instead, in October 2007 he got a surprise.
Enigma T-244. Photo by John Alexander, courtesy Bletchley Park.

The machine that GCHQ sent to BP was a non-Steckered machine with 4 adjustable wheels... After a closer examinition it turned out to be the extremely rare Enigma T, with serial number T-244. The machine was in pretty bad condition and its wooden case was missing completely.

Furthermore, there was no serial number plate on the machine's body and the paint had deteriorated somewhat. Nevertheless it is probably one of the best unexpected gifts that Bletchley ever received. The machine has since been restored and is now part of the permanent Engima showcase in B-Block. Another Enigma T machine is on display at the NCM.

The table below shows the wiring of the Enigma T wheels, the entry disc (Eintrittswalze, ETW) and the reflector (Umkehrwalze, UKW) [1]. The rightmost column shows what letter is visible in the window when the wheel causes a turnover of the wheel to the left of it. Please note that these positions are different from the actual position of the notches on the circumfere of the wheel.


  1. Note that this is the only machine with a different wiring for the ETW. All other Enigma machines have an ETW that is wired either in the order of the alphabet (ABCDEF...) or the order of the keyboard (QWERZU...).

JN-18 traffic (i.e. encrypted on Enigma-T) was not frequently intercepted by the Americans and was therefore very difficult to break. At the end of the war, Enigma T was used by the Japanese Naval Attachés and even for deplomatic traffic after the Japanese destroyed their PURPLE machines. It is known to have been used between stations in Tokyo, Berlin, Stockholm and Bern. The exact operating procedure is currently unknown. In 2009, Frode Weierud published some messages that were believed to have been enciphered on the Enigma-T [3].

The image above shows a real Japanese Enigma-T message as it was intercepted by the US Navy's signal intelligence and cryptanalytical group OP-20-G, on 10 March 1944. More sample messages and backgrounds are available from Frode's website [3].

  1. David Hamer, Geoff Sullivan and Frode Weierud,
    Enigma Variations: An Extended Family of Machines

    Cryptologia, July 1998, Volume XXII, Number 3, pp. 211-229.

  2. Frode Weierud, TIRPITZ OPAL
    TIRPITZ and the Japanese-German Naval War Communication Agreement.
    Cryptologia, July 1999, Volume XX, Number 3, p. 6-10.

  3. Frode Weierud, German Enigma T (Tirpitz) Messages,
    used for Japanese-German Intercommunications.
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Crypto Museum. Created: Tuesday 11 August 2009. Last changed: Friday, 23 February 2018 - 22:30 CET.
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