Use of the M-138-A cipher system by the US State Department in WWII

During WWII the US State Department used several cryptosystems in order to protect its radio communications from the Axis powers. The main systems used were the unenciphered Gray and Brown codebooks along with the enciphered codes A1, B1, C1, D1 and the new M-138 strip cipher. 

In the period 1940-1944 German, Japanese and Finnish codebreakers could solve State Department messages (both low and high level) from embassies around the world. The M-138-A strip cipher was the State Department’s high level system and it was used extensively during that period. Although we still don’t know the full story the information available points to a serious compromise both of the circular traffic (Washington to all embassies) and special traffic (Washington to specific embassy). In this area there was cooperation between Germany, Japan and Finland. The German success was made possible thanks to alphabet strips and key lists they received from the Japanese in 1941 and these were passed on by the Germans to their Finnish allies in 1942. The Finnish codebreakers solved several diplomatic links in that year and in 1943 started sharing their findings with the Japanese. German and Finnish codebreakers cooperated in the solution of the strips during the war, with visits of personnel to each country. The Axis codebreakers took advantage of mistakes in the use of the strip cipher by the State Department’s cipher unit.

After further (costly) research new information has come to light. Originally I thought that each US embassy had two sets of strips, the ‘specials’ for direct communications with Washington and the ‘circulars’ for messages sent to several embassies and for intercommunication between embassies.

Recently I also realized that there were additional sets of strips for enciphering the traffic of other US agencies such as the Office of Strategic Services, Office of War Information, Military Intelligence Service, Foreign Economic Administration, War Shipping Administration, Office of Lend-Lease Administration and War Refugee Board.

However there’s more to this story:

1). The circular strips were the 0 dash series. 0-1 was used from 1940 till August 1942. 0-2 from August 1942 till March 1943. From March 1943 a new set was used for each month, 0-3 for March 1943, 0-5 for May, 0-9 for September etc. The circular strips used in 1944 were numbered 0-13 to 0-24 for January-December 1944.

However there were two problems with this system.

One was that the embassy in Bern, Switzerland did not have access to the new strips so it seems that they continued to use the 0-2 strips for some time.

Another problem was that distributing the new circular strips to embassies around the world was not always possible, so some posts were told to continue using the old strips till the new ones arrived. This was clearly a security problem and Erich Huettenhain, chief cryptanalyst of OKW/Chi, said in his manuscript ‘Einzeldarstellungen aus dem Gebiet der Kryptologie’ that they relied on reencodements in their efforts to solve the strip system

‘Ein zweiter für die Entzifferung günstiger Umstand war, daß es wegen der U-Boot-Blockade nicht immer gelang, den auszuwechselnden Stabsatz rechtzeitig an alle Außenstellen zu bringen.  In solchen Fällen wurde z.B. ein cq-Spruch an die Stelle, bei der der neue cq-Stabsatz noch nicht eingetroffen war, mit dem bei der Stelle vorhandenen und seit längerer Zeit in Benutzung befindlichen Spezial-Stabsatz verschlüsselt. Wenn nun dieses Spezial-Verfahren gelöst war, - und das war in der Regel der Fall — so war der Klartext des cq-Spruches bekannt, und es lag ein Klar-Geheim-Kompromiß im neuen cq-Verfahren vor, aus dem die Stäbe des neuen cq-Verfahrens rekonstruiert wurden.’

2). A set of strips titled 00-1 (and key table C) were introduced in late 1943 for enciphering the confidential traffic of other US government agencies. In January 1944 the set 00-2 and 00-3 were sent to the embassies in Algiers (Free French), Turkey, Egypt, UK, Calcutta, Portugal, Spain, India, Sweden, Iran, Iraq, Beirut.

The 00-4 strips replaced set 00-3 in October 1944.

3). In April 1944 the strip system FRIBP was sent to Lisbon, Madrid, Tangier, Algiers, London, Dakar for Cross messages (US-British supply program).

In November 1944 a circular telegram said that the 000-1 strips were used for CROSS and Joint Economic missions messages.

4). In June 1944 Sweden, Spain, Portugal, Egypt, Turkey, Algiers (Free French) received strips to be used for the communications of the WRB - War Refugee Board.

5). In June/July 1943 the strip set 60-3 was introduced for intercommunication between the embassies in Bern, London, Lisbon, Algiers and Washington. From January 1st 1944 the strips 60-5 were used for this purpose.

Madrid also received the strips 60-5 in June 1944.

In July ’44 the 60-5 strips were sent to the US diplomatic facilities in Caserta (for Robert Daniel Murphy) and Rome (for Alexander Comstock Kirk).

6). The embassy in Bern, Switzerland received 6 new code systems in June 1943. In August they received systems 45 and six sixty, with key tables.

In late September 1944 Bern finally received the current circular strips 0-21 to 0-24 and thus use of the 60-5 strips was discontinued.

In early October ’44 Bern stopped using the 00-3 strips for sending messages of other US agencies.

7). During the war the State Department received information pointing to the compromise of the strip cipher system from the embassies in Casablanca, Vichy France, Helsinki, Stockholm and Bern.

8). The embassies in Panama, Turkey, India, Spain reported problems with the strip system. Similar problems (warping of the panel, defects in the paper strips) are mentioned in the military report SRH-366 ‘History of Army Strip Cipher devices’.

9). In August 1943  a strip system was forwarded to Harold J. Tittmann (US Charge d'Affaires to the Vatican).

10). In September ’44 a set of strips were sent to the Special mission of Taylor.

11). In November 1944 the ICSSY cryptographic material was sent to several embassies.

12). Several alphabet strips that are mentioned in decoded Japanese messages were used by embassies around the world. For example:

Strips 22-1 were used in Egypt and Baghdad in 1941, by Vladivostok in 1942-44, by Algiers in 1943.

Strips 38-1 were used by the embassies in Moscow, Ankara, China, Portugal, Australia in 1942-43.

Overall this is very interesting information and sheds some light into the use of the M-138-A strip cipher by the State Department. 

Sources: NARA - RG 59 - Purport Lists for the Department of State Decimal File 1910-1944 – microfilms 444 and 611 – 119.25/Strip Cipher