Their most modern and (in theory) secure system was the M-138-A strip cipher. Unfortunately for the Americans this system was compromised and diplomatic messages were read by the Germans, Finns, Japanese, Italians and Hungarians. The strip cipher carried the most important diplomatic traffic of the United States (at least until mid/late 1944) and by reading these messages the Axis powers gained insights into global US policy.
Germans, Finns and Japanese cooperated on the solution of the strip cipher. In 1941 the Japanese gave to the Germans alphabet strips and numerical keys that they had copied from a US consulate in 1939 and these were passed on by the Germans to their Finnish allies in 1942. Then in 1943 the Finns started sharing their results with Japan.
Finnish solution of State Department cryptosystems
During WWII the Finnish signal intelligence service worked mostly on Soviet military and NKVD cryptosystems however they did have a small diplomatic section located in Mikkeli. This department had about 38 analysts, with the majority working on US codes.
Head of the department was Mary Grashorn. Other important people were Pentti Aalto (effective head of the US section) and the experts on the M-138 strip cipher Karl Erik Henriksson and Kalevi Loimaranta.
Their main wartime success was the solution of the State Department’s M-138-A cipher. The solution of this high level system gave them access to important diplomatic messages from US embassies in Europe and around the world.
Apart from purely diplomatic traffic they were also able to read messages of other US agencies that used State department cryptosystems, such as the OSS -Office of Strategic Services Bern station, Military Attaché in Switzerland, Office of War Information representative in Switzerland, the Foreign Economic Administration, War Shipping Administration, Office of Lend-Lease Administration and the War Refugee Board.
Operation Stella Polaris
In September 1944 Finland signed an armistice with the Soviet Union. The people in charge of the Finnish signal intelligence service anticipated this move and fearing a Soviet takeover of the country had taken measures to relocate the radio service to Sweden. This operation was called Stella Polaris (Polar Star).
In late September roughly 700 people, comprising members of the intelligence services and their families were transported by ship to Sweden. The Finns had come to an agreement with the Swedish intelligence service that their people would be allowed to stay and in return the Swedes would get the Finnish crypto archives and their radio equipment. At the same time colonel Hallamaa, head of the signals intelligence service, gathered funds for the Stella Polaris group by selling the solved codes in the Finnish archives to the Americans, British and Japanese.
The Stella Polaris operation was dependent on secrecy. However the open market for Soviet codes made the Swedish government uneasy. In the end most of the Finnish personnel chose to return to Finland, since the feared Soviet takeover did not materialize.
The Higgs memorandum
In September 1944 colonel Hallamaa met with L. Randolph Higgs, an official of the US embassy in Sweden and told him about their successes with US diplomatic codes and ciphers.
This information was summarized in a report prepared by Higgs, dated 30 September 1944.
The report can be found in the US National Archives - collection RG 84 ‘Records of the Foreign Service Posts of the Department of State’ - ‘US Legation/Embassy Stockholm, Sweden’ - ‘Top Secret General Records File: 1944’.
Higgs met with colonel Hallamaa on September 29 and the OSS officials Tikander and Cole were also present during their discussion.
Hallamaa stated that he was an administrator, not a cryptanalyst and about 10-12 of his men worked on US diplomatic codes.
His unit had solved the US codes Gray, Brown, M-138-A strip cipher and enciphered codebooks (probably the A1, B1, C1).
The high level M-138-A system had been solved mostly by taking advantage of operator mistakes such as sending strip cipher information on other systems that had already been broken or sending the same message in different strips one of which had been broken.
The strip cipher was considered a strong encryption system and had been adopted by the Finns for some of their traffic.
Important diplomatic messages from the US embassies in Switzerland, Sweden and Finland were read by the Finnish codebreakers.
Regarding Bern, Switzerland most of the messages dealt with intelligence matters:
‘Replying to my request for information regarding the contents of the messages from our Legation in Bern to the Department, Col. Hallamaa said the great bulk of them were intelligence messages dealing with conditions in Germany, France, Italy and the Balkans. He spoke in complimentary terms about ‘Harrison’s’ information service’.
Regarding Helsinki, Finland Hallamaa stated that thanks to the decoded diplomatic traffic they were always informed of current US policy initiatives:
‘Col. Hallamaa said that they always knew before McClintock arrived at the Foreign Office what he was coming to talk about’.
Hallamaa revealed a lot of confidential information to the Americans and volunteered to have some of his experts interviewed.
The interview was conducted on friendly terms with Higgs stating; ‘Col. Hallamaa was most pleasant and seemed to be entirely frank and open regarding the matters discussed’.