Saturday, May 9, 2015

The compromise of the communications of General Barnwell R. Legge, US military attache to Switzerland

In the course of WWII both the Allies and the Axis powers were able to gain information of great value from reading their enemies secret communications. In Britain the codebreakers of Bletchley Park solved several enemy systems with the most important ones being the German Enigma and Tunny cipher machines and the Italian C-38m. Codebreaking played a role in the Battle of the Atlantic, the North Africa Campaign and the Normandy invasion. In the United States the Army and Navy codebreakers solved many Japanese cryptosystems and used this advantage in battle. The great victory at Midway would probably not have been possible if the Americans had not solved the Japanese Navy’s code.

On the other side of the hill the codebreakers of Germany, Japan, Italy and Finland also solved many important enemy cryptosystems both military and diplomatic. The German codebreakers could eavesdrop on the radio-telephone conversations of Franklin Roosevelt and Winston Churchill, they could decode the messages of the British and US Navies during their convoy operations in the Atlantic and together with the Japanese and Finns they could solve State Department messages (both low and high level)  from embassies around the world.
The State Department made many mistakes in the use of its cipher systems and thus compromised not only US diplomatic communications but also the messages of other organizations that were occasionally enciphered with State Department systems, such as the Office of Strategic Services and the Office of War Information. Another similar case concerns the communications of General Barnwell R. Legge, US military attache to Switzerland during WWII.

Legge was a veteran of WWI and recipient of the Distinguished Service Cross. In Switzerland he worked to promote US interests and he also cooperated in intelligence gathering activities with Allen Dulles, head of the local station of the Office of Strategic Services. The Swiss were officially neutral but they had close economic relations with the Axis countries and thus it was possible for the Allied intelligence agencies to gather information on political and military developments in Europe. Legge sent reports dealing with military developments and Axis war potential to the War Department in Washington but it seems that at least some of them were also read by the Germans and the Finns.
US military attaches used several cryptosystems during WWII. The basic systems were the Military Intelligence Code and the War Department Confidential Code. These were letter codebooks enciphered with the use of substitution tables. The US authorities were confident in their security but in 1941-42 the Italians and the Germans were able to get copies of the codebooks and some of the substitution tables and thus they could read US attache communications from Stockholm, Moscow, Cairo, Baghdad, Teheran and possibly other areas. The communications of colonel Bonner Fellers, US military attache in Cairo during 1940-2, were very important for the Germans and they provided them with valuable information during the fighting in N. Africa.

It is reasonable to assume that General Legge also used these codebooks at least in the period 1941-42 but it’s clear that he also had the M-138-A strip cipher and in late 1944 he was given one time pads. A report found in the US National Archives and Records Administration (1) has the results of a security study of his messages sent in the period April-June 1944. The system he was using was the strip cipher and the report says ‘While many violations were found in the traffic, it may be concluded that security has been maintained because of the relatively small number of groups enciphered each day’.

Apart from the standard cryptosystems (Military Intelligence Code, War Department Confidential Code. M-138-A strip cipher) US attaches also had an emergency double transposition cipher. According to the instructions for this system, found in the files of Pers Z (decryption department of the German Foreign Ministry) (2):

Use of cipher. To enable M/As to exchange safely secret or confidential messages with other attaches or with assistants or agents acting under their direction, the double transposition cipher is prescribed.

Keys. The keys will be determined by the M/A. They will consist of short phrases consisting of from five to twenty letters. They will be changed at frequent intervals.

However the same numerical sequence was used for both cages, which means that this system would have been vulnerable to cryptanalysis.

According to the postwar interrogations of German intelligence officers operating in Switzerland (3) in 1941 they were able to recruit a spy inside the US embassy in Bern. This person, named Fuerst, had access to the office of the US military attaché General Legge and he was able to take documents plus the used carbon paper and give it to the Germans. 

The stolen reports revealed some of Legge’s sources and showed that he got information from his British, Polish and French counterparts. The used carbon paper also contained valuable information but it had to be examined by experts in Germany. The information uncovered from these sources was also used to decipher some of his messages.

The German spy was arrested in March 1942 but this doesn’t seem to have ended the compromise of General Legge’s communications. In the Finnish national archives, in collection T-21810/4, there are a few messages signed Legge from March and April ’43. The originals are from NARA, collection RG 319 'Records of the Army Staff'.

Other US messages from Bern, found in the Finnish national archives, have information on German war production and mobilization data. Although these are not signed Legge they must have originated either from his office or from the OSS station. These messages were enciphered with State Department systems that the Germans and the Finns could solve cryptanalytically. So even if US attache ciphers were secure it was still possible for the Axis powers to read some of Legge’s communications in the period 1943-44. For example message No 4.926 of August 1st 1944 and the original from NARA, collection RG 59:

Also message No. 4973 of August 3rd 1944 and the original from NARA, collection RG 59:

(1) NARA - collection RG 457 - Entry 9032 - box 1.019 - ‘Working papers on strip cipher systems, 1943-1947’

(2). TICOM DF-15 ‘Reports of group A’, p1-2

Acknowledgments: I have to thank Randy Rezabek of TICOM Archive for the strip cipher security report.

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