Compromise of US M-209 cipher machine prior to the invasion of Normandy

Sometimes the answer to an interesting question is right in front of us but we can’t see it because we’re not paying attention…

In my essay German intelligence on operation Overlord I said about the M-209 cipher machine:

The M-209 cipher machine was used extensively by the US armed forces in the period 1943-45. Army units in England sent training messages on the M-209 which the Germans decoded.

The USAAF used it in operational and administrative networks.

M-209 traffic together with D/F may have allowed the Germans to discover the concentration of US forces in the South.

After having a look at the report E-Bericht Nr. 3/44 der NAASt 5 (Berichtszeit 1.4-30.6.44) it is clear that the Germans were in fact able to get order of battle intelligence on the US forces in the UK. In pages 2-3 it says:

 

 

Activity report before the invasion

…………………………………………

1). AM1:

Focused on decoding the AM1. Ten absolute settings were recovered, which brought the deciphering of 1,119 messages. This cipher-material, mostly composed by the U.S American Expeditionary Corps, gave valuable insights into the location of enemy groups.

AM1 (Amerikanische Maschine 1) was the German designation for the M-209.