ISOS and ISK Series Reports

Intelligence from decrypts of the Abwehr (German Military Intelligence) were largely responsible for enabling Britain to control the entire German espionage network in the country, keeping track of the messages of double agents (captured Abwehr agents turned to work for the Allies under the Double Cross system) both on home soil and abroad, and to identify any other German spies entering or leaving the country. Amongst other successes, the intelligence revealed that the Germans believed the information fed to them via the double agents, in particular that the Abwehr controllers had been fooled into believing that the D-Day landings would take place at Pas de Calais rather than Normandy.

ISOS Series
ISOS issued the ISOS series reports of intelligence obtained by breaking hand enciphered radio traffic. Suspected German agent radio nets were initially monitored by Radio Security Service at Hanslope Park, and when decrypts became available it became clear that they were indeed Abwehr traffic. Eventually, a new section was set up at Bletchley Park under the charge of Oliver Strachey, which was known as Illicit (or Intelligence) Services Strachey (ISOS). The first ISOS decrypt was issued 14 April 1940. Double agents' reports could be designed to allow them to be followed through the Abwehr radio network and help break the keys of the Enigma messages German controllers were sending on to Hamburg by using them as cribs.

ISK Series
When daily Abwehr Enigma traffic increased intercepts were given to Bletchley Park codebreaker Dillwyn Knox to deal with. In Oct-Dec 1941, Knox and his team broke the cipher (an account of how it was broken can be downloaded here). The first Abwehr Enigma decrypt was issued on 25 Dec 1941 in the first of a series of reports on Enigma-based Abwehr traffic. These reports were issued by the newly formed Illicit (or Intelligence) Services Knox (ISK) section. By the end of the War, over 140,000 ISK decrypts had been circulated.