Bletchley Park and ULTRA
At the Government Code and Cypher School (GC&CS) in Bletchley Park, high-level encrypted enemy radio and teleprinter communications were decrypted to yield wartime signals intelligence that British military intelligence designated with the codename ULTRA.
Group Captain F. W. Winterbotham claimed in his book The Ultra Secret from 1974 that the British government had received advance notice of the Coventry attack thanks to ULTRA: intercepted German radio messages encoded with the Enigma cypher machine and decrypted by British cryptanalysts at Bletchley Park.
He added that Winston Churchill had instructed against taking any defensive steps to safeguard Coventry for fear that the Germans would think their cypher had been cracked.
This is a major accusation – the idea that the Prime minister was in effect, prepared to allow major death and destruction to occur to a major British city in order to protect a valuable intelligence asset.
Was Coventry actually named?
One of Ultra's major figures, Winterbotham oversaw the "Special Liaison Officers" who brought Ultra supplies to field commanders. So he could be considered a reliable source of information on this matter.
However, historians and other Ultra participants have refuted Winterbotham's assertion.
They claim that although Churchill was aware of the impending big bombing raid, no one knew what the target would be.
The Air Section at Bletchley Park, which analysed and translated all deciphered Luftwaffe communications, was led by Peter Calvocoressi. "Ultra-never mentioned Coventry," he said. "Churchill was under the notion that the raid was to be on London, so far from debating whether to save Coventry or safeguard Ultra."
Target London?
Scientist Reginald Victor Jones, who commanded the British team in the Battle of the Beams, said that Coventry was not the original target because "Enigma signals to the X-beam stations were not destroyed in time." British were also not aware that the Luftwaffe switched from having its pilots manually listen to the signals to having an automatic narrow-band receiver on board, which rendered jamming countermeasures useless.
Jones also recorded Churchill's return to London that afternoon, which suggested Churchill thought London was the raid's most likely target. Since 1996, the UK National Archives has had access to the Ultra decrypts for the pertinent time period.
Mondschein Sonate
A German signal was decoded and assigned the serial number CX/JQ/444 paragraph 4, between 07:35 GMT on November 10, 1940, and 05:00 GMT on November 11. The telegram did not specify Coventry as the target or the date, but it did provide code words that planes would use during the "Mondschein Sonate" operation.
In retrospect, it has become clear that "KORN," which was designated by the transmission of a figure 9, is the code word for Coventry.
KORN
However, even though PAULA had been sent to Paris and LOGE to London, this was not realised at the time. In fact, two reports from an aircraft participating in a raid on Southampton on November 30, two weeks after the Coventry Blitz, contained the word "KORN." In a subsequent decrypt on November 11 or early November 12, navigational beam settings for Wolverhampton, Birmingham, and Coventry were provided, but dates were not provided.
Churchill could not have taken action on new Ultra intelligence on the afternoon or evening of the attack because there was none to provide him. There was a pause in Ultra decrypts from 01:15 GMT on November 13 to 02:40 GMT on November 15, by which time the operation was well under way. Documents and information from captured airmen did not provide a clear picture either.