General Schellenberg, head of this department, describes the whole affair in his memoir. After learning the codename of this operation the Germans were naturally keeping an eye for messages that referred to it.
In page 390 of his memoirs he says about the files that Cicero copied from the ambassador’s safe:
The contents so impressed me that at first I devoted myself entirely to the study of the documents and almost forgot to initiate those measures which must be carried out by the chief of a Secret Service in such cases. However, I then ordered:
(a) the immediate presentation of the reports to Hitler through Himmler.
(2) General Thiele (Chief of the Wireless Security and Decoding section of the Wehrmacht Supreme Command) to visit me at once to receive the material, which would enable him to start work on deciphering the British diplomatic code (The four greatest decoding experts in Germany, two professors of mathematics among them, worked on this material for weeks until finally they were able to 'crack' a part of the code. It was a tremendous achievement. Especially revealing were a number of handwritten notes on the margins of the documents, technical data on code messages from London to Ankara. Such things were of the greatest value to our experts.)
In page 393 he says:
In the meantime, by using the documents we had been able to decipher part of the British diplomatic code. One of the first important pieces of information we found in 'Cicero's' material was that the planned invasion of France was to carry the code name Operation 'Overlord'. After the first appearance of these words in the document, I immediately conferred with General Thiele. He at once started operations that would enable us to determine where and when the code word 'Overlord' appeared in the enemy's short-wave communications.
However his postwar interrogation by the Allies, summarized in ‘Hitler's Last Chief of Foreign Intelligence: Allied Interrogations of Walter Schellenberg’, p251 says:
Schellenberg maintains that he has never seen deciphered British messages. He has learnt that the last successful deciphering was that of British messages which went by W/T from Cairo to London. After that a coding machine was introduced in Cairo which abruptly prevented all further deciphering. General Thiele had continuously asked Schellenberg to provide him with an English code machine or an English diplomatic or military attache code still in use, but Schellenberg never succeeded in getting these.
So which version is correct?