Dear Editor

Sir Harry Hinsley's interesting "The Counterfactual History of No ULTRA" ( Cryptologia, 20(4): 308 324) provides considerable food for thought. However, I was puzzled about a few items in it, probably because it was a talk, given to an unspecialised audience. I therefore wrote to Sir Harry, enclosing a draft of this letter, and later spoke to him.

In fairness to Sir Harry, readers should be aware that, when he gave the talk in October 1993, he did not know that it would be submitted to Cryptologia for publication. He was not even aware that it had been published until I contacted him.

I turn now to some detailed points for the record.

Page 317: "We read all the Enigma signals of the German Abwehr which meant that we captured every spy that arrived in the United Kingdom by having advance knowledge of his arrival". However, while the Abwehr attempted to infiltrate agents into the United Kingdom from September 1940 onwards, the Government Code and Cypher School ("GCCS") did not start to read Abwehr Enigma until over a year later, on 25 December 1941, when it issued ISK 1 (Intelligence Series Knox, 1), the first decrypt derived from one of several versions of the Enigma machine used by the Abwehr, which had been broken by Dillwyn Knox: Ralph Erskine, "Eavesdropping on Bodden: ISOS v. The Abwehr in the Straits" Intelligence and National Security (forthcoming); cf. F. H. Hinsley and C. A.