12 Apr 1994, Bill Sommerfeld wrote: (about the strategical impact of codebraking in WWII)
Sources: the book "Bodyguard of Lies". Unfortunately, my copy of the book is at home; I don't recall the name of the author, but it's a book on deception campaigns in World War II; the title is a shortened form of the (approximate) quote "In wartime, the truth is protected by a bodyguard of lies".
With some effort I found that book deep inside my private library. By Anthony Cave Brown, 1975. It has been a while since I read it but I recollect that it is a straightforward tale of spying and deception incidents without much of a critical analysis. For those who want to read a rather different conclusion (i.e. negative) regarding the importance of the spooks in WWII (and whatever) I warmly recommend: The Second Oldest Profession by Phillip Knightley, 1986. Some quotes from the cover description: He shows how, once it had gained a toehold within a single government bureaucracy, the espionage industry expanded remorselessly and firmly established itself at the very heart of the modern state. Do they make any difference - even in wartime? Over the years intelligence work has probably attracted more con-men, fantasists and sheer incompetents than any other field of human endeavour and, stripped of their mystique, the secret world and the antics of its inhabitants are as much the stuff of farce as of melodrama.