This book has grown out of reporting on issues related to intelligence and cyber security over a number of years. It based in part on interviews with many individuals who have worked in the field and I am grateful for their assistance. Endnotes indicate where people have spoken on the record but there are many others who have provided advice and thoughts who would likely not thank me for naming them here. But they too have my gratitude.
I am particularly indebted to those who read drafts of parts of the book and provided comments. Other people and institutions I would like thank for help in my research include: the National Museum of Computing, the National Cryptologic Museum and its librarian René Stein, Bletchley Park, the Imperial War Museum and its sound library, the National Archives at Kew, the Charles Babbage Institute at the University of Minnesota, Satu Haase-Webb for research assistance in the US National Archives, Charlotte Dando at the Porthcurno Telegraph Museum, David Hay at the BT Archive, Erich Schmidt-Eenboom in Germany and my colleagues at the BBC, especially Mark Savage. My agent Georgina Capel has provided support and encouragement, while this book would not be what it is without the guidance and patience of my editor Bea Hemming. My greatest debt is owed to my family.