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"This booklet, which is based upon a Bletchley Park Lecture given in January 1998, describes three situations that came very close to revealing the Allies most precious secret in World War II, the ULTRA intelligence based on the decryption of the German Enigma cipher messages."
"January 2011."
"In association with Royal Holloway University of London."
Title from cover.
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How the ultra secret was nearly revealed by John Gallehawk. ISBN 9781906723156. Published by Bletchley Park Trust in 2011. Publication and catalogue information, links to buy online and reader comments.