Primary Sources: World War II: Code Breaking

A Codemaker's War, 1941-1945
"... begins with his failure to get into British Intelligence's cryptographic department. As everyone else on his course heads off to Bletchley Park ("the promised land"), he is sent to what his sergeant terms "some potty outfit in Baker Street, an open house for misfits." In fact, the Special Operations Executive's mandate was, in Churchill's stirring phrase, to "Set Europe Ablaze," and Marks's was to monitor code security so that agents could could report back as safely as possible."

Calvocoressi worked as a code breaker for the RAF in WWII

"Operation Ultra was designed to intercept and decode German signals sent using Enigma, the top-secret German cypher machine. F.W. Winterbotham, was the man responsible for the organization, distribution and security of Ultra. This is his personal account of the operation. "