BROKEN AND UNBROKEN

During the U.S. Congressional investigation of the Pearl Harbor attack held immediately after the war, a stunning fact emerged: even before the attack, American cryptologists had mastered the Japanese cipher machine they had codenamed “Purple” and had been reading the most secret Japanese diplomatic communications. The open irony of this disclosure was that since Japanese diplomats had been kept in the dark about their government’s intentions, they did not reveal the planned attack. The concealed irony was that the Japanese communications disclosed far more than Japanese secrets.

Nearly 30 years transpired before a groundbreaking book, F. W. Winterbotham’s The Ultra Secret (1974), revealed the vast triumph of Allied codebreakers against German secret communications, most famously the German Enigma machine. Since then, the declassification of thousands of documents detailing Allied success against coded Axis communications has generated shelves of works celebrating these accomplishments.

That mass of literature hints at a huge reciprocal question: just how secure were Allied secret communications? During the decades after the war, disclosures divulged some Axis success, perhaps most notably German penetration of the code used through 1943 to route Allied convoys. That such disclosures are rare, though, points to a vital Allied supremacy in communications security—yet no great library elaborates on this crucial achievement. What has emerged from that void is that behind the two most significant American cryptography achievements—the codebreaking success against the sophisticated Japanese Purple cipher machine and the development of the most secure cipher machine of the war, the United States’ SIGABA—was the same man, William F. Friedman, in parallel stories that unwind along a train of improbable coincidences and bizarre twists.

FIRST, SOME KEY TERMINOLOGY. The term “codebreaking” is often loosely applied to decrypting both codes ciphers, but these are two very different categories. Take, for example, the word “battleship”—which in the world of codebreakers is termed “plain text.” In a code, typically there would be a specific multiple-letter or numerical sequence standing for “battleship,” like “ABCDE” or “12345.” A cipher, on