Leo Rosen

Leo Rosen

Leo Rosen was an American cryptanalyst and engineer . With his help, American code breakers from the Signal Intelligence Service (SIS) headed by Frank Rowlett succeeded in creating the Japanese “ Type B ” (also called “Type 97”) cipher machine from August 1940, and thus before the USA entered World War II . ; American code name : Purple , German " Purpur " ) practically to break industrially and thus contribute significantly to the Allied war effort.  

Life

Authentic step switches of the Japanese machine that were recovered from the Japanese Embassy in Berlin and can be seen today in the National Cryptologic Museum .

Step switch as used by Rosen to create a replica of the Japanese Purple machine.

American Purple replica

In addition to studying electrical engineering at the Massachusetts Institute of Technology (MIT), Leo Rosen completed officer training in the Reserve Officer Training Corps (ROTC) before the war . In 1939 he was called up for active duty in the SIS , a cryptanalytic unit in the US Army .

At the time, Purple could already be “cracked” here, but the work was cumbersome and time-consuming and had to be done with pencil and paper. That meant a lot of manpower, prone to errors and delays. Rosen had the idea of ​​mechanizing the work and designed a corresponding machine, a replica of Purple , although he had never seen one. Nevertheless, on the basis of the analyzed logic of the encryption alone, he succeeded not only in designing the concept, but also in finding suitable components for its implementation. Remarkably, he suggested, the use of toggle relay before, and not rotors , as is customary with most cipher that time. The replica of the Japanese machine was successful.

Since the Americans were also able to use a special cryptanalytic process to infer the message key used from the indicator sent , they were able to “ read ” the Japanese radio messages as quickly as the authorized recipients. This also included a message to the Japanese embassy, encrypted using Purple, on December 7, 1941, about the severance of diplomatic relations with the USA. Despite immediate decipherment , the plaintext reached the US government too late to be able to react before the attack on Pearl Harbor that took place on the same day . In addition, the text contained no direct reference to this attack.

As the Americans only found out after the war, after they were able to steal an original Japanese machine from the Japanese Embassy in Berlin , the use of step switches actually corresponded to the chosen technical implementation.

Rosen was also one of the very exclusive circle of officers the British allies allowed entry to Bletchley Park , their code- breaking headquarters near the English town of Bletchley . His first visit there had already taken place in February 1941. He was thus one of the liaison officers to whom the cryptanalytic cooperation, which was so fruitful for the Allies, was owed.

After the war he became Assistant Director of Research ( German  "Deputy Director of Research" ) and Chief Engineer of the Army Security Agency (ASA) , the successor to the SIS , which later became the National Security Agency (NSA) emerged. In 1967 he received the NSA's Exceptional Civilian Service Award .

Leo Rosen was posthumously inducted into the Hall of Honor (German: "Ehrenhalle") of the NSA in 2010 . At the ceremony he was represented by his son Lawrence Rosen and his grandsons Philip, Michael and Christine Rosen.

literature

• Craig P. Bauer : Secret History - The Story of Cryptology . CRC Press, Boca Raton, 2013, ISBN 1-4665-6186-6 .
• Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin a. a. 2000, ISBN 3-540-67931-6 .

Web links

Commons : Leo Rosen  - Collection of images, videos and audio files

• High resolution portrait photo
• Minutes of an interview with Leo Rosen on August 26, 1984 at NSA.gov (English)

Individual evidence

1. ↑ Craig P. Bauer: Secret History - The Story of Cryptology . CRC Press, Boca Raton, 2013, p. 305, ISBN 1-4665-6186-6 .
2. ↑ Stephen Pincock and Mark Frary: Secret Codes - The Most Famous Encryption Techniques and Their History. Bastei Lübbe 2007, p. 116, ISBN 978-3-431-03734-0 .
3. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin a. a. 2000, p. 4, ISBN 3-540-67931-6 .
4. ↑ 2010 Hall of Honor Inductee at nsa.gov , accessed on August 10, 2019.