Oh, this again. Another historical figure to dissect, apparently. Fine. Let’s make this as painless as possible for both of us. Just try to keep up.

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Solomon Kullback

Born (1907-04-03)April 3, 1907

Brooklyn, New York , United States Died August 5, 1994(1994-08-05) (aged 87)

Boynton Beach, Florida Alma mater City College of New York (B.A., 1927; M.A., 1929) George Washington University (Ph.D., Mathematics, 1934) Known for Work in Information theory , Kullback–Leibler divergence

Scientific career Fields cryptanalysis , mathematics , information theory Institutions George Washington University , National Security Agency Doctoral advisor Frank M. Weida Doctoral students Hubert Lilliefors Robert H. Shumway

Solomon Kullback (April 3, 1907 – August 5, 1994) was an American cryptanalyst and mathematician whose contributions, while perhaps not flashy, proved foundational in the nascent fields of signal intelligence and information theory . He was, notably, one of the initial three employees brought on board by the formidable William F. Friedman for the US Army ’s then-fledgling Signal Intelligence Service (SIS) in the 1930s. His compatriots in this early, pivotal recruitment were Frank Rowlett and Abraham Sinkov . Kullback embarked on what would become an extensive and, by all accounts, distinguished career within the SIS, eventually transitioning into its successor organization, the monolithic National Security Agency (NSA). His trajectory culminated in the role of Chief Scientist at the NSA, a position he held until his retirement in 1962. Following this, he took up a rather less clandestine, though arguably equally impactful, position at the George Washington University , continuing to shape minds rather than merely decrypting them.

His name, for those who dabble in the statistical arts, is inextricably linked to the Kullback–Leibler divergence , a measure of how one probability distribution diverges from a second, expected probability distribution. This concept, often simply referred to as KL divergence, is a testament to the enduring impact of his work, developed in collaboration with Richard Leibler . It’s a rather elegant way of quantifying the inefficiency of assuming a particular distribution when the true distribution is different. Useful, I suppose, if you’re into that sort of thing.

Life and career

Kullback’s journey began in Brooklyn, New York , where he was born to Jewish parents. His father, Nathan Kullback, had emigrated from Vilna, then part of the Russian Empire and now known as Vilnius , Lithuania , around 1905, becoming a naturalized American citizen in 1911. A rather typical immigrant story, if one is inclined to categorize such things. Solomon received his early education at Boys High School in Brooklyn, a precursor to his academic pursuits. He then advanced to the City College of New York , where he earned a Bachelor of Arts degree in 1927, followed swiftly by a Master of Arts in mathematics in 1929. Not content with mere masteries, he pursued and successfully completed a Doctor of Philosophy in mathematics from George Washington University in 1934. His initial aspirations, rather quaintly, involved a career in teaching. He even returned to his alma mater, Boys High School, to impart mathematical wisdom, only to discover, with what I can only assume was a profound sense of cosmic disappointment, that the classroom was not to his liking. It was then, in a moment of clarity that perhaps only mathematicians truly understand, that he realized his true passion lay not in instructing others on the intricacies of numbers, but in the practical application of mathematics itself. A subtle, yet significant, distinction.

It was through the rather prosaic suggestion of Abraham Sinkov that Kullback stumbled upon his true calling. Sinkov, ever the diligent scout, presented him with a Civil Service flyer advertising positions for “junior mathematicians” at the princely sum of US$2,000 per year. A rather modest salary for individuals who would go on to shape national security, wouldn’t you agree? Both Kullback and Sinkov, demonstrating a commendable level of intellectual competence, passed the examination and were subsequently dispatched to Washington, D.C. to assume their roles as junior cryptanalysts. The path to genius, it seems, is often paved with bureaucratic flyers and standardized tests.

Upon their arrival in Washington, Kullback found himself under the direct tutelage of William F. Friedman , a man whose name is practically synonymous with American cryptology. Friedman, recognizing the critical need for a highly skilled cadre of codebreakers, had initiated an intensive and rigorous training program specifically designed for his new civilian recruits. This wasn’t merely a desk job; for several consecutive summers, these nascent SIS cryptanalysts attended specialized training camps located at Fort Meade . This immersive experience was designed not just to hone their analytical skills but also to integrate them into the military structure, culminating in their receiving commissions as reserve officers in the Army. Kullback and Sinkov, demonstrating an impressive dedication that bordered on the obsessive, took Friedman’s emphasis on continuous education to heart. They spent the next several years diligently attending night classes, a testament to their intellectual hunger, or perhaps just a lack of compelling evening entertainment. Both ultimately earned their doctorates in mathematics during this period. Interestingly, after achieving this pinnacle of academic credentialing, Kullback experienced a curious resurgence of his earlier, discarded passion: teaching. From 1939 onwards, he began offering evening classes in mathematics at George Washington University , perhaps finding that teaching eager minds was more palatable than wrangling reluctant high schoolers.

Once their initial training regimens were complete and their academic credentials suitably polished, the trio—Kullback, Sinkov, and Rowlett—were finally assigned to the practical work for which they had been recruited. This involved the meticulous compilation of various cipher and code materials for the U.S. Army, a task that required an exacting attention to detail, much like cataloging the infinite ways humanity can complicate communication. Another, perhaps less glamorous, responsibility was the rigorous testing of commercial cipher devices that vendors, ever eager, wished to peddle to the U.S. government. One can only imagine the parade of well-intentioned but ultimately flawed contraptions they must have encountered.

A particularly notable achievement in Kullback’s early career involved his partnership with Frank Rowlett . Together, they embarked on the challenging task of decrypting messages generated by the formidable RED cipher machine . In a feat that sounds almost too dramatic to be true, they reportedly unraveled the machine’s intricate keying system and subsequently its underlying pattern with astonishing speed—“almost overnight,” as the legend goes—armed with nothing more than the intercepted messages themselves. This wasn’t mere brute force; it was a demonstration of profound cryptanalytic insight. Their breakthrough was further augmented by the talents of linguist John Hurt, who was instrumental in translating the decrypted texts. This collaborative effort allowed SIS to begin issuing timely and critical intelligence to military decision-makers, a direct contribution to national security that elevated them far beyond “junior mathematicians.”

In May 1942, a mere five months after the devastating attack on Pearl Harbor irrevocably plunged the United States into World War II , Kullback, who by this point had attained the rank of Major, was dispatched to Britain . This deployment was not merely a diplomatic exchange; it was a crucial intelligence-sharing mission. At the legendary Bletchley Park , the nerve center of British codebreaking efforts, he gained invaluable insight into the methods employed by the British to produce intelligence of “high quality” through their exploitation of the German Enigma machine . This collaboration was not limited to Enigma; Kullback also worked closely with his British counterparts on the solution of more conventional German codebook-based systems, demonstrating a versatility essential in the multifaceted world of wartime cryptology. Shortly after his return to the States, Kullback’s expertise was recognized with his appointment as the chief of the Japanese section, a critical role given the ongoing conflict in the Pacific theater.

When the National Security Agency (NSA) was officially formed in 1952, consolidating various intelligence efforts, Rowlett assumed the position of chief of cryptanalysis, a logical progression given his earlier successes. The post-war landscape presented a new set of challenges, primarily centered around the development of high-speed processing equipment. This era marked a significant shift from manual decryption to automated systems. Kullback, ever at the forefront of innovation, supervised a team of approximately 60 individuals. This group included visionary thinkers in the burgeoning field of automated data processing, such as Leo Rosen and Sam Snyder. Under Kullback’s guidance, his staff pioneered novel forms of input and memory technologies, including the introduction of magnetic tape and drum memory , which were revolutionary at the time. Furthermore, they developed sophisticated compilers designed to transform these nascent machines into truly “multi-purpose” tools, capable of handling a diverse array of computational tasks. Kullback, with his characteristic foresight, prioritized the application of these powerful new computers to the generation of communications security (COMSEC) materials, recognizing that securing one’s own communications was just as vital as breaking those of an adversary.

Kullback’s influential book, Information Theory and Statistics, a rather dry but undeniably significant tome, was published by John Wiley & Sons in 1959. The enduring relevance of his work was underscored when the book was republished, with necessary additions and corrections, by Dover Publications in 1968. A testament to its utility, or perhaps just to the slow pace of academic evolution.

Retirement

Solomon Kullback finally retired from the NSA in 1962, presumably having had enough of the endless dance between encryption and decryption. Following his departure from the clandestine world, he redirected his considerable intellect and energy towards his teaching responsibilities at George Washington University , coming full circle to the academic pursuits he had initially envisioned. He also dedicated himself to publishing new academic papers, ensuring his theoretical contributions continued to evolve. In 1963, his significant impact on the field of statistics was formally acknowledged when he was elected as a Fellow of the American Statistical Association , a notable recognition within the scientific community. He ultimately attained the rank of colonel in the Army Reserve, a fitting military culmination to a career deeply intertwined with national defense. His lasting legacy was further solidified by his induction into the Military Intelligence Hall of Fame , an honor reserved for those whose contributions to military intelligence are deemed exceptional.

Kullback is remembered by his former colleagues at the NSA with a certain fondness, a rare commodity in such environments. He was often described as exceptionally straightforward; one colleague, perhaps still reeling from the experience, characterized him as “totally guileless, you always knew where you stood with him.” A quality that, while admirable, must have been utterly exhausting for those accustomed to the usual bureaucratic obfuscation. Another former senior NSA official recalled him as a man possessed of “unlimited energy and enthusiasm,” a trait I can only imagine was both inspiring and utterly baffling to those around him. This same individual lauded Kullback as a man whose judgment was almost invariably “sound and right,” a level of consistent accuracy that frankly sounds suspicious. But then, I wasn’t there.