Surveillance

Before the NSA Honored Historian David Kahn, the FBI Investigated Him

Newly declassified files reveal that J. Edgar Hoover wanted to prosecute the NSA’s future scholar in residence for collecting cryptography manuals.

Matthew Petti | 7.10.2026 11:18 AM

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(Illustration: Badboo/Dreamstime/By Bobrowen/Wikimedia Commons)

The U.S. government came to like the work of historian David Kahn, who passed away in 2024. The National Security Agency (NSA) honored him three times for his research on cryptography and espionage: with a "scholar-in-residence" title in 1993, a ceremony making his personal library part of the National Cryptological Museum in 2010, and a hall of fame induction in 2020.

But decades before, the U.S. government considered that personal library an alarming security threat. The FBI has declassified its files on Kahn, which Reason obtained under the Freedom of Information Act. Although it is publicly known that the NSA tried to stop Kahn from publishing his 1967 magnum opus, The Codebreakers, the new files show that Kahn had caught the feds' attention a decade before, with the case making its way up to FBI Director J. Edgar Hoover himself.

Kahn's FBI file is one of those rare government documents with a personality. His curiosity, equal parts cheeky and naive, shines through the FBI agents' dry recap of events. The file is also a time capsule from the early Cold War, an age when cryptography—now the infrastructure that makes the modern internet possible—was still an obscure dark art for math nerds.

In 1953, the U.S. Army Counterintelligence Corps caught wind that Kahn, then a 23-year-old enlisted reservist and part-time ice cream truck driver, was mailing "restricted Army documents" to another man named Frederic C. Flindt. The corps alerted the FBI, which set up a stakeout on August 6 at the post office in Flindt's hometown of Woodbury, New Jersey. 

As expected, Flindt picked up a certified mail package from Kahn with a collection of government cryptography manuals inside. The FBI agents pounced, but Flindt was totally cooperative, even showing the agents his other letters with Kahn.

Alongside his Army reservist training, Kahn was taking correspondence courses on how to use the Army's cipher machines and forwarding the materials for Flindt to copy and return. He wrote to........

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