The Codebreakers comprehensively chronicles the history of cryptography from ancient Egypt to the time of its writing. It is widely regarded as the best account of the history of cryptography up to its publication. Most of the editing, German translating, and insider contributions were from genius and World War II American cryptographer, Bradford Hardie III. William Crowell, the former deputy director of the National Security Agency, was quoted in Newsday as saying "Before he (Kahn) came along, the best you could do was buy an explanatory book that usually was too technical and terribly dull."

Kahn, then a newspaper journalist, was contracted to write a book on cryptology in 1961. He began writing it part-time, at one point quitting his regular job to work on it full time. The book was to include information on the National Security Agency (NSA), and according to the author James Bamford writing in 1982, the agency attempted to stop its publication, and considered various options, including publishing a negative review of Kahn's work in the press to discredit him. A committee of the United States Intelligence Board concluded that the book was "a possibly valuable support to foreign COMSEC [communications security] authorities" and recommended "further low-key actions as possible, but short of legal action, to discourage Mr. Kahn or his prospective publishers". Kahn's publisher, the Macmillan company, handed over the manuscript to the federal government for review without Kahn's permission on March 4, 1966. Kahn and Macmillan eventually agreed to remove some material from the manuscript, particularly concerning the relationship between the NSA and its British counterpart, the GCHQ.

The Codebreakers did not cover most of the history concerning the breaking of the German Enigma machine (which became public knowledge only in the 1970s). Nor did it cover the advent of strong cryptography in the public domain, beginning with the invention of public key cryptography and the specification of the Data Encryption Standard in the mid-1970s. The book was republished in 1996, and the new edition included an additional chapter briefly covering the events since the original publication.

The Codebreakers was a finalist for the non-fiction Pulitzer Prize in 1968.

Kahn was born in New York City to Florence Abraham Kahn, a glass manufacturer, and Jesse Kahn, a lawyer.

Kahn has said he traces his interest in cryptography to reading Fletcher Pratt's Secret and Urgent as a boy. Kahn is a founding editor of the Cryptologia journal. In 1969 Kahn married Susanne Fiedler; they are now divorced. They have two sons, Oliver and Michael.

He attended Bucknell University. After graduation, he worked as a reporter at Newsday for several years. It was during this period that he wrote an article for the New York Times Magazine about two defectors from the National Security Agency. This article was the origin of his monumental book, The Codebreakers. Subsequently, Kahn was an editor at the International Herald Tribune in Paris for two years in the 1960s.

Kahn was awarded a doctorate (D.Phil) from Oxford University in 1974 in modern German history under the supervision of the then Regius professor of modern history, Hugh Trevor-Roper.

He worked as a reporter and an op-ed editor for Newsday until 1998, and a journalism professor at New York University. In 1995, Kahn was selected as the scholar-in-residence at the National Security Agency.

Kahn lives (as of 2005) in Great Neck, Long Island, a suburb of New York City. He has lived in Washington, D.C.; Paris, France; Freiburg, Germany; and Oxford, England.

Kahn has donated a collection of books, papers, and artifacts on codes and ciphers to the National Cryptologic Museum.