The Book That Should Never Have Been Published: “The American Black Chamber” by Herbert O. Yardley

A history piece. Read for 1931, not for now — see the note below.

In 1931 a former United States government cryptologist published a book explaining, in detail, that America had been secretly reading the diplomatic cable traffic of its allies and adversaries for a decade.

He named the countries. He described the methods. He explained how the Japanese diplomatic codes had been broken in 1919, and how reading Tokyo's traffic throughout the Washington Naval Conference of 1921–22 let the American delegation hold firm at exactly the right moment.

It was a bestseller. It was also, by any reasonable measure, one of the most damaging disclosures in the history of American intelligence — and it changed the law.

What the book actually covers

Yardley ran what he called the American Black Chamber — a codebreaking unit funded jointly by the State Department and the Army, operating quietly out of New York after the First World War, reading the diplomatic traffic of a long list of governments.

The book is his account of it: how the unit was created, how the codes were attacked, what was learned, and what it was worth. The Washington Naval Conference material is the centerpiece and remains genuinely instructive — a negotiation in which one party knew the other's fallback position in advance and behaved accordingly. Anyone who has ever wondered what perfect information is actually worth in a negotiation has a case study here.

Then in 1929 the Secretary of State, Henry Stimson, shut the operation down. The line attributed to him — that gentlemen do not read each other's mail — has outlived everything else about the episode.

Yardley, out of work in the Depression, wrote it all down.

What happened next

The consequences were substantial and rapid.

Japan, which already updated its diplomatic codes every couple of years, did not change them in direct response to the book; David Kahn's documentary research found no sign of the code revisions often attributed to it. American cryptologists had already lost the Black Chamber's output when it closed in 1929, and spent years rebuilding a capability that had taken a decade to develop.

Congress responded with the Act of June 10, 1933 — prompted by this book, and passed in haste when Yardley's follow-up manuscript, Japanese Diplomatic Secrets, was seized from his publisher — making it a criminal offense to willfully disclose, without authorization, specified diplomatic codes or intercepted correspondence obtained through US government employment. It is a foundational piece of American secrecy law.

Yardley's own reputation never recovered inside the profession. David Kahn's biography records that William Friedman, who became the dominant figure in American cryptology, regarded the disclosure as a betrayal, and that Yardley spent the rest of his career at the edges of the field.

That is the arc worth taking from it: a capable insider, dismissed and financially exposed, converted institutional knowledge into a commercial product, and the damage was measured in years.

Why it matters for your risk posture

Read it as the founding insider-disclosure case. The elements are all there and all recognizable: genuine expertise, a termination handled without regard to what the person carried in their head, financial pressure, no enforceable constraint, and a receptive market for the material. Every modern version of this problem is a variation on the same structure.

Read it for the negotiation material, which is the transferable part. The Washington Conference chapters are a rare documented account of what asymmetric information is actually worth at a negotiating table — and, read from the other side, an argument for assuming your own position may be less private than you believe.

Finally, read it as a period document, which is what it is. The tradecraft is a century obsolete. The institutional lesson is not.

Yardley kept some of the Cipher Bureau's records when it closed, and the Army's later demand for them seems never to have been met, which places the failure in the handling of his dismissal. In most organizations that decision rests with line managers and human resources rather than with security. When we run insider threat workshops for managers, the departure of someone who holds sensitive knowledge is one of the decisions worth rehearsing. That includes which obligations are confirmed in writing and how the person’s circumstances after leaving are taken into account.

Key takeaways

• The knowledge leaves with the person. Document control protects documents. Yardley kept some records, but what he carried in his head was the larger asset.
• How a termination is handled is a security decision. A capable person dismissed carelessly and left financially exposed is a risk profile, not a personnel footnote.
• Asymmetric information is worth more than most negotiators assume. The Naval Conference chapters quantify it about as clearly as any account ever has.
• Read the date on the cover. The methods here are a century out of date. Anyone treating this as current tradecraft is badly mistaken.

About the author

Herbert Osborn Yardley was born in Indiana in 1889 and began his career as a telegrapher, joining the State Department code room and there discovering how weak American cryptographic security was. During the First World War he headed the Army's cryptologic section, and afterwards led the joint State-Army codebreaking unit he later called the American Black Chamber.

The unit was closed in 1929. The American Black Chamber was published in 1931 and became an international bestseller, translated widely — including, inevitably, into Japanese.

He later worked as a cryptologic adviser abroad, including in China and Canada, and wrote further books, among them a well-regarded volume on poker. He died in 1958 and is buried at Arlington.