The one-time pad

Gilbert Vernam of AT&T patents a teleprinter cipher combining each character of a message with a key character (US Patent 1,310,719). Joseph Mauborgne of the US Army adds the crucial insight: if the key is truly random, as long as the message, and never reused, the cipher is unbreakable. The one-time pad is born, and with it the logic of numbers stations.

In a wartime report conventionally dated to 1941 though no verified source confirms the year, Kotelnikov proves the perfect secrecy of the one-time pad, independently of Shannon. The proof reportedly remains classified. Two men on opposite sides of an emerging Iron Curtain reach the same conclusion.

American codebreakers begin a secret effort, later named VENONA, to read Soviet intelligence cables. Their success comes not from breaking the one-time pad itself but from exploiting the Soviets' reuse of key material, the one flaw that turns an unbreakable cipher into a readable one.

In 'Communication Theory of Secrecy Systems' Shannon proves that a correctly used one-time pad leaks no information whatsoever, the mathematical bedrock of the numbers-station method. First written as a classified 1945 report, it is now public.

Beyond secrecy and information, Shannon builds a maze-solving mechanical mouse, a chess machine and a juggling robot. The man who proved the one-time pad unbreakable is, at heart, a tinkerer delighted by machines that think.

The Soviet illegal known as Rudolf Abel is arrested in New York, his shortwave radio and one-time pads among the evidence. He is later exchanged for U-2 pilot Gary Powers, the classic Cold War spy swap the 2010 exchange consciously echoed.

The United States releases the VENONA decrypts, revealing how it read Soviet cables by exploiting reused one-time-pad keys. The public learns, decades late, both the power of the numbers method and the single mistake that undid it.

A Miami jury convicts five Cuban agents, three of them intelligence officers, including spymaster Gerardo Hernandez. The trial enters into the public record how a numbers station works in practice; the sequence is documented in detail for the parallel Montes case: a voice reads groups, an agent types them into a laptop, and one-time-pad software turns them into orders.

After some twenty years in prison Montes is released, an unrepentant symbol of how a Cold War method, one-way voice links and one-time pads, survived intact into the twenty-first century.