The Secret ChannelsThe human history of keeping — and breaking — secrets

The historian Herodotus, writing around 440 BC, preserved two of the oldest secret-message tricks we know of. One was Histiaeus's tattooed courier, sent to spark the Ionian Revolt against Persia. The other was the exiled Spartan king Demaratus, who warned Greece of Xerxes' coming invasion by scraping the wax off a writing tablet, scratching his message into the bare wood, and re-coating it — a blank tablet that anyone could inspect and no one would suspect. This is steganography, from the Greek for "covered writing": you don't scramble the message, you hide that a message exists at all.

Steganography has one fatal weakness — discovery is total defeat. Find the hidden ink, the buried wood, the scalp beneath the hair, and the secret is simply gone. So a second, deeper strategy grew up beside it: cryptography, scrambling the meaning so that even a message found in plain view stays unreadable. The Spartans wound a strip of leather around a rod of fixed diameter — a scytale — so the letters only lined up on a matching rod. Centuries later Julius Caesar simply shifted every letter three places down the alphabet. Crude as they were, these were the first moves in a contest that has never stopped: the maker of secrets against the breaker of them.

Around 850 CE, the polymath al-Kindi wrote On Deciphering Cryptographic Messages and changed the game forever. His insight was statistical: in any language some letters are far more common than others, so the most frequent symbol in a scrambled message probably stands for the most frequent letter, and so on down the list. Frequency analysis turned the unbreakable substitution cipher into a puzzle a patient clerk could solve — and made codebreaking a science rather than a lucky guess. From then on, every cipher had to assume a hostile, intelligent reader.

The stakes were rarely abstract. In 1586 the imprisoned Mary, Queen of Scots, plotted Elizabeth I's overthrow in enciphered letters smuggled in a beer barrel — not knowing that Elizabeth's spymaster, Francis Walsingham, had already broken the cipher. His code-breaker Thomas Phelippes read every word, even forging a postscript to draw out the conspirators' names; the decrypts sent Mary to the executioner in 1587. A broken code had become a death warrant. By then Europe's powers ran permanent black chambers — government offices that quietly opened, copied, decoded, and resealed the diplomatic mail passing through their cities. The cipher and its breaker were locked into an arms race that has never let up since.

In January 1917, German foreign secretary Arthur Zimmermann wired a coded proposal to Mexico. Room 40's analysts decoded it — then faced the spy's eternal dilemma: how to use a secret without revealing that you can read the enemy's mail. They engineered cover for the leak, handed the telegram to Washington, and watched American opinion turn. Within weeks President Wilson asked Congress for war. The Zimmermann Telegram showed that in the industrial age, codebreaking was no longer a court curiosity — it was a weapon that could move nations.

The Second World War made that lesson total. At Bletchley Park, a few thousand codebreakers — Alan Turing among them — built electromechanical machines to break the German Enigma cipher, and the resulting intelligence, codenamed Ultra, is widely credited with shortening the war by a year or more (the exact figure is debated, but the impact is not). On the other side of the world, the US Marines used Navajo code talkers, whose unwritten language formed a battlefield code the Japanese never broke. Secrecy had become an industry — staffed by mathematicians, linguists, and machines, and decisive enough to bend the outcome of total war.

The Cold War turned secrecy into a permanent apparatus. Two streams fed it: signals intelligence — the mechanized descendant of Room 40, now run by sprawling agencies like America's NSA and Britain's GCHQ, sharing intercepts through the "Five Eyes" alliance — and human intelligence, the older world of agents, defectors, and moles. The two crossed constantly. The codebreaking project Venona, quietly decrypting Soviet cables, helped expose deep penetrations of the West, including the "Cambridge Five" ring of British insiders who spied for Moscow for decades.

Mitrokhin's archive opened a rare window onto the other side of that war, detailing KGB operations across the globe. It comes with an honest caveat worth stating plainly: what survives are Mitrokhin's handwritten copies, not original documents — the originals remain locked in Russia — so while the FBI and his collaborating historian regard the material as extraordinarily valuable, some scholars urge caution about details that can't be independently checked. That tension is the whole texture of the secret state: a world built on information no outsider can fully verify, where the line between intelligence and disinformation is exactly the thing each side is fighting to control.

For all of history, secret messaging had a chicken-and-egg flaw: to share a secret you first had to share a key — secretly. The companion's one-way functions dissolved that problem. With public-key cryptography — Diffie, Hellman and Merkle in 1976, then the RSA algorithm in 1977 (and, unknown to them, GCHQ's classified prior version) — you could publish a "lock" for all the world to use while keeping the only "key" to yourself. For the first time, two strangers could speak in perfect secrecy without ever having met. The secret channel, for five thousand years the property of kings and spies, was suddenly available to anyone with a computer.

Governments fought it. The United States classified strong encryption as a munition, illegal to export like a weapon. When programmer Phil Zimmermann released free encryption software called PGP in 1991 and it spread worldwide within days, he faced a three-year criminal investigation; supporters printed PGP's source code as a hardback book, daring the government to ban a book. The case collapsed in 1996, and the NSA's "Clipper chip" — a backdoor built into every phone — was beaten back too. These were the Crypto Wars, and strong encryption won. Today the same mathematics quietly guards the padlock on every web address and the card number behind every purchase — even as the Snowden disclosures of 2013 and the ongoing "going dark" debate prove the old fight never really ends. The tools have changed beyond recognition; the human question is exactly the one Histiaeus faced — who gets to keep a secret, and who gets to read it.