A History of Secret Codes: Timeline

This is a timeline about a narrower subject than it might sound: not signaling systems like Morse code or the phonetic alphabet, which exist to get a message through reliably, but ciphers — systems built specifically to keep a message unreadable to anyone but its intended recipient. The two threads run in parallel through history, but the driving problem is different, and so is the story.

Roughly 100 BCE: Caesar's Shift Cipher

According to the Roman historian Suetonius, Julius Caesar used a simple substitution cipher for sensitive military correspondence, shifting each letter of the alphabet a fixed number of positions. If Suetonius's account is accurate, it's roughly two thousand years old — a genuinely remarkable run for a technique still taught to schoolchildren today, and the fuller story of its origins is worth reading for the parts of that account historians still debate.

9th Century: The First Documented Codebreaking Method

The Arab polymath Al-Kindi wrote what's considered the earliest known description of frequency analysis — the technique of counting how often each letter appears in a piece of ciphertext and comparing that pattern against the known letter frequencies of ordinary language to crack a substitution cipher. It's a genuinely foundational moment in the history of cryptanalysis: for the first time, breaking a cipher was described as a systematic, repeatable method rather than a matter of luck or brute trial and error.

1553–1586: The Cipher Named After the Wrong Person

The polyalphabetic cipher popularly known today as the Vigenère cipher — which uses a repeating keyword to shift each letter by a different amount, defeating simple frequency analysis — is commonly credited to the French diplomat Blaise de Vigenère, who published a description of it in 1586. But the technique had already been described several decades earlier, in 1553, by the Italian cryptographer Giovan Battista Bellaso. The misattribution has stuck for centuries anyway, which is a small, telling example of how a catchy name can outlast historical accuracy.

17th Century: The Cipher That Took Two Hundred Years to Break

The Great Cipher, created for King Louis XIV of France by father-and-son cryptographers Antoine and Bonaventure Rossignol, was so effective that after the Rossignol family's knowledge of it was lost, French archives full of encrypted royal correspondence sat unreadable for roughly two centuries. It wasn't broken until the 1890s, when military cryptanalyst Étienne Bazeries worked out its structure — the cipher didn't substitute individual letters but whole syllables, an approach unusual enough for its era that it resisted every attempt at analysis for generations after the people who built it were gone.

1917: A Decrypted Telegram That Helped Change the Course of a War

British codebreakers in the intelligence unit known as Room 40 intercepted and decrypted the Zimmermann Telegram, a coded German diplomatic message proposing a military alliance with Mexico against the United States. Its publication was a significant factor in shifting American public opinion toward entering the First World War — a rare, concrete example of a single act of cryptanalysis genuinely reshaping a major historical outcome, rather than just settling a battlefield skirmish.

1918: A Field Cipher Broken After 26 Sleepless Hours

Late in the First World War, the German army introduced a field cipher known as ADFGX, later strengthened into ADFGVX, to protect frontline radio traffic. French military cryptanalyst Georges Painvin broke the original ADFGX cipher on April 5, 1918, and when the Germans complicated it further that May, he worked for roughly 26 straight hours to reconstruct the new system on June 2, decrypting a message that revealed German resupply movements ahead of a planned offensive near Compiègne. The intercepted warning reached French command in time to help blunt the attack — a direct, documented case of one cryptanalyst's exhausting manual work changing the outcome of a specific battle, not just providing background intelligence.

1939–1945: Enigma, and a Code That Was Never Broken by the Enemy

The Second World War produced two of cryptography's most famous stories at once. Allied codebreakers, working from Polish foundational work before the war, eventually broke the German military's Enigma machine cipher — covered in full separately — while in the Pacific theater, the United States relied on Navajo Code Talkers, U.S. Marines who transmitted messages using a code built on the Navajo language. Japanese cryptanalysts never broke it, and the program remained classified for decades after the war ended before its role was publicly recognized.

1943–1980: A Cold War Project Kept Secret for Decades

The Venona project, a joint American and British effort, decrypted portions of Soviet intelligence communications intercepted over decades, eventually exposing multiple real espionage networks operating inside the United States. The project itself remained classified until 1995, meaning the fact that any of this decryption work had happened at all was a closely guarded secret for roughly half a century — an unusually long gap even by the standards of intelligence history.

1976–1977: The Shift From Secret Keys to Public Ones

Every cipher up to this point in the timeline, no matter how sophisticated, shared one structural requirement: both parties needed to already possess the same secret key before they could communicate securely. That changed with Whitfield Diffie and Martin Hellman's 1976 paper describing a method for two parties to establish a shared secret over an insecure channel without ever transmitting the secret itself, followed in 1977 by Ron Rivest, Adi Shamir and Leonard Adleman's RSA algorithm, the first practical public-key cryptosystem. Together, these two developments are the direct ancestors of the encryption protecting essentially every secure website and messaging app in use today.

What Changed, and What Didn't

The tools got vastly more sophisticated across this two-thousand-year span, but the underlying contest didn't fundamentally change: someone builds a system meant to keep a message unreadable, and someone else works out how to read it anyway. Frequency analysis cracked simple substitution within a few centuries of Caesar; the Great Cipher held for two hundred years before Bazeries worked it out; RSA-scale public-key cryptography, so far, has held for nearly half a century against everything short of a hypothetical quantum computer. The pattern across every entry above is the same one running through the rest of this site's history pages: no code stays unbroken forever, and the interesting history is almost always in exactly how, and how long it took.