A cipher earns its place in history either by protecting something that mattered or by failing to. Both kinds are here, along with the small set of messages that nobody has yet read.
The ciphers§
- The Great Cipher France, 1660s-1890s solved
The Rossignol family's code of 587 numbers, many standing for syllables and some for nothing at all. It protected Louis XIV's secrets and then stayed unread for two hundred years, until Bazeries broke it in the 1890s. - The Zimmermann Telegram 1917 solved
German code 0075, decrypted by Room 40. Britain's problem was not reading it but explaining how it came by the text without admitting it read neutral cable traffic. - ADFGVX Germany, 1918 solved
A Polybius square over six letters chosen to be unmistakable in Morse, followed by a columnar transposition. Painvin broke it in the field. - Enigma Germany, 1920s-45 solved
Rotors, a reflector and a plugboard. Its fatal property was that no letter could ever encrypt to itself, which turned a guessed crib into a mechanical test — the principle the bombe ran on. - Lorenz SZ40/42 (Tunny) Germany, 1940-45 solved
Hitler's strategic teleprinter cipher, twelve wheels, far stronger than Enigma. Broken by Tutte's analysis and Colossus's speed. - Purple Japan, 1939-45 solved
A stepping-switch machine for diplomatic traffic, reconstructed in Washington without a physical example ever being seen. - Navajo code talkers US, 1942-45 unbroken
Not a cipher at all: a natural language with no written form, spoken by people the enemy had no speakers of, with a coined vocabulary for military terms. Never broken. - The one-time pad Vernam, 1917; Shannon, 1949 proven
The only cipher with a proof of perfect secrecy — provided the key is truly random, as long as the message, and used exactly once. Venona is what happens when the last condition slips. - The Voynich Manuscript c. 1404-1438 unsolved
Two hundred and forty pages in an unknown script, carbon-dated to the early fifteenth century. Statistically it behaves somewhat like language and somewhat not. Hoax, unknown language and unknown cipher all remain live possibilities. - The Beale Ciphers published 1885 disputed
Three number ciphers said to locate buried treasure in Virginia. The second decodes against the Declaration of Independence; the first and third do not decode against anything. Many researchers think the whole thing is a nineteenth-century fiction. - The Dorabella Cipher Elgar, 1897 unsolved
Eighty-seven squiggles in three orientations, sent by the composer to a young friend. Short enough that almost any proposed solution fits, which is precisely the problem. - Zodiac Z408 and Z340 1969-70 solved
Z408 was a homophonic substitution broken in a week by two schoolteachers, Donald and Bettye Harden. Z340 held out until 2020, when it turned out to be a transposition wrapped around a homophonic substitution. - Kryptos Jim Sanborn, CIA headquarters, 1990 partly unsolved
Four panels. K1 and K2 are Quagmire III over a keyed alphabet, K3 is a transposition, and K4 — ninety-seven characters — has resisted public solution for over three decades despite published clues (BERLIN, CLOCK, NORTHEAST, EAST). Treat any claimed break as needing verification.
Why unsolved ciphers stay unsolved§
Rarely because the cipher is strong. Usually the message is too short: with eighty-seven symbols, as in the Dorabella cipher, many different keys produce something that reads plausibly, and nothing distinguishes the right one. That is a problem of evidence rather than of computing power, and it does not go away with a faster machine — which is why this solver caps its own confidence when a key is large relative to the text it was recovered from.
The other common reason is a wrong assumption held collectively for years. Z340 resisted for half a century partly because analysts assumed the transposition ran one way when it ran another.
More: the timeline of codebreaking and the people who did it. To try the classical ciphers yourself, start at the cipher wiki.
Frequently asked questions§
What is the most famous unsolved cipher?
The Voynich Manuscript is the best known. Kryptos K4, the Beale ciphers and Elgar's Dorabella cipher are the other long-standing open cases.
Has the Zodiac cipher been solved?
Z408 was solved within a week in 1969 by Donald and Bettye Harden. Z340 held out until December 2020, when David Oranchak, Sam Blake and Jarl Van Eycke showed it was a transposition over a homophonic substitution.
What is Kryptos?
A sculpture by Jim Sanborn at CIA headquarters carrying four encrypted panels. Three are solved; the fourth, ninety-seven characters long, has resisted public solution since 1990 despite several clues released by the artist.
Was any cipher never broken?
The Navajo code talkers' system was never broken during the war — though it was a language plus a code vocabulary rather than a cipher. A correctly used one-time pad is provably unbreakable.