Chaocipher
Two alphabet wheels that reshuffle themselves after every letter — a cipher whose workings stayed secret for 92 years.
John F. Byrne invented the system in 1918 and spent decades trying to sell it to the US Navy and the State Department, without success. Two wheels each carry a scrambled alphabet; you find the plaintext letter on one, read the ciphertext letter off the other, then permute both alphabets in a fixed way. The result is a substitution cipher whose table changes with every character, related to Alberti's disk but without its predictable stepping.
Byrne printed challenge texts in his 1953 autobiography, yet the method itself remained unknown until 2010, when his family gave his papers to the National Cryptologic Museum. The algorithm could finally be described in public — and turned out to be simple enough to fit in a cigar box, which had been Byrne's selling point all along.