The Chaocipher is a mechanical encryption algorithm invented in 1918. No one was able to reverse-engineer the algorithm, given sets of plaintexts and ciphertexts—at least, nobody publicly. On the other hand, I don’t know how many people tried, or even knew about the algorithm. I’d never heard of it before now. Anyway, for the first time, the algorithm has been revealed. Of course, it’s not able to stand up to computer cryptanalysis.