Polyalphabetic cipher

[1] However, it has been claimed that polyalphabetic ciphers may have been developed by the Arab cryptologist Al Kindi (801–873) centuries earlier.

For this encipherment Alberti used a decoder device, his cipher disk, which implemented a polyalphabetic substitution with mixed alphabets.

Even Alberti's implementation of his polyalphabetic cipher was rather easy to break (the capitalized letter is a major clue to the cryptanalyst).

The principle (particularly Alberti's unlimited additional substitution alphabets) was a major advance—the most significant in the several hundred years since frequency analysis had been developed.

It was not until the mid-19th century (in Babbage's secret work during the Crimean War and Friedrich Kasiski's generally equivalent public disclosure some years later) that cryptanalysis of well-implemented polyalphabetic ciphers got anywhere at all.