The first polyalphabetic cipher: two rotating discs switch alphabets every few letters.
polyalphabetic substitution cipher
easy to break by hand
15th century
Devised by Leon Battista Alberti around 1467 on his cipher disk, Formula.
encrypt · decrypt · cryptanalysis
The discs are the tool’s fixed ones, so the unknowns are the period and the disc step. Exact for ciphertexts whose plaintext letters all sit on the disc (like the practice challenges) — dropped letters shift the chunks otherwise.