Based on what, a form of polyalphabetic substitution uses a series of interwoven Caesar ciphers for encrypting alphabetic text?
Polyalphabetic cipher A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though it is a simplified special case. The Enigma machine is more complex but still fundamentally a polyalphabetic substitution cipher.
Iota subscript The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota placed below the letter. It can occur with the vowel letters eta , omega , and alpha . It represents the former presence of an [i] offglide after the vowel, forming a so‐called "long diphthong". Such diphthongs (i.e., ηι, ωι, ᾱι )—phonologically distinct from the corresponding normal or "short" diphthongs (i.e., ει, οι, ᾰι  )—were a feature of ancient Greek in the classical era. The offglide was lost in pronunciation during the Hellenistic period with the result that from approximately the 1st century BC onwards the former diphthongs were no longer distinguished from the simple long vowels η, ω, ᾱ respectively. During the Roman and Byzantine eras, the iota, now mute, was sometimes still written as a normal letter but was often simply left out. The iota subscript was invented by Byzantine philologists in the 12th century AD as an editorial symbol marking the places where such spelling variation occurred.