The first letter shifts by nothing, the second by one, the third by two, and so on. It is a Vigenere cipher whose key word is simply the alphabet in order, and it was written down in 1508, long before anybody had a name for it.
It fixes the great weakness of the shift cipher — the same letter no longer looks the same twice — without needing a key at all. Which is also its weakness: no key means no secret.
A book that got its author accused of magic
Johannes Trithemius was an abbot who wrote about ciphers in a book so strange that people assumed it was witchcraft: it was disguised as a work on summoning spirits, with the real content hidden inside. The shifting alphabet in it was a genuine step forward, and it took three hundred years for scholars to agree the book was cryptography rather than sorcery.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.