Porta cipher
A polyalphabetic cipher from 1563 in which encrypting and decrypting are exactly the same operation.
Giovan Battista della Porta, a Neapolitan polymath whose interests ran from optics to physiognomy, published it in De Furtivis Literarum Notis. It has thirteen alphabets, one per pair of key letters (AB, CD, EF …), and each one swaps the first half of the alphabet with the second.
Since every alphabet is its own inverse, the recipient simply runs the same procedure again – the property that makes ROT13 and Enigma convenient. The price is that a letter can never encrypt to anything in its own half of the alphabet, a regularity that helps the analyst. Beyond that, it breaks like Vigenère.