Hello Emiliano

Did you get LTI 1.3 working in Totara? I was also stuck with Totara only accepting public key and LTI 1.3 platforms only proving keyset URL. I ended up using a key converter: I got the key by accessing the keyset URL in a browser and converting it into the format Totara recognised https://8gwifi.org/jwkconvertfunctions.jsp

It's unfortunate that Totara doesn't support keyset URL method, while most LTI vendors use it.