Several sources and links for more info. Check my profile for more, I don't want to spam the same links all over this topic, but I'd start here:

https://community.signalusers.org/t/proper-secure-value-secu...

https://web.archive.org/web/20210126201848mp_/https://palant...

https://www.vice.com/en/article/pkyzek/signal-new-pin-featur...