CertFP, short for Certificate Finger Print, is another method to log you in to NickServ. Instead of a password, it uses a Client Certificate.
In order to use it, you
first need a certificate,
then configure Irssi to use this certificate,
and finally register the certificate with NickServ.
Irssi does not have built-in certificate management commands, so you need to use an external tool like
openssl to create the certificate.
A step-by-step guide for the Libera Chat network can be found here => https://github.com/shabble/irssi-docs/wiki/liberachat_certfp