Comment by jwr

That was my first thought, too — this might work as one of additional methods of logging in, but not as the only one.

I have been using YubiKeys for many years now, following this guide: https://github.com/drduh/YubiKey-Guide

This means I can have secure private keys for ssh and gpg (cue the crowd saying gpg is insecure without offering good modern alternatives), which I can also back up. And I can set up multiple YubiKeys, which I do.

An additional advantage of this solution is that you can set your YubiKey to require a touch confirmation before performing an operation. This makes it much harder for runaway AI agents to wreak havoc on your servers: there is a physical limitation, which I really like.