So I've been trying to create more secured passwords now that I have employment where I have responsibility. They require us to change our passwords every 3 months. I used to use the same passwords for multiple sites. Then I used a password manager and got rid of those memory passwords. With this job I don't want to mix my personal password manager with my work computer and I also don't want to remember a complicated 15 character long password to log in every day.
That brings me to my question. I've been using Yubikeys for years. I store a challenge response, use it for 2FA on all sites that allow, and I use it for TOTP on most sites (there's a limit to how many entries in the Yubikey 5). You can also store a password in one of it's two slots. My thinking is this: Is it secure to store a base password that is long and complicated, say 40 characters long with all the characters, and use a different "prefix" for each application? Example: On my banking site I type in "bank" then press the Yubikey to type the rest. Same thing with social media and other accounts. Each one has a prefix and I don't know the actual password. Of course I store all passwords, including the Yubikey, in a password manager that's backed up in the cloud (I use KeePassXC).
Your thoughts? Is this secure or stupid?
This. And "figuring out one password" can mean stealing it from some 3rd party server with bad security practices. The password complexity and the other OP's practices aren't relevant.
You also can't change passwords easily, what again is a problem on those sites that have bad security practices.
It's possible to make an algorithmic derived password that doesn't have that first flaw (losing one doesn't lead to losing all), but the second one will always be around.