Can someone explain how TFA (or any security feature that relies on my phone) works when the phone is unresponsive -- dead battery, no cell or internet reception, hardware failure.
I am trying to develop a security process that I can rely on. It only has to be better than what I have now, it doesn't have to be bulletproof.
I store my printed backup codes for most of my services in an encrypted file in my Dropbox (encrypted with a different password than the password used for Dropbox).
I then also have printed backup codes for my primary email account and for my Dropbox account that I carry with me on an unmarked piece of paper stashed deep in a semi-hidden pocket in one of my bags. I also have printed backup codes for my email and Dropbox stashed in a semi-hidden place in my home, with the thought that in a last case scenario (or I lose my bags or something like that), I can phone my roommate and have him read me the code.
It isn't perfect and I feel like it could be improved, but so far it works fine.
It's not magic, there isn't any other way.
What happens when your first and only Yubikey gets dropped in a puddle? You're also back to paper and backup codes.
Of course it won't work if your battery is dead. :)