[Liskni_si]

I really don't think this is true for FIDO2 like Yubikey. My understanding is that your ssh client gets a challenge from the server, reads the key "handle" from the private key file, and sends both to Yubikey. The device then combines its master key with the handle to get the actual private key, signs the challenge, and gives the result back to your ssh client. At no point does the private key leave the Yubikey.

What am I missing?

[wang_li]

I don't know if you are missing anything. That's why I'm asking and making statements about how I understand the various processes to work. I want to understand how it is that the only device that interacts with the yubikey/tpm, when compromised, can't be subverted to the attackers ends.

Thank you for your reply.

[Liskni_si]

Perhaps one extra bit to add: you've mentioned consuming slots on the device - that's what happens if you generate a resident key. Those keys live on the device and can be used from any computer you plug them into, without having to retain/copy any files. A non-resident key, on the other hand, is derived from the master key on the device, and a "handle" that's stored as a file on your computer. You can have as many as you want, but if you lose either the file or the hardware device, they're gone.

(Others in the thread have confirmed that both resident and non-resident keys never leave the hardware. If you generate one that requires touch, they're fairly secure - you need physical presence and confirmation for every operation.)