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by fhenneke 2237 days ago
The new OpenSSH keys are very convenient if used correctly, but have a crucial disadvantage compared to the PIV-based approach of yubikey-agent: They currently can't be protected effectively with a PIN, so you should take good care of your security key or require multiple authentication methods on your server.
1 comments
StavrosK 2237 days ago
Why can't they? Using a PIN works fine with my key.
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fhenneke 2237 days ago
You can set a FIDO2 PIN on your security key and it will prevent ssh-keygen/ssh-add from regenerating the key files without it. But the relevant information (the key handle) can also be retrieved from the key in other ways that don't require the PIN. This is likely going to be fixed in a future version of OpenSSH, but may require a more recent kind of FIDO2 key. Until then, you should consider resident OpenSSH keys to provide only single-factor authentication ("possession"), even if a PIN is set on the security key.
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StavrosK 2237 days ago
Hmm, does the token provide signing without the PIN? That seems like a very big oversight, but mine doesn't sign in without the PIN.

What can someone with the handle do? They can't log in without the USB token, right?

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fhenneke 2237 days ago
The tokens are engineered to protect the private key material stored inside them very well, so you can be quite certain that nobody will ever be able to log in without physical access to the key (to touch/press the button).

However, the SSH protocol differs quite substantially from the FIDO2/WebAuthn spec in how it uses the PIN set on the token. Depending on how the SSH server is configured and which defaults your security token's manufacturer chose, it may be the case that the PIN is not needed to log in (assuming physical access to the token).

I hope that all of this will be clarified in the OpenSSH documentation at some point as it is quite vague about security guarantees at the moment. It's probably best to use the non-resident version of the new key type together with a passphrase on the key file for now, or rely on the PIV applet instead.

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StavrosK 2237 days ago
Oh huh, that's unfortunate. I was under the impression that the Yubikey would not sign anything without a PIN, period, and that it wipes its contents after three (ten?) wrong attempts. This greatly reduces the security of the token, and it's a shame since we were so close to perfect...
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