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by jlgaddis
2178 days ago
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> ... so a remote SSHD can (if it wanted) demand to see signed evidence of user presence or refuse login, ... I'd like to be able to, server-side, 1) require that the private key used for authentication be stored on a hardware device and 2) require user presence, but I've never read or heard that this is possible (granted, I haven't looked into too much). If you have links to any documents that discuss how to implement this, I'd appreciate you sharing them. The only relevant documentation I can find is regarding FIDO/U2F support and even the release notes for 8.2 mention that "OpenSSH does not currently make use of this [attestation certificate]". |
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> 1) require that the private key used for authentication be stored on a hardware device
So for this you're going to need the attestation data, which as you observe OpenSSH currently doesn't do anything with. It is willing to stash it in a file, and because it's a certificate it's safe for the token's owner to send this somewhere. So you could construct a mechanism to examine the attestation file and decide whether to accept the proffered public key based on that attestation. e.g. "This is a genuine Yubikey 5C, so OK" or "This is a Crap Corp Funky Fake, no thanks".
I expect this would be a bunch of work, and I don't expect Free Software people to help build something to do it any time soon. But from what I can see it's possible with the components that already exist. Maybe somebody who really wants this will do all the work and put it on GitHub.
> 2) require user presence
The FIDO "options" bitflags have a flag for "user presence" (sometimes labelled UP). If this isn't set, user presence was not checked by the authenticator.
The FIDO authenticator includes this options parameter as part of the message it's signing.
So this means if a SSH client/ agent sends you an options field with UP set, but actually user presence wasn't checked it won't be able to provide a signature that matches. The SSH server doesn't need to do anything special except look at the UP bitflag if it cares (all WebAuthn servers are required to check this, it's on the long list of steps to perform WebAuthn authentication properly).
To be fair this isn't OpenSSH (though I'm sure they have at least somebody paying attention) it's FIDO itself that designed the signatures to also sign the user presence indication, they couldn't have avoided it.