Re:OpenSSH's Support for U2F/Fido Security
by tialaramex on Tuesday November 05, 2019 @01:26PM
(#21455146)
That is exactly how it works for WebAuthn, yes. The server sends over this cookie and then the client uses it.
But SSH is defined with the _Client_ picking how to authenticate first, so they can't do this.
There isn't a step in the SSH Authentication protocol where the server can say "Oh, here's the FIDO cookie you need". The SSH design says that (for public keys) the client starts by proposing "Hi, I can prove I know the private key that goes with this public key K1" and then the server either says "OK prove it" or "No, what else?". You can see this corresponds to the authorized_keys file on the server.
So instead you'll see the document calls this value "key_handle" and you'll see it is in the "private key" stored on the client, not with the "public key" stored on servers.
But SSH is defined with the _Client_ picking how to authenticate first, so they can't do this.
There isn't a step in the SSH Authentication protocol where the server can say "Oh, here's the FIDO cookie you need". The SSH design says that (for public keys) the client starts by proposing "Hi, I can prove I know the private key that goes with this public key K1" and then the server either says "OK prove it" or "No, what else?". You can see this corresponds to the authorized_keys file on the server.
So instead you'll see the document calls this value "key_handle" and you'll see it is in the "private key" stored on the client, not with the "public key" stored on servers.