openssh unix dev - Jan 2020 - u2f seed

On Fri, 3 Jan 2020, Stuart Henderson wrote:
> As said in James Bottomley's message and djm's reply, doing similar
in
> ssh is not possible without significantly changing the protocol:
>
>
https://lists.mindrot.org/pipermail/openssh-unix-dev/2020-January/038092.html
so how does Google change the protocol to support u2f?

not supporting authentication from multiple machines seems to defeat the purpose
of adding u2f support.

David Lang

David Lang:
> not supporting authentication from multiple machines seems to defeat the
> purpose of adding u2f support.
It works just like other SSH key types.  You have a private SSH key
and a public one, and you can copy the private key to multiple
machines or load it into ssh-agent and use agent forwarding.

The only difference is that the private SSH key on its own is
insufficient and requires the cooperation of the FIDO/U2F authenticator.

-- 
Christian "naddy" Weisgerber                          naddy at
mips.inka.de

On Fri, 3 Jan 2020, Christian Weisgerber wrote:
> David Lang:
>
>> not supporting authentication from multiple machines seems to defeat
the
>> purpose of adding u2f support.
>
> It works just like other SSH key types.  You have a private SSH key
> and a public one, and you can copy the private key to multiple
> machines or load it into ssh-agent and use agent forwarding.
>
> The only difference is that the private SSH key on its own is
> insufficient and requires the cooperation of the FIDO/U2F authenticator.
part of the value of u2f is that there is not anything that you need to install 
on every system.

turning u2f into just a way to unlock ssh keys may be an easy way to use a u2f 
key, but it's missing out on the value of u2f.

As I said, Google has a modified sshd that they use with u2f keys that does not 
require anything be copied or stored on the client machine.

Yes, it modifies the protocol to pass a server/application name, but why is it 
bad to add a new authentication mechanism? There is provision for the ssh 
protocol to issue a prompt for a password, that could be (ab)used to pass the 
name needed for u2f to work properly.

David Lang