OpenSSH not requesting touch on FIDO keys (was: OpenSSH not requesting PIN code for YubiKey)

[Domenico Andreoli]

On Mon, Jul 13, 2020 at 01:34:37PM +1000, Damien Miller wrote:
> On Fri, 10 Jul 2020, Frank Sharkey wrote:
> 
> > I set up the YubiKey with OpenSSH 8.2 (Ubuntu client and server) and it
> > works. However, it does not do PIN enforcement at SSH login.  It only
> > requests the PIN during the set-up process (when the key is being
> > generated). Is that the way it's supposed to work?
> 
> Assuming you are using this device as a FIDO token (and not PKCS#11),
> this is expected. OpenSSH doesn't yet support requiring PINs for keys
> except for a couple of corner cases (e.g. resident keys).
> 
> I hope to add this before OpenSSH 8.4.

Somewhat related: touching the FIDO key to authorize the operation.

The user is prompted to touch the FIDO key when generating an ssh key
but later on (eg. ssh-add -T ...) this does not happen any more.

I guess it's due to the agent server not having any means to call back
the client for notifying that user action is required [0].

Is it maybe an idea to add some 'touch required' constraint to such
ssh keys?

If the client could query for such constraint (via some protocol
extension yet to be implemented), then it would show a prompt just
before requesting the operation to the agent server.

Dom

[0] https://tools.ietf.org/html/draft-miller-ssh-agent-04

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