2-factor auth options

[Jason A. Donenfeld]

On Fri, Sep 22, 2017 at 4:45 PM, Konstantin Ryabitsev
<konstantin at linuxfoundation.org> wrote:
>  The upside is that it would make sure we
> don't have 2-factor stuff happening via a separate (potentially weaker)
> crypto channel than the rest of the session,

That's not actually the case. If a normal IP packet goes through
wireguard, if it a) comes from a wireguard interface (wg0), and b) is
from a particular IP address, then it will be unforgeably bound to the
peer public key that has that IP listed in his allowed-ips. There's a
strong binding here, in otherwords, between src IPs of packets within
a wireguard interface and the actual identity.

This means...

> On Fri, Sep 22, 2017 at 12:52:43AM +0200, Jason A. Donenfeld wrote:
> It may be the easiest and, with a web frontend, would allow doing something
> like U2F (bonus!), but it'll probably be fairly
> unpopular because it would require a wholly disconnected step before
> establishing a connection.

This means that you actually wouldn't even need to do it with a
webpage. You could add this as a step that just uses tcp via ncat in a
simple script to send the challenge. That's the kind of thing you
could even add as a PostUp section in a wg-quick(8) configuration, so
that immediately after starting up the tunnel, the OTP script runs
that prompts the user for their 2FA, ncats it to a trivial little
daemon on the server, that then validates the pin and adjusts the
firewall (or more trivially, adds an entry to an ipset object).

There are a lot of very easy ways to structure this. I think next week
I'll try to put together a little OTP example for the contrib/
directory. Do you have a favorite OTP command line challenge response
tool I should use for this?

> For example,
> establishing an initial connection would satisfy one set of permitted IP
> routing, but performing some API action would apply additional rules. It
> would be hard to do this in a non-kludgy way, though.

I actually think this wouldn't be too difficult to accomplish.
Live-changing IP address ACLs usually isn't too difficult. If you're
really into it, you can even do this all with `ip-rule` instead of
iptables/ipset.

> I don't have concrete plans at this time, but we do use openvpn for
> admin-level access (that requires 2-factor auth), and I was looking for
> feature parity to see if we should eventually plan to switch.

Cool. You should certainly plan to switch. We'll make this happen.

Jason