SSH multi factor authentication

Nico Kadel-Garcia nkadel at
Sun Jul 10 09:11:23 AEST 2016

On Sat, Jul 9, 2016 at 12:00 PM, Ben Lindstrom <mouring at> wrote:

> We've all written horrible stuff to satisfy business requirements.  Our
> universe, as admins,
> is  scattered with many "yet another tool"  or "yet another set of packages"
> to maintain for
> security or for our users to do their jobs.

Ben, I'm not trying to say it's not wise and sensible to go write a
non-standard, unsupported, and potentially root access destabilizing
non-standard workaround to shove user keys in a separate, locked,
down, root owned repository managed by someone other than the
individual user.

Actually, come to to think of it, I am saying it's an idea that many
admins do reject and should reject.. It's easy to do at first glance.
But taking manual control of sshd_config can be fragile, and can
breadk restarts of sshd. I don't recommend it for the faint-hearted
and those without test environments with local console access.

> So if the above is a requirement for your fictional admin then a tool will
> need to be written.
> And OpenSSH has already provided the means today by which it can easily
> integrate in.

Yes, you can hand modify or plug in a wrapper to modify sshd_config
and try to keep it consistent with OpenSSH release changes. It's
sometimes proven a finicky task.

> However, I would hope that it is a well written tool, and it can be shared
> with the community
> so it doesn't become a one-off burden.

Me, too. I haven't noticed any good ones. Even the chef "openssh"
cookbook doesn't have good default options for the different OpenSSH
variants, and tends to screw up SFTP settings if you use the cookbook.

> Anyways, the question still boils down to is Google Authenticator +
> Authorized Keys (with or
> without password) 2FA.  And honestly I'm going to have to state it is 2FA in
> both cases.

> The argument is simple.  If I use a password management tool (that requires
> me to unlock
> it with a different password) that auto fills in my Blizzard
> login/password then I use
> the Blizzard's authenticator and type in the random code do I have 2FA?

I agree with this argument.

> Clearly, I've not typed my password.  I may NOT EVEN KNOW my password (which
> is true
> in this case as it is a nasty auto-generated beast =).  So it is basically
> the same as the
> 2048bit RSA authorized key I use in SSH.   I use a password to decrypt
> access,
> but I only really possess both as I don't have them memorized.

> To complete the comparison I can write myself a password manager that
> doesn't require
> me to unlock (e.g. most "remember my password" crap in browsers).   So the
> fact it is if
> my password is protected or not doesn't change my knowledge (or lack of) of
> the
> password.

This is the kind of thing I really, really don't recommend for the
faint of heart or the wise of maintenance. This sort of trick quickly
leds to the infamous XKCD "automation" chart, shown at .

> The same goes for RSA keys.  Someone can steal my password manager, hack it,
> and
> gain access.  Just like they could steal steal my RSA keys off my laptop.
> Ben

People *do* steal private SSH keys and other keys off computers, both
laptops and servers, all the time. I've certainly done it as a proof
of concept.

The last time, it was a "Linux architect" who erected an SSH tunnel
between servers at work and his home machine, with unencrypted SSH
private keys stored in his home direcotry on ech end, and published on
NFS shares. I was pretty shocked, but that kind of thing is not that
unusual. Keys get stolen by this kind of silliness *all the time*.

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