Request for a Lockdown option

Simon Josefsson
Thu, 04 Jul 2024 13:21:55 -0000

Jochen Bern <Jochen.Bern at binect.de> writes:

> (And since you mention "port knocking", I'd like to repeat
how fond I
> am of upgrading that original concept to a single-packet
> crypto-armored implementation like fwknop.)

I am reluctantly considering to use some kind of port knocking mechanism
on some machines, however I really don't want to carry around shared
symmetric keys or setup yet another public/private key infrastructure
for that purpose.  I already have a working infrastructure for SSH
authentication.

Does anyone know of any implementation that allows me to configure a
PGP/SSH/FIDO/TPM/whatever public key on the server side, and it then
only listens to signed port knocks from the corresponding private keys?

I notice fwknop has PGP support, but it requires a private key on the
server side, and that's really annoying.  Instead of using public-key
encryption, shouldn't be possible to rely only on public-key signing
instead?  I already carry around a physical device with a public/private
keypair in it, and I need that for SSH public-key authentication anyway.
To avoid replay attacks, the signed data needs to be an ever increasing
counter or timestamp a'la HOTP/TOTP.

I think this could be a good builtin functionality of OpenSSH, it
already has all of the public/private key trust infrastructure
available, what is missing is just the plumbing to connect it the
firewall.  Maybe it could go into a separate binary and not in the
default sshd though.  How about a sshfwkd?

/Simon
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