Security of ssh across a LAN, public key versus password

[Jochen Bern]

On 21.10.24 20:26, Chris Green wrote:
> I have a small LAN at home with nine or ten systems on it running
> various varieties of Linux.  I 'do things' on the LAN either from my
> dekstop machine or from my laptop, both run Xubuntu 24.04 at the
> moment.
> 
> There's a couple of headless systems on the LAN where login security
> is important to me and I've been thinking about the relative merits of
> password and public-key authentication.
> 
> [...] If someone 'breaks in' (in the physical or computer sense) to my
> desktop then how might they attack another system on the LAN? [...]
> 
> [...] If I went back to all passwords life would be so much easier!

As has already been pointed out, if someone manages to break into your 
workplace machine, the (past) security of the rest of the network is 
usually not the most pressing concern, much less your only one. But 
let's stick to that, for the sake of the argument.

Subverting your workplace machine - the same account you log in as, or 
even a superuser - gives the attacker a lot of possibilities, 
essentially getting his hands on all data that passes through that 
computer, from keystrokes to (before-/after-encryption) network 
communication to the contents of your screen. If that's a scenario 
probable enough to make it a concern, and the consequences for the other 
hosts in your LAN important enough to consider, the question to answer 
is not "which auth protocol spoken *by the subverted machine* is a bit 
harder to catch as well" but "how do get I get the relevant secrets 
*off* that machine and into an *actually* secure location".

Thanks to Yubikeys and similar devices, that is actually feasible, but 
it also makes it quite clear why *then* using keypair auth is vastly 
preferable: A password still needs to travel through the insufficiently 
secure machine whenever you use it, and can get snarfed there; a private 
key, on the other hand, never leaves the extra device and the worst the 
attacker can achieve is to *somehow* piggyback onto *your* activity 
(which will definitely not be as easy as copy-pasting a password out of 
a data stream recorded way-back-when).

>> do people use a password manager
> Not for passwords, I remember all the ones I use a lot.

Then I have a thought experiment for you: Let's assume that you found 
your workplace machine to be hacked and are now busy setting up a new 
one from scratch. Which of these two subtasks sounds easier to do:

a) Create new keypairs, possibly reusing the old passphrases for them 
(because either you succeeded in making the new setup more secure, then 
reusing the passphrases won't matter, or you didn't, then the attacker 
would likely eavesdrop new ones soon-ish, anyway)

b) Replace *every* password and *memorize* them all, preferably from one 
day to the next

Kind regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH
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