Why is using an SSH key more secure than using passwords? [duplicate]

Question

If people use a password to log in to a UNIX server, then it could be forced to expire the password, then they change it.

If people use an ssh key and have no passwords, no password expiry, then nothing forces them to change their SSH key regularly.

Question: Which solution is more secure? Why do the "howto"'s for hardening a server always advise to use an ssh key, not passwords?

UPDATE: not counting the brute-force weakness - regarding passwords, since they could be guessed if there is no Fail2ban-like solution.

Answer 1

Both keys and passwords have their pros and cons. The reason that "howtos" and the like advise using the SSH key is that they find their cons less worrisome than passwords' cons.

SSH keys are long and complex, far more than any password could be. But as you state, they don't have expiry, and they sit on disk where they can be stolen from. On the other hand, they don't get transmitted to the remote system (except key forwarding, natch) which passwords need to be.

Passwords are generally, predictably, unavoidably weak. While it is possible to have strong passwords, time and again it has been shown that people will use weak passwords and have poor password practices... short, simple, word-based, simple patterns ("p@ssw0rd!"), write them down, use them on multiple sites, base them on their phone number, their children's birthdate, their own name. You point out that keys don't expire, but why do passwords expire? To ensure that a brute-force attack is less likely to crack a password before it's been replaced. Not an issue that impacts keys.

And, bad passwords aside, even "good" passwords are vulnerable to brute-force (online or offline) under the right conditions. They have to get transmitted to the other system, or to any other place that the user can be fooled into sending them by mistake.

The balance of evidence strongly suggests that passwords are weaker and keys are stronger.

Answer 2

With passwords, then the password is sent to the server, so the safety of the password is relative to how well the server protects whatever it uses to verify passwords (e.g. the /etc/shadow file). When you use an SSH key, then your private key remains on the client side, and no secret value is ever sent to the server. Even if the server is under hostile control, or you are somehow induced into connecting to a fake server, then your SSH key remains safe. A fake server does not gain enough information on your key to recover it or do some MitM. In that sense, SSH keys are more robust than passwords against compromises on the server side.

On the other hand, a SSH key must be stored somewhere, on a computer, and this can be a vulnerability. You have to protect your private key with a passphrase; otherwise, a stolen laptop turns into an account compromise. Conversely, a password can be stored in your brain only, which (supposedly) makes it less likely to leak.

Thus, it can be argued that SSH keys are "more secure" than passwords, but the opposite can also be argued. It depends on the context. Most HowTos will take the stance that keys are better because it so happens that, on average, human users have a terrible track record for passwords. Users choose weak passwords, and reuse them (password reuse is very very bad).

Answer 3

If what I am understanding in your update, if your can provide the same entropy in your passwords vs keys, than as far as its security it's moot. You could make tthe case that keys are better because it would provide a better "user experience" because your not typing in passwords every time.

I think more to your original point, we are discussing the manageability of SSH passwords vs SSH Keys. Yes passwords can be assigned policies, which allow basic security measures (IE Password expiration, password history, mix-max password length, etc) but this doesn't change human habit, and it also doesn't please users where security becomes encumbering to production (A balance at an organizational level).

Keys provide method for maintaining a degree of entropy and manageability while also providing a method of access that doesn't encumber a user (Once your key pair is setup you can authenticate without a password prompt until it's revoked.)

On top of this, know that Keys can be password protected and providing another layer.