> passwords are a symetric key, hence if the server is compromised, so is the password

Pretty sure that’s not how it works, iirc passwords are stored one-way encrypted. And if it were true, then anyone with root access to a box could comprise every other (Unix) user’s key, which seems like a potentially bigger problem…

Passwords are (or rather should be) indeed stored using crypt. However at login the provided password needs to be compared to the hashed one, which means the clear text password needs to be rehashed. I am not sure this happens on the client.

It definitely doesn’t happen on the client. If it did then the server’s password file would effectively be plaintext.

> If it did then the server’s password file would effectively be plaintext.

Send seed and hashing parameters to the client, then client does hashing, client sends hash, server compares hashes. It's vulnerable to replay attacks, but it's the same with client sending plaintext password to server (assuming that you're not using SSH or similar).

You two are working your ways towards defining a zero-knowledge password proof:

https://en.wikipedia.org/wiki/Zero-knowledge_password_proof

I think SRP is the most widely implemented version. https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco...

Note that SRP is an older protocol, and things have moved on from there. See various PAKE algorithms:

* https://en.wikipedia.org/wiki/Password-authenticated_key_agr...

Quick google led me to RFC4252[0], section 8 of which (as far as I understood) describes ssh auth sending password as UTF8 plaintext string (and the whole packet is encrypted at transport layer). While passwords in /etc/shadow are hashed, if someone got access to your server he can just put malicious listener that will catch this UTF8 string.

I'm not a SSH guru, so if I'm mistaken please shout at me ;D

[0] https://datatracker.ietf.org/doc/html/rfc4252#section-8

Anything that is not PAKE (which SSH is not) generally sends the password to the server:

* https://en.wikipedia.org/wiki/Password-authenticated_key_agr...

* https://blog.cryptographyengineering.com/2018/10/19/lets-tal...

   A Password-Authenticated Key Exchange (PAKE) attempts to address this
   issue by constructing a cryptographic key exchange that does not
   result in the password, or password-derived data, being transmitted
   across an unsecured channel.

* https://datatracker.ietf.org/doc/html/rfc8125

I'm sure there are other zero-knowledge protocols besides PAKE-like ones, but I'm not an expert here.

I wouldn't worry about storage. Anyone with root access can modify the sshd daemon (along with imap, pop3, and whatever else) to log all the passwords received.

pretty sure passwords are sent to the server in a way that reveals the actual password to the server. any chance you are thinking about key-agent?