Feature - Password over Pubkey auth

[Jochen Bern]

On 03.08.25 11:54, Eduardo Suarez-Santana wrote:
> I've observed that password authentication typically passes through the
> server-side PAM authentication modules. This may be useful for instance to
> unlock an encrypted home directory using the user's password.
[...]
> I wonder whether an hybrid authentication method could be implemented, where
> the password of the user is stored along with the authorized public key in the
> server, but instead of storing it in plain text, it would be stored encrypted
> with the public key.
> 
> So that, I'm proposing a new authentication method that would send the
> encrypted password to the client, so the client could decrypt it with the
> private key, and then send it back to the server.

My .02:

I'm wondering whether there actually is a use case for (or, much 
resource savings to be obtained by) SSH logins that do *not* make $HOME 
available.

Because if
-- there are none,
-- you *want* people to do *keypair* auth to log into the server, and
-- sshd defers the password auth to PAM (as you said it does, above),
I would expect that setting "AuthenticationMethods publickey,password" 
already does everything that's really required from the *server* side.

It would differ from your proposal in that the password would actually 
come only from the client side, eliminating the need for the (completely 
new) "server sends encrypted password to client for decryption" part. 
I'd *guess* that that greatly reduces the amount of coding required, and 
it would also mean that the security model remains "*all* sensitive 
material is stored only on the client", as it is with only-keypair-auth 
currently; might save a lot of thinking about potential new attack 
surfaces opened by a change.

What would remain to be done is to (decrypt as needed on and) send the 
password from the client to the server in a (semi-)automated way. As the 
manpage for sshpass(1) will tell you
	https://linux.die.net/man/1/sshpass
that's not something that the OpenSSH devs are terribly fond of, but it 
*can* be done with third party software (like, surprise, sshpass). In 
particular, if you have an agent running for GnuPG as well as for SSH, 
you may want to look at "Example 4" on
	https://www.redhat.com/en/blog/ssh-automation-sshpass
for an idea how to connect the pipes ...

Kind regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH
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