Hello,

Using the same public+private set of keys, unlike other servers in the lab, when connecting from Windows with the Kitty terminal application to a new Debian 11 host, I'm prompted for the passphrase:

Before removing the passphrase as a quick work-around, I'd like to understand why this happens, as I don't see differences on the server and the client sides that might explain the difference in behavior.

FWIW, it's OpenSSH_8.4p1

Thank you.

You might check the permissions and ownership on the /home, home folder, ~/.ssh and its files, and on the /etc/ssh folder and files.
SSH will, and change its behavior due to differences in those permissions and ownership.

Passphrases and, for that matter, private keys have nothing to do with the server. Neither the passphrase nor the key itself ever leave the client. The authentication process uses them in a different way. Thus if the client is a legacy system with Windows and it is the only one misbehaving then what you have is a Windows problem, maybe check with a Windows forum instead.

But since you ask here, my approach would be to back up the client data, download Linux Mint, wipe the client hard drive with a fresh install of Linux Mint, and then restore the backed up data into that fresh installation. That will save a lot of work over time anyway. From there check about what kind of SSH agent you have running and whether the private key is loaded into it, some of that can be configured in ~/.ssh/config.

(snip since can't delete error)

Thanks for the tip.

Here's /etc/ssh/sshd_config:
root's .ssh/authorized_keys
and the contents of sshd_config:

You have multiple hosts and one client. Are those folders and files on the client, a host with the behavior, or a host without the behavior?
You need to be very specific and clear here, because you are dealing with security software with very specific requirements and behavior.

Those files above are on the Debian host.

The behavior occurs only with that freshly installed Debian 11 host, connecting from a Windows client that works fine with other Linux servers.

Being prompted for the passphrase is no biggie, but enough to want to turn off that "feature", especially since it doesn't occur when connecting to other Linux servers.

Found it: I was using a copy of the private key elsewhere, that might have its passphrase disabled (I don't know how to tell.)

Pointing the SSH client to that other file solved the issue.

Thanks all.

--
Edit: To check if the private key is passphrase-protected, check what it says for "Encryption".

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Not sure if you know this, or even if you're interested, but Windows comes with OpenSSH client built in. You can just open a cmd prompt and type:

et viola