Debian/OpenSSH || SSH Key-Pair Authentication

DerMeldos

Configure SSH server to login with Key-Pair Authentication.
Create a private key for client and a public key for server to do it.
Create Key-Pair by each user, so login with a common user on SSH Server Host and work like follows.

root@AWX0708:~# ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa
Your public key has been saved in /root/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:pqzEJ90xi63D5M9GqH/iG3fDOjc53XXK1nqS/1rnPUI root@AWX0708
The key's randomart image is:
+---[RSA 3072]----+
|                 |
|                 |
|                 |
|                 |
|       .S        |
|   . oo*.=   E  o|
|    +=B.= +o.o =+|
|   ..+==+o=...B+=|
|    .o=B+o o .+*B|
+----[SHA256]-----+
root@AWX0708:~#

root@AWX0708:~# ls -l ~/.ssh
total 20
-rw-r--r-- 1 root root  566 Jun  1 05:03 authorized_keys
-rw------- 1 root root 2602 Nov  2 16:09 id_rsa
-rw-r--r-- 1 root root  566 Nov  2 16:09 id_rsa.pub
-rw------- 1 root root 2684 Nov  2 16:05 known_hosts
-rw------- 1 root root 1848 Nov  2 16:05 known_hosts.old
root@AWX0708:~#

Transfer the private key created on the Server to a Client, then it's possible to login with Key-Pair authentication.

ssh-copy-id username@remote_host

if ssh-copy-id is not available use plain ssh:

cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys && chmod -R go= ~/.ssh && cat >> ~/.ssh/authorized_keys"

OPTIONAL:
Disable Password Authentication on your Server

sudo nano /etc/ssh/sshd_config

...
PasswordAuthentication no
...

this will disable it for all users, to only restrict specific users try matching;
Try Match in sshd_config:

Match User user1,user2,user3,user4
    PasswordAuthentication no

Or by group:

Match Group users
    PasswordAuthentication no

Or by negation:

Match User !root
    PasswordAuthentication no

Reload the SSH service for changes to take affect.

sudo systemctl reload ssh