Openssh server setup for public key login only

Aim

OS: ubuntu 18.04 64 bit

We want to have a ssh server that only accept login through rsa public key.

Installation

$ sudo apt-get install openssh-client openssh-server openssl

Server setting

$ sudo sed -i 's/^PasswordAuthentication yes/PasswordAuthentication no/' /etc/ssh/sshd_config

$ sudo sed -i 's/^PubkeyAuthentication no/PubkeyAuthentication yes/' /etc/ssh/sshd_config

$ sudo vim /etc/ssh/sshd_config
------------------------------------
PubkeyAuthentication yes
PasswordAuthentication no
------------------------------------
$ sudo systemctl restart sshd.service 

Generate rsa key pairs

$ ssh-keygen -t rsa -b 4096 -C "testing@gmail.com"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/sulfred/.ssh/id_rsa): id_testing
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in id_testing.
Your public key has been saved in id_testing.pub.
The key fingerprint is:
SHA256:6F/Pnyj3A96+qgSGrw4hQc+O2x2xrdBGA5lmznqllOY testing@gmail.com
The key's randomart image is:
+---[RSA 4096]----+
|  . .o           |
| . o=.           |
|  .=o.+          |
|   +*o.B         |
|  o=+oB S        |
|  .+E* = .  .    |
|  ..o + . o. o   |
|     . o o.oo.o. |
|     .o . .+==*o |
+----[SHA256]-----+

Add Authorized keys to server

$ cat id_testing.pub >> ~/.ssh/authorized_keys
$ ls -hal ~/.ssh
------------------------------------
-r--r--r--  1 you you  405 Jun  7 09:42 authorized_keys
-rw-------  1 you you 3.2K Jun  7 10:48 id_testing
-rw-r--r--  1 you you  743 Jun  7 10:48 id_testing.pub

------------------------------------
$ sudo service ssh restart