Ubuntu 21.04
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OpenSSH : SSH Key-Pair Authentication
Configure SSH server to login with Key-Pair Authentication.
Create a private key for client and a public key for server to do it.
[1] Create Key-Pair by each user, so login with a common user on SSH Server Host and work like follows.
# create key-pair


Generating public/private rsa key pair.
Enter file in which to save the key (/home/ubuntu/.ssh/id_rsa):   # Enter or input changes if you want
Created directory '/home/ubuntu/.ssh'.
Enter passphrase (empty for no passphrase):   # set passphrase (if set no passphrase, Enter with empty)
Enter same passphrase again:
Your identification has been saved in /home/ubuntu/.ssh/id_rsa
Your public key has been saved in /home/ubuntu/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:NyP461lUypP7j4Js9ERyxP8fjWbrLg5O80X+VBzUGKo ubuntu@dlp.srv.world
The key's randomart image is:

ll ~/.ssh

total 16
drwx------ 2 ubuntu ubuntu 4096 Apr 27 05:50 ./
drwxr-x--- 4 ubuntu ubuntu 4096 Apr 27 05:50 ../
-rw------- 1 ubuntu ubuntu 2655 Apr 27 05:50 id_rsa
-rw-r--r-- 1 ubuntu ubuntu  574 Apr 27 05:50 id_rsa.pub

mv ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys

[2] Transfer the private key created on the Server to a Client, then it's possbile to login with Key-Pair authentication.
mkdir ~/.ssh

chmod 700 ~/.ssh
# transfer the private key to the local ssh directory

scp ubuntu@ ~/.ssh/

ubuntu@'s password:
id_rsa                                        100% 2655     1.8MB/s   00:00

ssh ubuntu@

Enter passphrase for key '/home/ubuntu/.ssh/id_rsa':   # passphrase if you set
Welcome to Ubuntu 21.04 (GNU/Linux 5.11.0-16-generic x86_64)

ubuntu@dlp:~$     # logined
[3] If you set [PasswordAuthentication no], it's more secure.
vi /etc/ssh/sshd_config
# line 58 : change to [no]

systemctl restart ssh

SSH Key-Pair Authentication on Windows Client #1
This is the example to login to SSH server from Windows Client.
On this example, it uses Putty.
Before it, Transfer a private key to Windows Client.
[4] Run [Puttygen.exe] that is included in [Putty]. (placed in the folder [Putty.exe] is also placed)
If not included, Download it from official site (www.chiark.greenend.org.uk/~sgtatham/putty/).
After starting [Puttygen.exe], Click [Load] button on the following window.
[5] Specify the private key that you transfered from SSH server, then passphrase is required like follows, answer it. (if not set passphrase, this step is skipped)
[6] Click [Save private key] button to save it under a folder you like with any file name you like.
[7] Start Putty and Open [Connection] - [SSH] - [Auth] on the left pane, then specify your private key on the [Private key file] field.
[8] Back to the [Session] on the left pane and specify your SSH server host to Connect.
[9] When SSH key-pair is set, the passphrase if it is set is required to login like follows, then answer it.
SSH Key-Pair Authentication on Windows #2
[10] If you are using Windows 10 Version 1803 or later like here [7],
OpenSSH Client has been implemented as a Windows feature, so it's possbile to authenticate with SSH Key-Pair without Putty and other 3rd party softwares.
Transfer your private key to your Windows 10 and put it under the [(logon user home).ssh] folder like follows, then it's ready to use Key-Pair authentication.
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