Ubuntu 19.04
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OpenSSH : SSH Key-Pair Authentication2019/04/22
 
Configure SSH server to login with Key-Pair Authentication. Create a private key for client and a public key for server.
[1] Create Key-Pair for each user, so login with a user you'd like to set Key-Pair Auth and work it like follows.
# create key pair

ubuntu@dlp:~$
ssh-keygen 

Generating public/private rsa key pair.
Enter file in which to save the key (/home/ubuntu/.ssh/id_rsa):   # Enter if you don't need to change the default
Enter passphrase (empty for no passphrase):   # set passphrase (if not set, 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:fj3Bk+PtY/Li5CpcYyqXKV/aJs37UpiG/MUfOPxTdxA ubuntu@dlp.srv.world
The key's randomart image is:
ubuntu@dlp:~$
mv ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys

ubuntu@dlp:~$
chmod 600 ~/.ssh/authorized_keys

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

ubuntu@www:~$
chmod 700 ~/.ssh
# copy the secret key to the local ssh directory

ubuntu@www:~$
scp ubuntu@10.0.0.30:/home/ubuntu/.ssh/id_rsa ~/.ssh/

ubuntu@10.0.0.30's password:
id_rsa
ubuntu@www:~$
ssh ubuntu@10.0.0.30

Enter passphrase for key '/home/ubuntu/.ssh/id_rsa':    
# passphrase

Last login: Sat Apr 21 20:30:53 2018 from www.srv.world
ubuntu@dlp:~$    
# just logined
[3] If you set "PasswordAuthentication no", it's more secure.
root@dlp:~#
vi /etc/ssh/sshd_config
# line 56: uncomment and turn to [no]

PasswordAuthentication
no
root@dlp:~#
systemctl restart ssh

SSH Key-Pair Authentication from Windows Client #1
 
It's the example to login to SSH server from Windows Client. It uses Putty on here.
Transfer a secret key to Windows Client first.
[4] Download [Puttygen.exe] from Putty Site and save it under the Putty directory. Next execute it and click [Load] button.
[5] Specify the secret key which you downloaded, then passphrase is required like follows, answer it.
[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 menu, then select the [private_key] which was just saved above.
[8] Back to the [Session] on the left menu and connect to the SSH server.
[9] The passphrase is required to login, then answer it. If it's correct, it's possible to login normally like follows.
SSH Key-Pair Authentication from Windows Client #2
[10] If your Windows is Windows 10 Version 1803 or later like here, OpenSSH Client has been implemented as a Windows feature, so it's possbile to authenticate with SSH Key-Pair without Putty and others. Transfer the secret key to your Windows 10 and put it uder the [(logon user home)\.ssh] folder like follows, then it's ready to use Key-Pair login.
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