Debian 8 Jessie
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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 for each user, so login with a common user and work it like follows.
# create key pair

ssh-keygen -t rsa

Generating public/private rsa key pair.
Enter file in which to save the key (/home/jessie/.ssh/id_rsa):  
# Enter

Enter passphrase (empty for no passphrase):  
# set passphrase (set no passphrase to Enter with empty)

Enter same passphrase again:
Your identification has been saved in /home/jessie/.ssh/id_rsa.
Your public key has been saved in /home/jessie/.ssh/
The key fingerprint is:
30:ee:f0:47:38:9d:8a:24:5e:01:c8:13:df:15:40:b4 jessie@dlp
The key's randomart image is:

mv ~/.ssh/ ~/.ssh/authorized_keys

chmod 600 ~/.ssh/authorized_keys

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

chmod 700 ~/.ssh
# copy the secret key to local ssh directory

scp jessie@ ~/.ssh/

jessie@'s password:
ssh jessie@

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

Last login: Wed April 24 21:37:19 2014 from
# logined

[3] If you set "PasswordAuthentication no", it's more secure.
vi /etc/ssh/sshd_config
# line 52: turn to "no"

systemctl restart ssh

SSH Key-Pair Authentication from Windows Client
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 input, then answer it. If it's correct passphrase, it's possible to login normally like follows.