OpenSSH : SSH Key-Pair Authentication2026/04/24

	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

ubuntu@client:~$

ssh-keygen

Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/ubuntu/.ssh/id_ed25519):  # Enter or input changes if you want
Enter passphrase for "/home/ubuntu/.ssh/id_ed25519" (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_ed25519
Your public key has been saved in /home/ubuntu/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:mU2GVu3ZgC6Xcab6XQ2KIazz9JOeDzQBOTdyxwHet7E ubuntu@client.srv.world
The key's randomart image is:
.....
.....

ubuntu@client:~$

ll ~/.ssh

total 16
drwx------ 2 ubuntu ubuntu 4096 Apr 24 07:16 ./
drwxr-x--- 4 ubuntu ubuntu 4096 Apr 24 07:09 ../
-rw------- 1 ubuntu ubuntu    0 Apr 24 04:15 authorized_keys
-rw------- 1 ubuntu ubuntu  464 Apr 24 07:16 id_ed25519
-rw-r--r-- 1 ubuntu ubuntu  105 Apr 24 07:16 id_ed25519.pub

# copy the SSH public key you created to the server you want to log in to

ubuntu@client:~$

ssh-copy-id dlp.srv.world

/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/ubuntu/.ssh/id_ed25519.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
ubuntu@dlp.srv.world's password:

umber of key(s) added: 1

Now try logging into the machine, with: "ssh 'dlp.srv.world'"
and check to make sure that only the key(s) you wanted were added.

# verify whether you can log in to the server to which you copied the public key using key authentication

ubuntu@client:~$

ssh dlp.srv.world hostname

Enter passphrase for key '/home/ubuntu/.ssh/id_ed25519':   # if you set a passphrase, enter it
dlp.srv.world

[2]	If you set [PasswordAuthentication no], it's more secure.

root@dlp:~#

vi /etc/ssh/sshd_config

# line 78 : uncomment and change to [no]

PasswordAuthentication

# line 85 : make sure the setting below

KbdInteractiveAuthentication no

# If the following file exists,
# change the content in the same way or delete the file itself

root@dlp:~#

cat /etc/ssh/sshd_config.d/50-cloud-init.conf

PasswordAuthentication yes
root@dlp:~#

rm /etc/ssh/sshd_config.d/50-cloud-init.conf

root@dlp:~#

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 shows with Putty. Before it, Transfer a private key to Windows client computer.
[3]	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.

[4]	Specify the private key that you transferred from SSH server, then passphrase is required like follows, answer it. (if not set passphrase, this step is skipped)

[5]	Click [Save private key] button to save it under a folder you like with any file name you like.

[6]	Start Putty and Open [Connection] - [SSH] - [Auth] - [Credentials] on the left pane, then specify your private key on the [Private key file] field.

[7]	Back to the [Session] on the left pane and specify your SSH server host to Connect.

[8]	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

[p]

OpenSSH Client has been implemented as an Windows feature, so it's possible to authenticate with SSH Key-Pair without Putty and other 3rd party softwares. Transfer your private key to your Windows 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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