Ubuntu 25.04
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OpenSSH : SSH Key-Pair Authentication2025/04/21
 

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] Key pairs are created by each user, so the user who creates the key pair log in to any client computer and work as that user.
# 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:saW0mmPf1r0HjphkQ1+VGsCK1zqr4U7VUXKuV57pSkY ubuntu@client.srv.world
The key's randomart image is:
.....
.....
ubuntu@client:~$
ll ~/.ssh 

total 16
drwx------ 2 ubuntu ubuntu 4096 Apr 21 00:44 ./
drwxr-x--- 4 ubuntu ubuntu 4096 Apr 17 23:53 ../
-rw------- 1 ubuntu ubuntu    0 Apr 17 23:49 authorized_keys
-rw------- 1 ubuntu ubuntu  444 Apr 21 00:44 id_ed25519
-rw-r--r-- 1 ubuntu ubuntu   95 Apr 21 00:44 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: 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:

Number 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 use key pair authentication, you can make your server more secure by disabling password authentication on the SSH server side as follows.
root@dlp:~#
vi /etc/ssh/sshd_config
# line 78, 83 : uncomment and change to [no]

PasswordAuthentication
no

.....
.....
KbdInteractiveAuthentication
no
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.
It uses Putty on this example.
Beforehand, transfer the SSH private key ([id_ed25519] in the above example) to the Windows client.

[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
[9] On Windows 11, OpenSSH Client has been implemented as a 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 11 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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