Ubuntu 24.04
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OpenSSH : SSH Key-Pair Authentication2024/04/26

 
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@dlp:~$
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 (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:VPFjxEDeLKJLbKQurhgh0VIZxIrq8K+C+fg1iow8PmY ubuntu@dlp.srv.world
The key's randomart image is:
.....
.....

ubuntu@dlp:~$
ll ~/.ssh

total 24
drwx------ 2 ubuntu ubuntu 4096 Apr 26 06:47 ./
drwxr-x--- 4 ubuntu ubuntu 4096 Apr 26 06:45 ../
-rw------- 1 ubuntu ubuntu    0 Apr 26 01:09 authorized_keys
-rw------- 1 ubuntu ubuntu  464 Apr 26 06:47 id_ed25519
-rw-r--r-- 1 ubuntu ubuntu  102 Apr 26 06:47 id_ed25519.pub
-rw------- 1 ubuntu ubuntu  978 Apr 26 06:28 known_hosts
-rw-r--r-- 1 ubuntu ubuntu  142 Apr 26 06:28 known_hosts.old

ubuntu@dlp:~$
cat ~/.ssh/id_ed25519.pub >> ~/.ssh/authorized_keys

[2] Transfer the private key created on the Server to a Client, then it's possible to login with Key-Pair authentication.
ubuntu@node01:~$
mkdir ~/.ssh

ubuntu@node01:~$
chmod 700 ~/.ssh
# transfer the private key to the local ssh directory

ubuntu@node01:~$
scp ubuntu@10.0.0.30:/home/ubuntu/.ssh/id_ed25519 ~/.ssh/

ubuntu@10.0.0.30's password:
id_rsa                                        100% 2655     1.8MB/s   00:00

ubuntu@node01:~$
ssh ubuntu@10.0.0.30

Enter passphrase for key '/home/ubuntu/.ssh/id_ed25519':   # passphrase if you set
Welcome to Ubuntu 24.04 LTS (GNU/Linux 6.8.0-31-generic x86_64)

.....
.....

ubuntu@dlp:~$     # logined
[3] If you set [PasswordAuthentication no], it's more secure.
root@dlp:~#
vi /etc/ssh/sshd_config
# line 57 : uncomment and change to [no]

PasswordAuthentication
no
# line 62 : 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.
[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 transferred 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] - [Credentials] 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] 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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