Fedora 41
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OpenSSH : SSH Key-Pair Authentication2024/10/31
 
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

[fedora@client ~]$
ssh-keygen 

Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/fedora/.ssh/id_ed25519):  # Enter or input changes if you want
Created directory '/home/fedora/.ssh'.
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/fedora/.ssh/id_ed25519
Your public key has been saved in /home/fedora/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:PMBZoNIrzv8y3ldJWzDGUS1D9pnmtL/khDYfC9PY67I fedora@client.srv.world
The key's randomart image is:
.....
.....
[fedora@client ~]$
ll ~/.ssh 

total 8
-rw-------. 1 fedora fedora 464 Oct 31 11:25 id_ed25519
-rw-r--r--. 1 fedora fedora 105 Oct 31 11:25 id_ed25519.pub
# copy the SSH public key you created to the server you want to log in to

[fedora@client ~]$
ssh-copy-id dlp.srv.world 

/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/fedora/.ssh/id_ed25519.pub"
The authenticity of host 'dlp.srv.world (10.0.0.30)' can't be established.
ED25519 key fingerprint is SHA256:d6bbyQgjLbqg2eTAsxEgNeZX1b6w/3iotKQi54eFaOM.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
/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
fedora@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

[fedora@client ~]$
ssh dlp.srv.world hostname 

Enter passphrase for key '/home/fedora/.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 65, 69 : uncomment and change to [no]

PasswordAuthentication
no

.....
.....
KbdInteractiveAuthentication
no
[root@dlp ~]#
systemctl restart sshd

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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