How to configure RSA SSH Key based authentication in Ubuntu server

Step 01:Tools and Key Generation

• Download and install the Putty MSI installer, as it includes essential utilities like puttygen and pageant.
• Open PuttyGen, click “Generate,” and move your mouse over the blank area to create a public and private key pair.
• Save both the public and private keys to your computer. It is highly recommended to set a passphrase for the private key for an additional layer of human-use security.

Step 02: Server-Side Configuration

• On the Ubuntu server, navigate to (or create) the .ssh directory in the target user’s home folder. Open the authorized_keys file and paste the public key code generated in PuttyGen.
• Initially, the server will recognize the keys. If you use Pageant (the Putty authentication agent) to load your private key, you can log in without entering a password.

Step 03: Disabling Password Authentication

• Once you’ve verified that key-based login works, you can further secure the server by disabling passwords entirely:
• Open the SSH configuration file (/etc/ssh/sshd_config) and change PasswordAuthentication to “no”.
• Restart the SSH service to apply changes. After this, only users with a valid private key can access the server.

Step 04: Configuration for Non-Root Users

• The same process applies to non-root users (e.g., a user named “test”). You must ensure the public key is placed in that specific user’s home directory (/home/[username]/.ssh/authorized_keys) for the authentication to work.

Step 05: Alternative Method (Without Pageant)

• If you prefer not to use an authentication agent, you can manually point Putty to your private key file within the session settings under Connection > SSH > Auth > Credentials. However, this will require you to enter your key’s passphrase every time you log in.