How to Set Up SSH Keys on Ubuntu 16.04

Run the following command on your local machine to generate a 2048-bit RSA key pair (use -b 4096 for a stronger 4096-bit key):

$ ssh-keygen

When prompted for a file path, press Enter to accept the default (~/.ssh/id_rsa). Optionally set a passphrase for an extra layer of security.

Two files are created: ~/.ssh/id_rsa (private key — keep this secret) and ~/.ssh/id_rsa.pub (public key — this goes on the server).

Method 1 — ssh-copy-id (recommended):

$ ssh-copy-id username@remote_host

Enter your password when prompted. The public key will be appended to ~/.ssh/authorized_keys on the server.

Method 2 — via SSH pipe:

cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

Method 3 — manually: Display your public key with cat ~/.ssh/id_rsa.pub, then append its output to ~/.ssh/authorized_keys on the remote server.

Test that key-based authentication works by connecting to your server:

$ ssh username@remote_host

If you set a passphrase in step 1, you will be prompted to enter it. If you did not, you will be logged in immediately without any password prompt.

Once you have confirmed key-based login works, you can disable password authentication to harden the server against brute-force attacks. Open the SSH daemon config:

sudo nano /etc/ssh/sshd_config

Find PasswordAuthentication, uncomment the line if needed, and set it to no:

PasswordAuthentication no

Save the file (Ctrl+X, then Y, then Enter) and restart SSH:

$ sudo systemctl restart ssh

Open a new terminal and verify you can still connect before closing your current session.