Key-Based Authentication Setup

Password authentication is fundamentally flawed for SSH. Even strong passwords can be compromised through brute-force attacks, keyloggers, or credential stuffing. Key-based authentication eliminates these risks.

Generate SSH Key Pair

On your local machine (not the server):

# Generate ED25519 key (recommended in 2025)
ssh-keygen -t ed25519 -C "[email protected]" -f ~/.ssh/id_prod_server

# Alternative: RSA 4096-bit key for older systems
ssh-keygen -t rsa -b 4096 -C "[email protected]" -f ~/.ssh/id_prod_server

Why ED25519? It’s faster, more secure, and uses shorter keys than RSA. I’ve switched all my infrastructure to ED25519 and never looked back.

Deploy Public Key to Server

# Copy your public key to the server
ssh-copy-id -i ~/.ssh/id_prod_server.pub username@server_ip

# Manual method if ssh-copy-id isn't available
cat ~/.ssh/id_prod_server.pub | ssh username@server_ip "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

Test Key Authentication

Before disabling password authentication, verify key-based login works:

ssh -i ~/.ssh/id_prod_server username@server_ip

If you can log in without entering a password, you’re good to proceed.

Set Correct Permissions

SSH is strict about permissions. Incorrect permissions will cause authentication to fail:

# On the server
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chmod 600 ~/.ssh/id_ed25519  # if private key is on server

Next Steps

With key-based authentication working, proceed to the SSH Daemon Hardening guide to lock down your sshd_config with production-ready security settings.