How to Set Up SSH Keys Securely on Linux Servers

A junior sysadmin dumped their private SSH key into a public GitHub repository at 4:45 PM on a Friday. By 5:12 PM, automated scrapers had harvested it, bypassed the single-factor authentication on three production nodes, and dropped malicious mining binaries across the cluster. We were called in to audit the wreckage. The root cause wasn’t sophisticated malware; it was pure operational laziness.

Password authentication on Linux servers is completely dead for production environments. If you are still leaving port 22 exposed to the public internet with password logins enabled, you are inviting automated brute-force attacks. Let’s look at the actual forensic numbers from our recent server hardening audits:

Step 1: Generate Cryptographically Secure Keys on Your Local Machine

Stop generating legacy RSA 2048 keys because of outdated compliance checklists from 2012. In our security reviews, we mandate Ed25519. It is faster, more secure, and immune to many mathematical attacks targeting older algorithms.

Open your local terminal and run the key generation command:

ssh-keygen -t ed25519 -C "[email protected]"

When prompted for a file path, press Enter to accept the default location (~/.ssh/id_ed25519). Big mistake: skipping the passphrase. Always assign a strong passphrase to encrypt your private key at rest on your local disk. If your workstation is physically stolen or compromised, an unencrypted private key provides instant access to every server in your infrastructure.

Step 2: Copy the Public Key to the Remote Target

Never manually paste public keys into remote files unless you enjoy debugging permission errors. Use ssh-copy-id to push your public key safely to the remote Linux server:

ssh-copy-id -i ~/.ssh/id_ed25519.pub username@remote_server_ip

If your server uses a non-standard port (which you should implement immediately), specify it explicitly:

ssh-copy-id -i ~/.ssh/id_ed25519.pub -p 2222 username@remote_server_ip

Behind the scenes, this script appends your public key to the remote user’s ~/.ssh/authorized_keys file. If you must do this manually via an emergency rescue console, ensure the permissions on the remote server are locked down tightly:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

If those directory and file permissions are too open—for instance, group-writable (775)—the OpenSSH daemon will flat-out reject key-based authentication as a security precaution. We see this misconfiguration break deployments constantly.

Step 3: Hardening the SSH Daemon Configuration

Having keys on the server means nothing if password login is still permitted. You must edit the SSH daemon configuration file to enforce absolute compliance. Open /etc/ssh/sshd_config with root privileges:

sudo nano /etc/ssh/sshd_config

Update or uncomment the following directives to match these exact security thresholds:

PubkeyAuthentication yes
PasswordAuthentication no
PermitRootLogin no
ChallengeResponseAuthentication no
UsePAM yes

Do not close your active terminal session yet. Test the configuration syntax before restarting the service to avoid locking yourself out permanently:

sudo sshd -t

If the command returns no errors, restart the SSH service:

sudo systemctl restart sshd

Now, open a separate terminal window and verify that login fails when using passwords and succeeds smoothly via your Ed25519 key.

Step 4: Operational Best Practices and Maintenance

Keys are credentials, not magic talismans. Treat them with the same forensic rigor as physical master keys. Implement these operational rules across your team:

• Never share private keys: Every human administrator must generate their own unique key pair. Shared keys destroy accountability during log audits.
• Audit authorized_keys quarterly: Run automated scripts to scan for stale or unauthorized public keys left behind by departed contractors.
• Use SSH agents securely: Load your keys into ssh-add with a short lifespan if you use jump hosts, preventing credential hijacking.

Frequently Asked Questions

Why are password-based SSH logins a severe security risk?

Passwords are vulnerable to brute-force attacks, credential stuffing, and weak human creation habits. In our forensic audits, we routinely see automated botnets hammering exposed port 22 nodes thousands of times per hour until weak user passwords crack.

What is the recommended cryptographic algorithm for SSH keys?

Ed25519 is the modern industry standard. It offers significantly faster signature verification, smaller key sizes, and superior resistance to side-channel attacks compared to legacy RSA 2048 or 4096 keys.

Where should the private SSH key be stored on the client machine?

Your private key must remain strictly on your local machine inside the ~/.ssh/ directory with strict file permissions set to 600. Never copy, email, or transfer a private key across an insecure network.

How do I disable root login over SSH safely?

Open your /etc/ssh/sshd_config file, locate ‘PermitRootLogin’, set it to ‘no’ or ‘prohibit-password’, and restart the SSH service. Ensure you have tested sudo access on a non-root user account before doing this to prevent permanent lockouts.

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Last Audited: October 26, 2026

Marcus Vance — Senior Investigative Field Correspondent

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