Linux - SSH Key Authentication

SSH key authentication is a secure method of logging into a remote server without using a password. Instead, it uses a pair of cryptographic keys—a public key and a private key—that work together to verify your identity. This method is safer, faster, and more reliable than traditional password-based logins, especially in server management and DevOps environments.

How SSH Key Authentication Works

Key Pair Generation
You generate a key pair on your local machine using:
```
ssh-keygen
```
- Private Key → stays on your local machine (never shared).
- Public Key → uploaded to the server.
Public Key Added to Server
The public key is stored in the server’s ~/.ssh/authorized_keys file.
When you attempt to connect, the server checks this key to confirm your identity.
Authentication Process
When you run:
```
ssh user@server
```
- The server sends a challenge encrypted with your public key.
- Your local machine uses the private key to decrypt and respond.
- If the response matches, access is granted.

No password is transmitted, making the process highly secure.

Why SSH Key Authentication Is Better

✔ Stronger security – Keys are nearly impossible to brute-force compared to passwords.
✔ Passwordless login – Saves time and supports automation.
✔ Prevents phishing attacks – No password means nothing to steal.
✔ Supports automation tools – Essential for DevOps pipelines, Git operations, and infra management.
✔ Can be protected with a passphrase – Adds an extra layer of security to the private key.

How to Set It Up (Simplified)

Generate key pair
```
ssh-keygen -t rsa -b 4096
```
Copy public key to server
```
ssh-copy-id user@server
```
or manually add it to ~/.ssh/authorized_keys.
Connect
```
ssh user@server
```

That’s it — seamless, secure access without typing a password.

Best Practices

Never share your private key.
Use strong key types (RSA 4096 bits, ED25519).
Protect the private key with a passphrase.

Restrict permissions:

chmod 600 ~/.ssh/id_rsa
chmod 700 ~/.ssh

Disable password authentication on the server once keys work:
```
PasswordAuthentication no
```

SSH key authentication is a foundational skill for developers, system administrators, and DevOps engineers. It ensures secure, fast, and automated access to remote systems—making server management far more efficient and safe.