Securing Your Connections: A Guide to SSH Keys

SSH (Secure Shell) is a fundamental tool for securely connecting to remote servers. While traditional password authentication works, it can be vulnerable to brute-force attacks. SSH keys offer a more robust and convenient solution for secure access.
SSH authentication using SSH keys

This blog post will guide you through the world of SSH keys, explaining their types, generation process, and how to manage them for secure remote connections and how to configure SSH key authentication.

Understanding SSH Keys: An Analogy
Imagine your home has two locks:

  • Combination Lock (Password): Anyone can access your home if they guess the correct combination.
  • High-Security Lock (SSH Key): Only someone with a specific physical key (your private key) can unlock the door.

    Similarly, SSH keys work in pairs:

  • Private Key: A securely stored key on your local machine. You never share this.
  • Public Key: A unique identifier you share with the server you want to access.
    The server verifies the public key against your private key when you attempt to connect. This verification ensures only authorized users with the matching private key can access the server.

    Types of SSH Keys
    There are many types of SSH keys, we are discussing the two main ones:

    RSA (Rivest–Shamir–Adleman): The traditional and widely supported option. It offers a good balance of security and performance.
    Ed25519 (Edwards-curve Digital Signature Algorithm): A newer, faster, and potentially more secure option gaining popularity.

    RSA vs. Ed25519 Keys:

  • Security: Both are considered secure, but Ed25519 might offer slightly better theoretical resistance against certain attacks.
  • Performance: Ed25519 is generally faster for both key generation and signing/verification compared to RSA. This can be beneficial for slower connections or resource-constrained devices.
  • Key Size: RSA keys are typically 2048 or 4096 bits, while Ed25519 keys are 256 bits. Despite the smaller size, Ed25519 offers comparable security due to the underlying mathematical concepts.
  • Compatibility: RSA is widely supported by all SSH servers. Ed25519 is gaining popularity but might not be universally supported on older servers.

    Choosing Between RSA and Ed25519:

    For most users, Ed25519 is a great choice due to its speed and security. However, if compatibility with older servers is a critical concern, RSA remains a reliable option.

    Generating SSH Keys with ssh-keygen
    Here’s how to generate your SSH key pair using the ssh-keygen command:

    Open your terminal.

    Run the following command, replacing with your desired name for the key pair:

    ssh-keygen -t <key_type> -b 4096 -C "<your_email@example.com>"
  • <key_type>: Choose either rsa or ed25519.
  • -b 4096: Specifies the key size (4096 bits is recommended for strong security).
  • -C “<your_email@example.com”>: Adds a comment to your key (optional).

    You’ll be prompted to enter a secure passphrase for your private key. Choose a strong passphrase and remember it well (it’s not mandatory, but highly recommended for added security).

    The command will generate two files:

    <key_name>>.pub: The public key file (you’ll add this to the server).
    <key_name>>: The private key file (keep this secure on your local machine).

    Important Note: Never share your private key with anyone!

    Adding Your Public Key to the Server’s authorized_keys File

    1. Access the remote server you want to connect to (through a different method if you haven’t set up key-based authentication yet).
    2. Locate the ~/.ssh/authorized_keys file on the server (the ~ represents your home directory). You might need to create the .ssh directory if it doesn’t exist.
    3. Open the authorized_keys file with a text editor.
    4. Paste the contents of your public key file (.pub) into the authorized_keys file on the server.
    5. Save the authorized_keys file on the server.

    Permissions:

    Ensure the authorized_keys file has permissions set to 600 (read and write access only for the owner).

    Connecting with SSH Keys
    Once you’ve added your public key to the server, you can connect using your private key:

    ssh <username>@<server_address>

    You’ll be prompted for your private key passphrase (if you set one) during the connection. That’s it! You’re now securely connected to the server without needing a password.

    Benefits of SSH Keys:

  • Enhanced Security: More secure than password authentication, making brute-force attacks ineffective.
  • Convenience: No need to remember complex passwords for multiple servers.
  • Faster Logins: SSH key-based authentication is often faster than password authentication.

    By implementing SSH keys, you can significantly improve the security and convenience of your remote server connections. Remember to choose strong passwords and keep your private key secure for optimal protection.