Linux/Mac Tutorial: SSH Key-Based Authentication - How to SSH Without a Password

SSH key-based authentication replaces password prompts with cryptographic login, making remote access both more convenient and more secure. The core workflow is straightforward: generate an RSA key pair on the local machine, copy the public key to the remote account’s ~/.ssh/authorized_keys, lock down permissions, and then SSH in without typing a password.

The tutorial starts with a working baseline: SSH into a remote Linux virtual machine using a username and IP address, but the session requires a password every time. The goal is to eliminate that password step. To do so, it emphasizes a common point of confusion—only the public key needs to be placed on the remote machine. The private key stays on the local machine, where only the user should have access. Using ssh-keygen, the process creates ID_RSA (private) and id_rsa.pub (public). It recommends using RSA with a stronger key size (via options like -rsa-b 4096 instead of the default 2048) and optionally adding a passphrase; leaving the passphrase blank is shown for simplicity.

Next comes the manual “push the public key” method. Before copying anything, the remote account must have a ~/.ssh directory. The tutorial creates it with mkdir -p ~/.ssh, then transfers id_rsa.pub using scp. The key is placed into the remote user’s home directory under ~/.ssh, and then its contents are appended into a new (or existing) file named authorized_keys using cat >> ~/.ssh/authorized_keys. After the key is in place, permissions are tightened to satisfy SSH’s security checks: the ~/.ssh directory should be 700, and files inside it (including authorized_keys) should be 600 via chmod 700 ~/.ssh and chmod 600 ~/.ssh/*.

With those pieces set, the tutorial demonstrates that an SSH command like ssh corems@192.168.56.100 logs in successfully without prompting for a password. For stronger hardening, it then shows how to disable password authentication entirely. That involves backing up /etc/ssh/sshd_config, editing it (e.g., setting PasswordAuthentication no), and restarting the SSH service (on Ubuntu, using sudo service ssh restart). The result is that only key-based logins will work.

Finally, the tutorial presents an easier automation path using ssh-copy-id. On Linux it’s typically available by default; on macOS it can be installed via Homebrew (brew install ssh-copy-id). Running ssh-copy-id user@host prompts for the remote password once, then automatically creates the remote ~/.ssh directory, sets correct permissions, and installs the public key into authorized_keys. Even then, the SSH server configuration change to disable password logins still needs to be done separately if desired.