SSH key

A key pair that replaces passwords for remote login.

An SSH key pair is a private key you keep and a public key you install on servers. Logging in, your client proves possession of the private key by signing a challenge: the key itself never leaves your machine, which makes it both safer and more convenient than a password.

Generate with ssh-keygen -t ed25519, install with ssh-copy-id user@host. Permissions matter: SSH refuses private keys that others can read, hence chmod 600 on the key and 700 on ~/.ssh.

See it in practice

$ ssh-keygen -t ed25519 && ssh-copy-id user@server

Create the pair once, install the public half on each server. Passwords stop being part of your workflow.

Where you'll meet it

ssh

Log into remote machines, run remote commands, use keys instead of passwords, and tunnel ports.

chmod

Change file permissions with numeric (755, 644) and symbolic (u+x) modes: clearly explained.

File permissions

Who may read, write and execute: nine bits per file.

Errors that come down to this concept: Permission denied (publickey)

Learn these concepts by typing them: the interactive course builds them up lesson by lesson in a real simulated shell.