SSH & Key Authentication

Secure Shell (SSH)

Command	Purpose
`ssh user@host`	Connect
`ssh -p 2222 user@host`	Custom port
`ssh-keygen -t ed25519`	Generate a key pair
`ssh-copy-id user@host`	Install your public key
`scp file user@host:/path`	Copy over SSH
`sftp user@host`	Interactive file transfer

Key auth uses a private key (~/.ssh/id_ed25519, keep secret) and a public key (~/.ssh/id_ed25519.pub, goes in the server's ~/.ssh/authorized_keys).

Server hardening in /etc/ssh/sshd_config:

PermitRootLogin no
PasswordAuthentication no      # keys only
Port 2222

Apply with systemctl reload sshd.

Exam trap: ~/.ssh must be 700 and authorized_keys 600, or sshd refuses the key. Wrong permissions are the #1 cause of "key auth won't work".