SSH Key Authentication Setup Guide

This is an original planning guide. It is not a vendor manual, an official standard, or legal advice. Example addresses use documentation ranges. Review every command before you paste it.

What this hour is for

The key generator builds an Ed25519 or RSA pair in the browser. Its own line is that the private key never left the page. You still have to put that private key in a password store, not in chat and not in the ticket.

Prefer Ed25519 when both sides can use it. RSA is the fallback you write down, not a second key you also install. The fingerprint calculator is how you compare the public key you are about to trust. Read it.

authorized_keys options such as from= and command= restrict a key. They are not a backdoor and not a way to hide a login. A key without a person named beside it is a key you do not install.

Do not email the private key. Do not generate a key for a host you do not administer. 192.0.2.10/24 and example.com are documentation. They stay in the note.

The change you can finish

Apply nothing while you are writing the note. When the lab window opens, it is one change on the host named in the ticket. Read the draft, then the host, then decide.

Rollback is the file, the package version, or the snapshot you saved first. You do not invent a rollback after the only copy is gone.

Name the person and the host.
Generate one key in the browser.
Store the private key in the password store.
Read the fingerprint.
Draft authorized_keys with a restriction only if the ticket names one.
Do not install the key during the reading.

Mistakes that outlive the command

SSH Key Authentication Setup Guide goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.

The private key in chat or in git.
Two key types installed for one person.
A key with no named owner.
from= used to hide a login.
A passphrase left empty because the form allowed it, without a decision.

Decisions to write down

One decision for SSH Key Authentication Setup Guide: One person, one key. Ed25519 unless the ticket says RSA. The private key is in the store. The fingerprint was read. And the public key is the only part that may go on the host, later.

Checks before you call it done

Confirm this before you call the change done: The person is named. The fingerprint is written. The private key is not in the ticket. No key was installed during the reading. And a second person can say where the private key is. If one of those fails, stop.

# Private key stays in the password store.
# The generator does not upload it.
# Public key only, later, on a host you administer.

Before you leave the host

SSH Key Authentication Setup Guide ends when the hostname in the prompt matches the hostname in the ticket. If it does not, you are on the wrong host. Stop. A command that succeeded on the wrong host is not progress.

The draft stays a draft until a person has read the names in it. A generator does not log into the host and does not know which disk is empty. You delete a line that names a host, a path, or an account you did not write down.

192.0.2.10/24, gateway 192.0.2.1, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. They belong in the note. They are not values you leave on a host because a draft included them.

prompt-for-this is a reminder that a secret is missing. It is not a password, a key, or a token you save in the ticket or in git. The real secret stays in the password store.

One change is the window. If two things move and the result is wrong, you will not know which one did it. The next change waits for its own page.

If the same step fails twice, stop for the day. The failure is the result. Adding a forum command beside a half-applied change produces a host nobody can rebuild. Restore the copy you took first.

A second person reads the hostname and the change aloud. If they cannot find them in the ticket, the note is not done. Silence is not agreement. Then leave the host alone.

This page is a planning guide. It is not a vendor manual, not an official standard, and not permission to touch a host you do not administer. You do not disable a firewall, AppArmor, or SELinux to make a check pass.

Tools for this guide

These tools run in the browser on this site. They do not log into your machines.

Linux File Permissions Explained

More in this cluster

The pillar for this page, and the next published pages filed with it.

Companion: Security: SSH Hardening Guide.