What is a root?

Trust has to start somewhere. When your browser accepts the certificate of a website, it has not met the website. It follows a chain of signatures to a certificate that was already on your computer before you opened the browser. That certificate, and the key behind it, is a root.

Two places a root shows up

	X.509 (certificates, TLS, code signing)	OpenPGP (signatures on software, mail, git)
The root is	A root CA certificate and its private key.	The primary key, also called the master key, of an identity.
Outsiders trust it because	It is installed in their trust store: the operating system’s, the browser’s, or a file your company ships.	They hold its public key in a keyring, and they have checked the fingerprint some other way.
What it signs	Certificates of intermediate CAs.	Certifications of identities and the bindings of subkeys.
Looks like	A file of a few hundred bytes of public data.	A fingerprint of 40 hexadecimal characters.

In both worlds the root is a public thing and a private thing. The public part is given to everyone, because everyone needs it to verify. The private part is what this site is about.

Why a root is special

Three facts make a root unlike any other key.

1. Nobody above it vouches for it. An intermediate that is stolen can be revoked by the root. A root that is stolen can be revoked by nobody except itself, or by a new root that outsiders are persuaded to install. Persuading them takes weeks, or years.
2. It is trusted for everything below it. Whoever holds the private key of a root can sign an intermediate, or a binding, that every client of that root will accept. The key does not know who is asking.
3. It is used rarely. A root signs a few certificates or bindings a year. That rarity is what lets you keep it somewhere safer, and less convenient, than any other key.

What goes wrong

• Lost. The root key is gone, and nobody can sign under it again. Existing certificates keep working until they expire, and then everything has to move to a new root, which every client must learn to trust.
• Stolen. Someone else can sign things your clients accept, until the root is revoked and the revocation reaches the clients.
• Used by mistake. A person signs the wrong request, with a wrong lifetime or a wrong name. The signature is real, and cannot be taken back.
• Expired unnoticed. A root with a fixed end date runs out on a bad day.

Most of what the rest of Learn explains is a way to avoid one of these four. Next: root and intermediate.