Root Certificate (Root CA certificate, trust anchor)

A root certificate is the self-signed certificate for a certificate authority's top-level key: its issuer and subject are the same, and nothing above it vouches for it. Clients trust it only because an operating system, browser or administrator placed it in their trust store. Root keys are kept offline in hardware security modules and are used mainly to sign intermediate certificates and CRLs.

Why it matters

Every publicly trusted TLS certificate ultimately depends on a root in the client's store, so adding or removing a root changes what millions of devices accept. The Baseline Requirements forbid signing subscriber certificates directly with a root key, which is why chains have intermediates. Roots are long-lived, often 20 years or more, and root programs now push rotation: the Chrome Root Program removes roots whose key material is more than 15 years old on a schedule. Let's Encrypt, for example, introduced ISRG Root YR (RSA-4096) and ISRG Root YE (ECDSA P-384) in its Generation Y hierarchy.

Example

openssl x509 -in root.pem -noout -subject -issuer

For a root, the subject and issuer lines are identical.

Common misconceptions

• The server must send the root certificate. Clients already have it; a root sent by the server is not trusted just because it appears in the chain, and it only adds bytes to the handshake.
• Installing a root certificate is harmless. A root you install can vouch for any website on that device, which is how corporate TLS inspection and some malware intercept HTTPS.
• Root and intermediate certificates are interchangeable files. A root is a trust anchor stored on the client; an intermediate must be served by the server, and mixing them up causes chain errors.