Certificate Authority (CA)
A Certificate Authority issues and signs TLS certificates that browsers and operating systems trust. CAs verify domain ownership (DV), organization identity (OV), or extended validation (EV) before signing. The CA system relies on ~150 root certificates pre-installed in trust stores. Let's Encrypt automated DV issuance, issuing 400M+ active certificates.
Definition
Certificate Authorities are trusted third parties that sign TLS certificates, binding a public key to a domain name. Browsers and OSes ship with a trust store of ~150 root CA certificates. Any certificate signed by a trusted CA (or an intermediate CA chaining to a root) is accepted by default. CAs perform domain validation before signing: HTTP-01 (serve a file at a specific URL), DNS-01 (create a specific DNS TXT record), or TLS-ALPN-01. The ACME protocol (RFC 8555) automates this process – Let's Encrypt issues free 90-day DV certificates automatically. Certificate Transparency (CT) requires CAs to log all issued certificates to public append-only logs. Browsers reject certificates not logged in CT, preventing secret certificate issuance. If a CA is compromised or behaves improperly (DigiNotar 2011, Symantec 2017), it is removed from trust stores.
Examples
- –certbot certonly --webroot -w /var/www -d example.com (Let's Encrypt via ACME)
- –openssl x509 -in cert.pem -issuer -noout shows the issuing CA
- –Certificate Transparency: crt.sh searches CT logs for certificates issued for a domain
Related Protocols
Related Terms
Certificate Pinning
Certificate pinning restricts which TLS certificates a client accepts for a specific domain, beyond standard CA validation. The client stores expected certificate hashes and rejects connections presenting different certs – even if CA-signed. Pins prevent MITM via compromised CAs but cause outages if pins are not rotated before certificate renewal.
OCSP (Online Certificate Status Protocol)
OCSP is a protocol for checking whether a TLS certificate has been revoked in real-time. Instead of downloading entire CRL lists, clients query the CA's OCSP responder for a single certificate's status (good/revoked/unknown). OCSP stapling lets the server fetch and cache the response, avoiding client-side privacy leaks and latency.
CRL (Certificate Revocation List)
A CRL is a signed list of revoked certificate serial numbers published periodically by a CA. Clients download CRLs to check if a presented certificate has been revoked. CRLs are large (megabytes), updated infrequently (hours to days), and rarely checked by browsers. OCSP and short-lived certificates are the modern replacements.