CA (certificate authority)
also: certificate authority, trusted CA, root CA, intermediate CA
A trusted organization that issues and signs digital certificates to verify the identity of websites, servers, and users on the internet. CAs use cryptographic keys to digitally sign certificates, making them verifiable and trusted.
A Certificate Authority (CA) is a third-party entity that acts as a trusted intermediary in public key infrastructure (PKI). When a website or server requests a certificate, the CA verifies the applicant's identity and cryptographically signs the certificate with their private key, creating a digitally verifiable credential.
CAs operate in a chain of trust: root CAs are trusted by operating systems and browsers out-of-the-box, and they often issue intermediate certificates that then sign end-entity certificates. For example, when you visit https://example.com, your browser verifies that the certificate was signed by a CA it recognizes (like DigiCert or Let's Encrypt).
Without a valid CA signature, browsers display security warnings because there's no cryptographic proof that the server is who it claims to be. This prevents man-in-the-middle attacks and ensures encrypted connections are actually with the intended destination.