A certificate authority (CA) is a trusted third party that issues digital certificates. By signing a certificate with its own private key, the CA vouches that a particular public key belongs to a particular subject, such as the owner of a domain name. The role of CAs in Internet PKI is defined in the X.509 certificate profile, RFC 5280.
The CA’s signature is what gives a certificate its weight. RFC 5280 describes how certificates bind a public key to an identity through the issuing authority’s digital signature, and states that CAs are responsible for indicating the revocation status of the certificates they issue, for example by publishing certificate revocation lists.
Trust in the system rests on a relatively small set of root CAs. Web browsers and operating systems ship with a built-in list of trusted CA certificates; any certificate that chains back to one of those roots is accepted automatically. This is convenient, but it concentrates trust: every site you visit securely depends on the discipline of these authorities.
That concentration is also the weakness. A single compromised or negligent CA can issue fraudulent certificates for any website, letting an attacker impersonate it without tripping the usual warnings. Real incidents of CA compromise have led browser vendors to distrust offending authorities and to add mechanisms that make misissuance easier to detect.