Which entity acts as a trusted third party in the public key ecosystem?

In the public key ecosystem, the Certificate Authority (CA) serves as the trusted third party. The primary role of a CA is to issue digital certificates that authenticate the ownership of a public key by the entity that holds the corresponding private key. By doing so, the CA provides assurance that the public key can be trusted and is linked to the correct owner.

When users receive a digital certificate from a CA, they can verify that the certificate was issued by a legitimate entity, establishing trust in the identity of the owner. This process is crucial for secure communications over networks, such as those using HTTPS, where the identity of websites needs to be verified to prevent man-in-the-middle attacks.

While the Registration Authority (RA) plays a role in the initial validation and registration of entities requesting certificates, it does not directly issue certificates, but instead works in conjunction with the CA. The Certificate Revocation List (CRL) and Online Certificate Status Protocol (OCSP) are used for checking the revocation status of certificates but do not function as trusted parties themselves. Thus, the CA is central to establishing trust in the public key infrastructure.