How do certificate authorities verify digital certificates?

Certificate authorities verify digital certificates by checking the certificate's signature using the issuer's public key.

In more detail, a Certificate Authority (CA) is a trusted entity that issues digital certificates, which are data files used to cryptographically link an entity with a public key. The process of verification involves a series of steps that ensure the authenticity and integrity of the digital certificate.

Firstly, the CA checks the digital signature on the certificate. This signature is created by the issuer of the certificate using their private key. The CA uses the issuer's public key to decrypt the signature. If the decrypted signature matches the certificate's data, it confirms that the certificate is indeed issued by the claimed issuer and has not been tampered with.

Next, the CA verifies the certificate's validity period. Every digital certificate has a specific period during which it is considered valid. If the current date falls within this period, the certificate is deemed valid.

The CA also checks the certificate revocation list (CRL). This is a list of certificates that have been revoked by the issuer before their scheduled expiry date due to various reasons such as compromise of the private key. If the certificate is found on this list, it is considered invalid.

Furthermore, the CA verifies the certificate's usage. Certificates are issued for specific purposes, such as server authentication, client authentication, or code signing. The CA checks whether the certificate is being used for its intended purpose.

Lastly, the CA checks the certificate's binding. A digital certificate binds a public key to the entity that holds the corresponding private key. The CA verifies this binding to ensure that the entity presenting the certificate is the actual owner of the public key.

In summary, the verification of digital certificates by a CA involves checking the certificate's signature, validity period, revocation status, usage, and binding. This process ensures the authenticity, integrity, and trustworthiness of the digital certificate and the entity it represents.