An SSL certificate is a type of digital certificate, which is a set of keys in form. After the SSL certificate is installed on the server, the data between the server and the terminal is transmitted after being encrypted by this key to prevent the data from being stolen and tampered with during transmission. The installed SSL certificate has a certain life cycle and cannot be used indefinitely. One is that after the SSL certificate expires naturally, it will not be able to be used, and you need to find a CA organization to issue a new certificate; the other is that the SSL certificate is revoked by the CA organization before it expires due to some special reasons . But how to judge whether the SSL certificate is within the validity period or whether it has been revoked? At this time OCSP played a vital role.
OCSP (Online Certificate Status Protocol) is an online certificate status protocol, one of two common modes for maintaining the security of servers and other network resources.
When a visitor visits a website with an SSL certificate installed, it will verify whether the SSL certificate has expired through the server interface. Due to some objective reasons such as the network, each connection to verify the foreign server may bring some uncontrollable users Experience and access delay, which is not a small concurrent connection for CA. Therefore, OCSP is generally applied to the server, saving this part of time for the client. The server periodically connects to the CA's OCSP server to verify the validity of a certificate and store it locally. When the client sends a request for certificate status information to the OCSP response server, the server will reply with a response of "valid", "expired" or "unknown". After getting the response, the visitor can visit the corresponding webpage.
To sum up, we can use this analogy. An SSL certificate is like a security door for a website, and an OCSP is a protocol to check whether the security door is open, closed, or valid. Only with this agreement can we know Can this safety door be used normally?