OCSP

What is OCSP?

The OCSP is a network protocol used to verify the validity of digital certificates. It was created as an alternative to CRLs and solves the problem of frequently downloading updates to keep the list current.

When a user accesses a server, OCSP sends a request for the status of the certificate. The server returns the status of the certificate, telling it whether it is 'valid', 'expired' or 'unknown'. This protocol defines how the certificate server and the client communicate with each other.

OCSP checks the status of security certificates in real time, which is especially important for extended validation of Secure Socket Layer (SSL) and Transport Layer Security (TLS) certificates. In the case of establishing a HTTPS connection, browsers perform an OCSP check with a certificate authority (CA) in order to verify that the certificate is still active and hasn't been revoked. This check, however, might delay the SSL handshake, but it gives the users access to the server until the certificates are renewed because it allows access to the server with expired certificates.

How OCSP Works?

When the validity of a certificate needs to be verified, an OCSP request is sent to an OCSP responder managed by the CA. This responder validates the request and returns the status of whether the certificate is valid, revoked, or unknown. OCSP is supported by most popular browsers such as Apple Safari, Internet Explorer, Microsoft Edge and Mozilla Firefox.

OCSP vs. CRL

In order to confirm whether a website certificate has been revoked, browsers typically use both OCSP and CRL. The CRL work as is a list of serial numbers of  certificates, which are revoked by CA, but it can be out of date and needs to be downloaded periodically to stay up to date.

While unlike CRL, OCSP verifies the cert's revocation status in real time improving security and signing as it ensures that the cert is valid. By providing instant status updates, this process is so much more efficient than downloading the entire list.

Challenges of OCSP

Although OCSP is very effective, it faces some challenges such as increased costs for CAs and privacy issues. Real-time OCSP checking can compromise a user's private browsing data because requests are sent over unencrypted HTTP and are associated with specific certificates. It means that nobody intercepting the traffic between the browser and the OCSP responder would get to know which sites the user has visited. Furthermore, if you have to go through a third party for your identity, it can be a little slower browsing experience.

OCSP binding technology was just the thing to deal with these problems. This technology enables the existing OCSP response to be contained in an HTTPS connection and provides privacy (reducing the number of times the browser needs to send out a separated request) and time savings (reducing the transmission of data between server and browser).

Advantages of OCSP over CRLs

OCSP has several distinct advantages over CRLs in the certificate validation process:

Real-time checking: OCSP is able to instantly verify the certificate status, thus quickly blocking revoked certificates, whereas CRLs require periodic updates, which can result in delayed propagation of revocation information.

Flexible architecture: OCSP can scale independently to cope with high volumes of traffic, and Certificate Authorities (CAs) can also delegate request processing to third-party responders, thereby increasing efficiency.

Lightweight operation: OCSP requests are very small, compared to CRLs that require downloading large lists of revoked certificates with higher bandwidth requirements, thus reducing network burden.

Detailed Diagnostics: OCSP responders can provide specific information about the status of a certificate and the reason for revocation, not just a simple 'good' or 'revoked'.

No Fixed Renewal Cycle: OCSP allows continuous requests without having to wait for fixed renewals, making it faster to respond to changes in certificate status.

These advantages make OCSP the preferred solution in environments where up-to-date and efficient certificate validation is required.