How do you choose between CRL and OCSP in your PKI design? (2024)

CRL is a list of serial numbers of revoked certificates, signed by the certificate authority (CA) that issued them. The CA periodically publishes the CRL on a public location, such as a web server or a directory service. The relying parties download the CRL and check if the certificate they are verifying is on the list. If it is, they reject the certificate as invalid. If it is not, they accept the certificate as valid.

OCSP is a protocol that allows the relying parties to query the CA or a delegated responder about the revocation status of a specific certificate. The relying party sends an OCSP request, containing the serial number of the certificate, to the responder. The responder replies with an OCSP response, indicating whether the certificate is valid, revoked, or unknown. The relying party accepts or rejects the certificate based on the response.

CRL has several advantages compared to OCSP, such as reducing latency and bandwidth consumption, enhancing privacy and security, and improving reliability and scalability. However, CRL also has some drawbacks, such as potentially not reflecting the most recent revocation status of certificates, being large and cumbersome to download and store, and not supporting finer-grained revocation information.

OCSP provides real-time or near-real-time revocation status of the certificates and is more efficient and flexible than CRL. It can also provide more detailed revocation information, such as the reason or the time of revocation. However, OCSP requires a network connection to the responder for every certificate verification, which increases latency and bandwidth consumption. Additionally, it exposes the identity or activity of the relying party to the responder, compromising privacy and security. Furthermore, it depends on the availability and performance of the responder, which may affect reliability and scalability.

Choosing between CRL and OCSP depends on various factors, such as the size and frequency of certificate issuance and revocation, the network and storage resources, privacy and security requirements, and performance expectations. Generally, CRL may be preferred if there is a small or stable number of certificates, a low or infrequent rate of revocation, a limited or unreliable network connection, a high or strict demand for privacy and security, and a low or flexible tolerance for latency and stale data. Alternatively, OCSP may be preferred if there is a large or dynamic number of certificates, a high or frequent rate of revocation, a sufficient or reliable network connection, a low or relaxed demand for privacy and security, and a high or strict tolerance for latency and fresh data. Other approaches to consider include using both CRL and OCSP for different types of certificates, OCSP stapling to reduce load and exposure, OCSP must-staple to enforce verification and freshness, and CRL sets to reduce size and frequency of updates.

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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