eMudhra Blog OCSP vs CRL

OCSP vs CRL: What's the Key Difference?

By eMudhra Editorial on July 05, 2023
OCSP vs CRL: What's the Key Difference?

The use of digital certificates is the cornerstone of trust-building in the digital ecosystem. However, with mass deployments of these certificates, the potential threat posed by compromised certificates, and the consequent need to revoke a large number of SSL certificates, the topic of certificate revocation has taken center stage. Often overlooked, certificate revocation is a critical function of certificate lifecycle management.

Digital certificates serve as trusted credentials that validate the identity of individuals, websites, and other entities, facilitating secure communication and transactions over the Internet. However, the revocation of these certificates, when necessary, poses a significant challenge, necessitating the implementation of robust revocation methods. Certificate Revocation Lists (CRLs) and the Online Certificate Status Protocol (OCSP) have emerged as two prominent methods for managing and disseminating revocation information. As organizations strive to maintain a secure digital ecosystem, understanding the difference, advantages, and limitations of these revocation techniques becomes imperative.

What is a CRL? A Quick Overview of Certificate Revocation Lists

A Certificate Revocation List (CRL) is a foundational component of certificate revocation. It serves as a trusted source for disseminating information about invalidated digital certificates. A CRL is a digitally signed, periodically updated document that contains a list of revoked certificates issued by a Certificate Authority (CA). It serves as a reference point for relying parties to determine the validity and trustworthiness of a given certificate.

CRLs follow a standardized format and include essential details such as the serial numbers of revoked certificates, the date of revocation, and the reason for the revocation. By consulting CRLs, organizations can mitigate the risks associated with compromised or expired certificates, ensuring the integrity and security of their digital ecosystems. While CRLs offer a proven and widely implemented method of certificate revocation, their effectiveness depends on timely updates and efficient distribution mechanisms.

What Is OCSP? An Overview of the Online Certificate Status Protocol

The Online Certificate Status Protocol (OCSP) represents a dynamic, real-time approach to certificate revocation, addressing the limitations of traditional Certificate Revocation Lists (CRLs). OCSP allows relying parties to query a Certificate Authority (CA) or an OCSP responder to ascertain the status and validity of a particular digital certificate. Unlike CRLs, which require periodic updates and distribution, OCSP provides instantaneous responses, enabling prompt verification of certificates.

When a query is made, the OCSP responder checks its local cache or contacts the issuing CA to determine if the certificate is valid, revoked, or expired. This streamlined process offers advantages such as reduced bandwidth requirements and improved scalability, particularly in environments with a high volume of certificates. However, it is important to consider the potential privacy implications, as well as the availability and response time dependencies associated with OCSP.

Monitoring OCSP and CRL

Monitoring both Certificate Revocation Lists (CRLs) and the Online Certificate Status Protocol (OCSP) is crucial due to their critical role in maintaining the security and trustworthiness of digital certificates. By actively monitoring CRLs, organizations can promptly detect and respond to revoked or compromised certificates. Regular monitoring ensures that CRLs are up to date, enabling relying parties to accurately assess the validity and trustworthiness of certificates within their digital ecosystem. Similarly, monitoring OCSP responses allows organizations to verify the real-time status of certificates, ensuring their continued validity and identifying any potential revocations.

The Need for CRL and OCSP

The widespread adoption of digital certificates necessitates robust revocation mechanisms, fulfilled by Certificate Revocation Lists (CRLs) and the Online Certificate Status Protocol (OCSP). The importance of these revocation methods arises from the need to ensure ongoing trust and security in digital communications. CRLs and OCSP serve as indispensable tools to promptly invalidate compromised or expired certificates, thereby mitigating potential security breaches.

OCSP vs CRL: A Quick Comparison of the Two Revocation Check Methods

The Certificate Revocation List (CRL) is a traditional approach to revocation, established long before the advent of the Internet. It is essentially a periodically updated list of revoked certificates that are distributed by the certificate authority (CA) to relying parties. While CRLs have a well-defined structure and proven reliability, they suffer from certain inherent limitations, such as the need for frequent updates and the associated bandwidth requirements.

In contrast, the Online Certificate Status Protocol (OCSP) emerged as a response to the limitations of CRLs. OCSP offers real-time certificate revocation information by enabling clients to query the CA or an OCSP responder for the status of a specific certificate. However, its reliance on constant communication with the CA or OCSP responder introduces potential concerns related to privacy, availability, and response times.

OCSP vs CRL: Comparison Guide

Online Certificate Status Protocol (OCSP) and Certificate Revocation List (CRL) are two methods used to check the revocation status of digital certificates. Here's a quick comparison of the two:


OCSP: The OCSP is a real-time method of verifying the revocation status of a particular certificate. When a client needs to verify a certificate, it sends a request to the OCSP responder (a server), which in turn returns the status of the certificate.

CRL: A CRL is a list of certificates that have been revoked by a Certificate Authority (CA) before their scheduled expiration date. The CA makes this list publicly available. To verify the status of a certificate, clients download the latest CRL and check if the certificate is on the list.


OCSP: OCSP can be faster than CRL because it requires only a single, specific query to the OCSP responder, thus using less bandwidth.

CRL: Depending on the size of the CRL, downloading and parsing it could be a significant operation, taking time and consuming resources, especially for large CAs with many revoked certificates.

Freshness of Information:

OCSP: Since OCSP provides real-time responses, the information is usually very fresh, potentially only minutes or even seconds old. This is advantageous in cases where certificates may be revoked and need to be checked frequently.

CRL: CRLs are updated periodically, so there's a chance that a client could download a CRL just before it's updated, thus missing recent revocations.


OCSP: OCSP requests can reveal to the CA or OCSP responder which sites a user is visiting, which can be seen as a privacy concern.

CRL: With CRLs, the client downloads the entire list and checks it locally, which can provide more privacy.


OCSP: A downside of OCSP is that if the OCSP responder is down or unreachable, the client can't check the certificate status.

CRL: If a CRL can't be downloaded, the client can still use its cached CRL, although the information may be outdated.


OCSP Stapling: This is a method where the server, rather than the client, queries the OCSP responder, and then "staples" the response to the certificate it sends to the client. This technique reduces the privacy concerns associated with OCSP, cuts down on the client's needed bandwidth, and also handles the situation where the OCSP responder is unavailable.

CRL Sets/Short-Lived CRLs: Google's Chrome browser, for example, uses an approach called CRL Sets, where it pushes a list of revoked certificates to the browser. These are usually certificates for high-profile sites where an incorrectly issued or malicious certificate would be very damaging. Additionally, "short-lived" CRLs are another alternative, where CRLs have a very short validity period and are re-issued often.

Both OCSP and CRL have their pros and cons, and some of the downsides can be mitigated using additional techniques, like OCSP Stapling or short-lived CRLs. The best choice between the two depends on the specific needs and constraints of the situation.

Since this article is about OCSP and CRL which help provide information about revoked certificates, it becomes pertinent to write about Certificate Lifecycle Management (CLM) that ensures certificates never expire or get revoked without prior notice. emDiscovery is on such system that monitors and manages digital certificates. It ensures the security of digital transactions, prevents downtime due to expired certificates, helps avoid compliance issues, and promptly addresses potential vulnerabilities by identifying and revoking compromised certificates, thereby safeguarding the IT infrastructure.

At eMudhra, we understand the importance of trust in the digital landscape. We ensure the integrity and trustworthiness of digital transactions. eMudhra, a renowned provider of digital identity and trust services, offers a comprehensive suite of solutions aimed at simplifying and streamlining certificate lifecycle management. emDiscovery platform stands out as an indispensable tool for efficient and effective certificate management as it empowers organizations with robust capabilities to centrally manage and monitor their entire certificate ecosystem. With a user-friendly interface and advanced features, this platform enables businesses to oversee the entire lifecycle of digital certificates from issuance to discovery and deployment.

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