Certificate Revocation List

Definition of Certificate Revocation List

A Certificate Revocation List (CRL) is a digital document containing a list of certificates that have been revoked or deemed invalid by a Certificate Authority (CA) before their intended expiration dates. CRLs help maintain the security and integrity of communication between parties by ensuring that only trusted and valid certificates are used. Users and applications can check CRLs to verify the status of a certificate and avoid trusting compromised or revoked certificates.


The phonetic pronunciation of the keyword “Certificate Revocation List” is:/ˈsɜr.tɪ.fɪ.kət rɪˌvoʊ.keɪ.ʃən lɪst/Here are the individual word pronunciations broken down phonetically:- Certificate: /ˈsɜr.tɪ.fɪ.kət/- Revocation: /rɪˌvoʊ.keɪ.ʃən/- List: /lɪst/

Key Takeaways

  1. Certificate Revocation List (CRL) is a list of digital certificates that have been revoked and are no longer considered valid by the issuing Certificate Authority (CA).
  2. CRLs play a critical role in maintaining the security and reliability of internet communication, as they help to ensure that only trusted digital certificates are used for authentication and encryption purposes.
  3. Web browsers and other applications refer to the CRL to check whether a certificate has been revoked or not. If a certificate is found in the CRL, the application may reject the communication, protecting users from potential security risks.

Importance of Certificate Revocation List

The Certificate Revocation List (CRL) is a vital aspect of internet security because it provides a constantly updated record of digital certificates that have been revoked or are no longer trustworthy.

CRLs ensure that compromised or expired certificates are not used for secure online transactions, protecting the integrity of data and maintaining users’ trust in websites and online services.

By checking the CRL before accepting a certificate, systems can confirm that the certificate is still valid and safe, thus preventing potential cyber-attacks, fraudulent activities, and preserving the overall security of the digital environment.


The main purpose of a Certificate Revocation List (CRL) is to ensure the security and integrity of digital communications within a network, and to bolster the reliability of public key infrastructure (PKI). As a vital part of PKI, the CRL works in tandem with digital certificates issued by Certificate Authorities (CAs) to establish trust and authenticity among different entities engaging in online interactions. Digital certificates serve as credentials for websites, validating their authenticity and ensuring secure communication through encryption. However, when a certificate becomes compromised or invalid, it poses a threat to the security and privacy of the users.

This is where a CRL steps in, providing a comprehensive, up-to-date list of revoked certificates to prevent misuse and maintain the integrity of the entire PKI system. CRLs are primarily used to verify that a digital certificate, which is key to establishing secure connections, is still valid and has not been revoked – considering situations such as security breaches or changes in certificate ownership. When a user accesses a website or an application, their device fetches the website’s digital certificate for evaluation.

During this process, the user’s device can also check for the most recent CRL, which is periodically updated by the relevant CA, to ascertain whether the presented certificate has not been revoked. If a certificate is listed on the CRL, it flags a warning to the user, indicating that the website or application may not be safe, and helps them avoid potential security risks. In summary, Certificate Revocation Lists serve as crucial tools to enhance the security, integrity and trustworthiness of digital communications while reinforcing the overall efficiency of a PKI system.

Examples of Certificate Revocation List

A Certificate Revocation List (CRL) is a list of digital certificates that have been revoked by a Certificate Authority (CA) before their scheduled expiration date. Such a list ensures that the revoked certificates cannot be used for any fraudulent or unauthorized purposes. Here are three real-world examples:

Let’s Encrypt OCSP Stapling:Let’s Encrypt is a widely used free, automated, and open Certificate Authority that issues SSL/TLS certificates for website owners. With OCSP (Online Certificate Status Protocol) stapling, web servers query the CA’s certificate status and periodically attach the most recent CRL in the TLS handshake. This enables clients to efficiently check the validity of the server’s certificate without directly reaching out to the CA. Let’s Encrypt provides a CRL that contains the list of revoked certificates, which clients can access and verify alongside OCSP stapling.

Google Chrome CRLSet:Google Chrome uses a collection called CRLSet to revoke specific intermediate CA certificates. The browser ships with a pre-defined and frequently updated CRLSet that helps protect users from potentially compromised intermediates. Though not a traditional CRL, it serves a similar purpose: it maintains a list of revoked certificates for Chrome users to validate the authenticity and security of sites they visit. Google Chrome chooses to use CRLSet over traditional CRLs and OCSP for efficiency and performance reasons.

OpenSSL-based Certificate Validation:OpenSSL is a widely used open-source software library that provides cryptographic functionality, including SSL/TLS encryption and certificate validation. During the certificate validation process, OpenSSL enables developers to utilize CRLs issued by Certificate Authorities to check the revocation status of specific digital certificates. Developers often incorporate CRL checking into their applications, servers, and networking devices to validate certificates and ensure secure communications.

Certificate Revocation List FAQ

1. What is a Certificate Revocation List (CRL)?

A Certificate Revocation List (CRL) is a list of digital certificates that have been revoked by the issuing Certificate Authority (CA) before their scheduled expiration date. These certificates are considered untrustworthy and should not be used for secure communication or identification.

2. Why is a CRL important?

A CRL is important because it helps maintain the integrity and security of digital certificate systems. By providing a list of revoked certificates, it helps prevent unauthorized access, identity theft, and secure communication failures caused by using compromised or revoked certificates.

3. How does a CRL work?

A CRL works by providing a list of revoked certificates to systems that rely on SSL/TLS certificates for secure communication. When a certificate is revoked, its details are added to the CRL. Whenever a system receives a certificate for authentication, it checks the CRL to ensure the certificate is not on the list. If it is, the certificate is considered untrustworthy, and the communication is rejected.

4. How often are CRLs updated?

The frequency of CRL updates depends on the Certificate Authority’s (CA) policy and the specific certificate environment. Some CAs may update their CRLs daily, while others may have longer update intervals, such as weekly or monthly. Ensuring that the CRL is regularly updated is crucial to maintain the integrity and security of digital certificate systems.

5. What is the difference between CRL and OCSP?

Both Certificate Revocation List (CRL) and Online Certificate Status Protocol (OCSP) are methods to assess the validity of digital certificates. The key difference is in how they work. A CRL provides a list of all revoked certificates, while OCSP allows clients to query the certificate’s status individually. OCSP offers a more efficient and real-time approach to verifying certificate status, while CRLs may require additional processing and time to download the entire list.

Related Technology Terms

  • X.509 Digital Certificates
  • Public Key Infrastructure (PKI)
  • Certificate Authority (CA)
  • Online Certificate Status Protocol (OCSP)
  • Revocation Reasons

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