What is a Certificate Revocation List? CRL Explained

Digital certificates form the backbone of secure online interactions, verifying identities and ensuring encrypted communication. However, when these certificates are compromised or misused, they must be promptly revoked to maintain trust. This is where Certificate Revocation Lists (CRLs) come in. Maintained by Certificate Authorities (CAs), CRLs are essential for identifying and invalidating revoked certificates before they can cause harm.

In this guide, we’ll explore what CRLs are, how they function, and why they are critical for web security.

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Table of Contents

1. What is a Certificate Revocation List (CRL)?
2. How Do CRLs Work?
3. Why Are Certificates Revoked?
4. CRLs vs. Alternatives: OCSP and Certificate Transparency Logs

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What is a Certificate Revocation List (CRL)?

A Certificate Revocation List (CRL) is a digitally signed file created by Certificate Authorities (CAs) that lists revoked digital certificates. These certificates are revoked prior their scheduled expiration date due to reasons like security breaches, key compromises, or administrative changes. CRLs are critical in Public Key Infrastructure (PKI) for ensuring secure communication by preventing the use of compromised certificates.

Defined under the X.509 standard and RFC 5280, a CRL contains critical details such as the serial numbers of invalid certificates, revocation timestamps, the revocation date, and, in some cases, specific reasons for revocation. This mechanism ensures revoked certificates are flagged during the authentication process.

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How Do CRLs Work?

Certificate Revocation Lists operate as a trust-verification system within the Public Key Infrastructure (PKI). When a digital certificate is revoked, its details are added to the CRL maintained by the issuing Certificate Authority (CA). This list is updated periodically and distributed through specified CRL Distribution Points (CDPs), which can be accessed via URLs embedded in the certificate.

The CRL Process

When a web browser or application encounters a certificate, it retrieves the associated CRL from the CDP. The retrieved list is scanned for the certificate’s serial number to check certificate revocation status. If a match is found, the certificate is marked as revoked, and the user is warned about the potential risk, preventing improper certificate acceptance. This process helps block insecure connections before they occur.

CRLs are typically signed by the issuing CA to ensure their authenticity and to prevent tampering. They include a timestamp and details about the next scheduled update. Frequent updates are necessary to maintain reliability but can pose performance challenges, particularly for large CRLs.

Challenges in CRL Functionality

CRLs are not without limitations. Their reliance on periodic updates creates a latency window during which a revoked certificate might still be accepted. Furthermore, the size of CRLs can impact efficiency; larger lists require more resources to parse, leading to slower responses, particularly for devices with limited processing power. Caching CRLs locally mitigates some delays but introduces additional risks if updates are missed.

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Why Are Certificates Revoked?

Digital certificates are revoked when their integrity or validity is compromised, ensuring that they cannot be misused to breach secure systems. This proactive measure by Certificate Authorities (CAs) prevents malicious actors from exploiting revoked certificates to bypass security protocols or deceive users.

Certificates may be revoked for various reasons, including:

1. Key Compromise: If the private key associated with a certificate is exposed or suspected to be compromised, revocation is necessary to mitigate unauthorized access.
2. CA Compromise: When the issuing CA’s security is breached, all certificates issued by it may lose trustworthiness.
3. Misissued Certificates: Errors during issuance, such as incorrect domain validation, necessitate revocation and reissuance.
4. Ownership Changes: Transitions in domain ownership or organizational affiliation often require certificates to be revoked and replaced.
5. Cessation of Operations: When a certificate holder discontinues business or ceases control over the domain, revocation ensures the certificate is not misused.

Compromised certificates pose significant risks to cybersecurity. A revoked certificate that is not flagged in time can facilitate man-in-the-middle attacks, identity theft, data breaches, and malware distribution. For instance, large-scale revocations by CAs like Apple and Google underscore the criticality of addressing vulnerabilities swiftly.

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CRLs vs. Alternatives: OCSP and Certificate Transparency Logs

While Certificate Revocation Lists are widely used for managing revoked certificates, they are not the only method. Alternatives like the Online Certificate Status Protocol (OCSP) and Certificate Transparency (CT) Logs offer distinct approaches to addressing certificate trust issues.

CRLs vs. OCSP

• CRLs rely on periodically updated lists that browsers or applications download and parse. While effective, this offline approach can introduce delays, especially for large lists.
• OCSP provides real-time validation by allowing browsers to query the CA directly about a specific certificate’s status. Instead of downloading an entire list, the browser receives a simple response: “good,” “revoked,” or “unknown.”
• OCSP Stapling, an enhancement, shifts the burden from the client to the server. The server caches the OCSP response and “staples” it to the certificate during the TLS handshake, improving performance and privacy.

CRLs vs. CT Logs

• CRLs focus exclusively on revoked certificates and do not address expired certificates, ensuring compromised credentials cannot be trusted.
• Certificate Transparency Logs, on the other hand, record all issued certificates. These append-only logs enhance transparency by exposing misissued or rogue certificates, but they do not address revocation status.

Choosing the Right Approach

CRLs remain effective for batch management of revoked certificates but may struggle with scalability and real-time requirements. Certificate revocation checking through OCSP and its stapling variant offers faster and more dynamic checks, addressing some of CRL’s limitations. Meanwhile, CT Logs serve as a complementary tool, ensuring oversight of certificate issuance but not revocation.

For comprehensive security, organizations often combine these methods. CRLs are ideal for environments requiring bulk updates, while OCSP provides instantaneous status checks. CT Logs add an additional layer of transparency, ensuring trust across digital ecosystems.

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Keep Your Website Secure with Trusted Solutions

Certificate Revocation Lists play an essential role in safeguarding digital communications by ensuring revoked certificates cannot be used to compromise security.

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