What is a Certificate Revocation List (CRL)?

A Certificate Revocation List (CRL) is a critical security feature that maintains the trustworthiness of digital communications. Managed by a Certificate Authority (CA), a CRL is a directory of digital certificates that have been invalidated before their scheduled expiration dates. This list helps various endpoints, such as web browsers, determine whether a certificate should be trusted.

CRLs are essentially blocklists of certificates that should no longer be trusted. They are utilized by web browsers and other applications to verify the validity and trustworthiness of a certificate. The CRL itself is signed by the CA to prevent tampering and ensure its integrity.

The Role of Digital Certificates

Digital certificates are fundamental to the encryption process, securing communications, and establishing trust in online transactions. They are commonly used with protocols like Transport Layer Security (TLS) and Secure Sockets Layer (SSL). These certificates, issued and signed by a CA, serve as digital proof of the certificate owner’s identity.

When a web browser connects to a site using TLS, it examines the digital certificate for any issues. Part of this examination involves checking if the certificate appears on a CRL. These checks are vital for certificate-based transactions, as they help users verify the identity of the site owner and determine if the digital certificate is trustworthy.

Definition and Structure of CRLs

According to the National Institute of Standards and Technology (NIST), a CRL is a list maintained by a CA that includes certificates it has issued and subsequently revoked before their expiration dates. CRLs contain certificates that have been either permanently revoked or temporarily marked as invalid.

The X.509 standard defines the format and semantics of CRLs in a public key infrastructure (PKI). Each entry in a CRL includes:

• The serial number of the revoked certificate.
• The date of revocation.
• Potentially additional information such as the reason for revocation and any time limitations.

It’s important to note that the CRL does not include expired certificates, and the issuer of the CRL might not always be the same entity as the CA that issued the revoked certificate.

Reasons for Certificate Revocation

Certificates can be revoked for several reasons, including:

• Private Key Compromise: The most common reason for revocation is the compromise of a certificate’s private key.
• Misissuance or Counterfeit Certificates: If a CA discovers that it improperly issued a certificate or a certificate is counterfeit, it will revoke the certificate.
• Changes in Ownership or Status: A certificate may be revoked if the certificate owner no longer owns the domain for which it was issued, or if the certificate owner ceases operations.
• CA Compromise: If the issuing CA itself has been compromised, it may revoke affected certificates.

Revocations are frequent in scenarios where security is paramount. For example, in 2019, several CAs, including Apple and Google, revoked millions of certificates that were mistakenly issued with non-compliant serial numbers.

Importance of CRLs

CRLs are essential for maintaining the security and trustworthiness of digital certificates. They serve several critical functions:

• Alerting Users: CRLs help inform users that a site’s digital certificate is not trustworthy, warning them not to access potentially fraudulent sites.
• Preventing Man-in-the-Middle Attacks: By ensuring that compromised certificates are not trusted, CRLs protect users from various attacks, including man-in-the-middle attacks.
• Maintaining Trust: CRLs help maintain trust in digital certificates by ensuring that only valid and trustworthy certificates are used for secure communications.

Without CRLs, users may inadvertently trust compromised certificates, leading to risks such as data breaches, malware infections, identity theft, and financial loss.

Drawbacks of CRLs

Despite their importance, CRLs have several drawbacks:

• Maintenance Challenges: CRLs are difficult to maintain due to the need for frequent updates. They are often updated weekly, daily, or even hourly, creating a significant administrative burden.
• Inefficiency: Distributing CRLs in real-time is inefficient. When a browser requests a CRL, it receives the entire list of revoked certificates managed by the CA. The browser must then parse the list to check if a specific certificate has been revoked.
• Caching Issues: CRLs are often cached to reduce overhead from repeated downloads. This can create a time gap during which a revoked certificate might still be accepted.
• Inconsistent Handling: Different browsers and operating systems handle CRLs differently, leading to potential vulnerabilities. For instance, some browsers may only check the validity of the server’s certificate, not the entire chain of certificates required for validation.

These issues can cause vulnerabilities, such as denial-of-service (DoS) attacks if the CRL is unavailable or if the caching mechanism leads to outdated information being used.

Alternatives: OCSP and CT Logs

To address some of the inefficiencies of CRLs, alternatives like the Online Certificate Status Protocol (OCSP) and Certificate Transparency (CT) logs have been developed.

• OCSP: This protocol allows browsers to check the status of a certificate in real-time by querying the CA. The CA responds with the certificate’s status (good, revoked, or unknown). OCSP eliminates the need to download and parse entire CRLs.
• OCSP Stapling: This enhancement reduces data transfer and improves privacy. When a website sends its certificate to the browser, it includes (or “staples”) the OCSP response, negating the need for the browser to query the CA directly.
• CT Logs: These logs record all certificates issued for a domain but do not provide revocation information. They serve as an inventory, ensuring that all certificates are transparent and publicly visible.

Conclusion

CRLs are a vital component of web security, ensuring that compromised certificates are not trusted. Despite their challenges, they play an essential role in maintaining the integrity and trustworthiness of digital certificates. Alternatives like OCSP and CT logs offer more efficient ways to manage certificate status, enhancing the robustness of web security.