What is a Certificate Revocation List?

What is a certificate revocation list?

A Certificate Revocation List (CRL) is a mechanism used in the field of public key infrastructure (PKI) to check the validity and status of digital certificates. A digital certificate is a cryptographic document that binds an entity’s identity (such as a person, organization, or device) to a public key. The certificate provides assurance that the public key belongs to the entity it claims to represent.

However, there may be situations where a digital certificate needs to be revoked before its expiration date. This could happen if the private key associated with the certificate is compromised, the entity’s information changes, or the certificate is found to be invalid or fraudulent. In such cases, the entity responsible for issuing the certificate (known as the Certificate Authority or CA) generates a Certificate Revocation List.

A CRL is a time-stamped list that contains the serial numbers of revoked certificates, along with additional information such as the date of revocation and the reason for revocation. The CRL is signed by the CA to ensure its integrity and authenticity. The CRL is then made available to users and relying parties who need to verify the validity of certificates.

When a client or application receives a digital certificate, it can refer to the CRL to check if the certificate has been revoked. If the certificate’s serial number appears in the CRL, it means the certificate is no longer considered valid, and its use should be denied.

CRLs have been widely used as a method to manage certificate revocation, but they have some limitations, such as the need for regular updates and the potential for large file sizes as the number of revoked certificates increases. To address these limitations, alternative mechanisms like Online Certificate Status Protocol (OCSP) have been developed to provide more real-time certificate status checks.

What is the difference between certificate revocation list and OCSP?

The main difference between a Certificate Revocation List (CRL) and Online Certificate Status Protocol (OCSP) lies in how they provide information about the revocation status of digital certificates in a public key infrastructure (PKI). Here are the key distinctions:

Structure and Delivery:

• CRL: A CRL is a time-stamped list that contains the serial numbers of revoked certificates. It is typically published by a Certificate Authority (CA) periodically (e.g., daily or weekly). The CRL is a file that needs to be downloaded and checked locally by clients.
• OCSP: OCSP is a protocol that enables real-time validation of certificate revocation status. When a client wants to check the status of a certificate, it sends a query to an OCSP responder. The responder then provides an immediate response indicating whether the certificate is valid, revoked, or unknown.

Timeliness:

• CRL: The revocation information in a CRL is only as up-to-date as the last publication of the list. Clients need to periodically download and update the CRL to ensure they have the latest revocation information. There may be a delay between the revocation of a certificate and its appearance on the next CRL.
• OCSP: OCSP offers real-time or near real-time revocation status checks. When a client sends an OCSP query, it receives an immediate response from the OCSP responder indicating the current status of the certificate.

Network Communication:

• CRL: The CRL needs to be downloaded by clients from a predetermined distribution point. This can introduce additional network traffic and delays, especially if the CRL is large.
• OCSP: OCSP queries are sent over the network to OCSP responders, which are dedicated servers designed to provide revocation status information. The communication is typically faster and more efficient, as only the necessary data is exchanged.

Scalability:

• CRL: As the number of revoked certificates increases, the size of the CRL can grow significantly, making the download and processing of the list more resource-intensive.
• OCSP: OCSP queries and responses are typically smaller in size, making them more scalable, especially in environments with a large number of revoked certificates.

In summary, while CRLs provide a static list of revoked certificates that requires periodic updates, OCSP enables real-time certificate revocation status checks by querying dedicated OCSP responders. OCSP offers more timely information, reduces network traffic, and provides scalability advantages compared to CRLs. However, both CRLs and OCSP have their specific use cases and can coexist within a PKI infrastructure.

What is the major disadvantage of using certificate revocation lists?

One major disadvantage of using Certificate Revocation Lists (CRLs) is the potential delay in obtaining up-to-date revocation information. Here are the key reasons why CRLs can present a challenge:

• Timeliness: CRLs are typically published periodically, such as daily or weekly, by the Certificate Authority (CA). This means that the revocation status of a certificate may not be immediately reflected in the CRL. There could be a delay between the revocation event and its inclusion in the next published CRL. During this period, a revoked certificate may still be considered valid by relying parties.
• Distribution and Download: CRLs need to be downloaded by clients from a predetermined distribution point. This process introduces additional complexity and overhead, as clients need to actively retrieve the CRL and update it regularly to ensure they have the latest revocation information. The downloading process can consume network bandwidth and increase latency, especially if the CRL is large or if the network connection is slow.
• CRL Size and Performance: As the number of revoked certificates increases, the size of the CRL can grow significantly. This can lead to longer download times, increased storage requirements, and additional processing overhead when validating certificates against the CRL. Handling large CRLs can be resource-intensive and impact the overall performance of the system.
• Offline Validation: CRLs require clients to have a local copy of the CRL to perform certificate validation. This poses challenges for devices or systems with limited storage capacity or intermittent connectivity, as they may not be able to retrieve and update the CRL regularly. Consequently, relying solely on CRLs for certificate revocation checks may not be feasible in certain scenarios.
• Revocation Checking Overhead: Each time a certificate needs to be validated against the CRL, the entire CRL needs to be processed to find the relevant entry. This can become time-consuming and inefficient, especially when dealing with large CRLs. The processing overhead can impact the performance of certificate validation operations.

To address these limitations, alternative methods such as Online Certificate Status Protocol (OCSP) have been developed to provide more real-time certificate revocation checks. OCSP offers immediate responses about the current status of a certificate, reducing the delay and overhead associated with CRL-based revocation checks.

Who maintains a certificate revocation list?

Certificate Revocation Lists (CRLs) are maintained and published by the Certificate Authority (CA) that issued the digital certificates. The CA is responsible for managing the entire lifecycle of digital certificates, including revocation when necessary.

When a certificate needs to be revoked due to reasons such as compromise, expiration, or changes in the certificate holder’s status, the CA generates a CRL that includes the serial numbers of the revoked certificates along with additional information like the date of revocation and the reason for revocation.

The CA is typically an organization or entity entrusted with the task of verifying and attesting to the identities of individuals, organizations, or devices in a public key infrastructure (PKI) system. CAs play a vital role in establishing trust and security by issuing digital certificates and maintaining the revocation status information through mechanisms like CRLs.

It’s worth noting that there can be multiple CAs operating within a PKI system, each responsible for maintaining their own CRLs. Therefore, relying parties, such as clients or applications, need to obtain the appropriate CRLs from the respective CAs to check the revocation status of certificates issued by those CAs.