What are the best practices for implementing CRL and OCSP in a scalable and secure way? (2024)

Last updated on Aug 9, 2024

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CRLs: pros and cons

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OCSP: pros and cons

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CRL distribution points

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OCSP stapling

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Hybrid solutions

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Best practices

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Here’s what else to consider

If you use public key infrastructure (PKI) to secure your communications and transactions, you need to know how to revoke certificates that are compromised, expired, or no longer needed. Certificate revocation lists (CRLs) and online certificate status protocol (OCSP) are two methods for checking the validity of certificates, but they have different advantages and disadvantages. In this article, we will explain what CRLs and OCSP are, how they work, and what are the best practices for implementing them in a scalable and secure way.

Key takeaways from this article

Implement caching and prefetching:

These mechanisms reduce CRL and OCSP traffic, improving efficiency. By storing data locally, you avoid constant checks back to servers, saving time and resources.
Offline CRL use:

In environments without internet, like on ships or in military vehicles, using CRLs is a must. They're secure since they're signed by the certificate authority's private key – tamper-proof unless the root CA is compromised.

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☁️ Luke McAlpine Principal Cloud Consultant

1 CRLs: pros and cons

CRLs are files that contain a list of serial numbers of certificates that have been revoked by the issuing certificate authority (CA). CRLs are signed by the CA and published periodically on a web server or a directory service. Clients can download the CRL and check if a certificate is on the list before accepting it. CRLs are simple and widely supported, but they also have some drawbacks. They can be large and consume bandwidth and storage, they can be outdated and not reflect the latest revocations, and they can be tampered with or spoofed by attackers.

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☁️ Luke McAlpine Principal Cloud Consultant
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Pros:Universal Support: Standard in most certificate systems.Simple: If a certificate's on the list, it's revoked.Offline Use: No internet needed once downloaded.Cons:Size Issues: Growing list means more bandwidth and storage.Update Delays: Not real-time; can miss recent revocations.Security Risks: Potential for tampering and spoofing.

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Both methods have their Pros and Cons. But based on the usage scenario one method may be more suitable from the other. OCSP may be considered better option for browsers as the browsers deals with multiple websites and multiple CAs. So, fetching the complete CRL to check one website is inefficient. Hence OCSP will be a better choice here. On the other hand, CRL is better option for a server which gets multiple client requests. The certificate in the server is from a single CA. So CRL comparison is faster than OCSP as once the list is downloaded, the matching of serial number is done with a list which is existing on the disk. But in case of OCSP it has to fetch the details over a network from a different server for each request.

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See Also
What’s the Difference Between CRL and OCSP?What Is a Certificate Revocation List (CRL) and How Is It Used?Cryptography Flashcards by Oliver Crawford What Is the Online Certificate Status Protocol (OCSP)? | Fortinet
Implementing CRL (Certificate Revocation List) and OCSP (Online Certificate Status Protocol) securely and at scale involves several best practices. For CRLs, ensure frequent updates and distribute them via highly available and reliable servers, using CDN if necessary for global reach. Compress CRLs to minimize bandwidth usage. For OCSP, deploy multiple OCSP responders with load balancing to handle high traffic, and ensure they are geographically distributed to reduce latency. Implement strict security measures, including TLS, to protect the integrity and confidentiality of OCSP responses.

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CRLs (Certificate Revocation Lists) offer simplicity and broad support for certificate revocation checking, but they come with notable drawbacks. While CRLs are straightforward to implement and use, they can become quite large, impacting bandwidth and storage. The periodic updates mean they may not always reflect the most recent revocations, potentially leading to outdated status information. Additionally, CRLs are susceptible to tampering or spoofing, posing a security risk. Despite these challenges, CRLs remain a common choice due to their ease of use and compatibility with many systems.

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2 OCSP: pros and cons

OCSP is a protocol that allows clients to query the CA or a trusted responder for the status of a specific certificate. OCSP requests and responses are smaller and faster than CRLs, and they can provide real-time information about the revocation status of a certificate. OCSP also has some security features, such as nonce values and digital signatures, to prevent replay and forgery attacks. However, OCSP also has some limitations. It requires network connectivity and availability of the CA or the responder, it can introduce privacy risks by revealing the certificates that clients are checking, and it can be vulnerable to denial-of-service attacks.

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OCSP (Online Certificate Status Protocol) offers real-time certificate status checks with smaller, faster requests compared to CRLs. Its security features, like nonce values and digital signatures, help prevent replay and forgery attacks. However, OCSP requires reliable network connectivity and access to the CA or responder, which can be a limitation if the network is down. Privacy concerns arise as it can reveal which certificates are being queried, and it can be vulnerable to denial-of-service attacks. Despite these issues, OCSP is valued for its efficiency and timely status updates.

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Kablu Mandal Senior Manager | Team Lead | Full Stack Engineer @Protean eGov | Ex-Acoustic | NSDL | NDML | Tata Interactive System. 12.6 Years of Expertise In Java Technology | Spring Framework | Spring Cloud | Microservices
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OCSP reduce latency in comparison to CRL based validation.since OCSP needs to check or query the status of single certificate so it typically provide faster response compared with CRL based validation, where a client application may need to download and parse larger set of lists.

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3 CRL distribution points

One way to improve the scalability and performance of CRLs is to use CRL distribution points (CDPs). CDPs are URLs that are embedded in the certificates and indicate where the clients can find the CRLs for those certificates. CDPs can point to multiple locations, such as web servers, LDAP servers, or file shares, to provide redundancy and load balancing. CDPs can also be configured to use different scopes and frequencies for different types of certificates, such as root, intermediate, or end-entity certificates, to optimize the CRL size and update interval.

4 OCSP stapling

One way to enhance the security and privacy of OCSP is to use OCSP stapling. OCSP stapling is a technique that allows the server to obtain an OCSP response from the CA or the responder in advance and attach it to the certificate during the TLS handshake. This way, the client does not need to contact the CA or the responder directly, which reduces the network latency and the exposure of the client's certificate usage. OCSP stapling also ensures that the OCSP response is fresh and valid, as it is signed by the CA or the responder and has a limited lifetime.

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☁️ Luke McAlpine Principal Cloud Consultant
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Server-side Query: Instead of the client querying the OCSP responder directly, the server periodically queries the OCSP responder itself."Staples" the Response: The server then "staples" the OCSP response to the initial TLS handshake when a client connects. This means the client gets the certificate and its revocation status in one go.Privacy: Since the client doesn't directly query the OCSP responder, its browsing habits (i.e., which servers it's connecting to) aren't exposed to the Certificate Authority (CA) or OCSP responder.Performance: It reduces the number of round trips required during the TLS handshake, as the client doesn't need to separately fetch the OCSP response.

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5 Hybrid solutions

Depending on your needs and constraints, you may want to use a combination of CRLs and OCSP to achieve the best balance between scalability, security, and availability. For example, you can use CRLs as a fallback option when OCSP is unavailable or unreliable, or you can use OCSP as a supplement to CRLs when you need more timely revocation information. You can also use different methods for different roles or scenarios, such as using CRLs for internal certificates and OCSP for external certificates, or using CRLs for low-risk certificates and OCSP for high-risk certificates.

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6 Best practices

No matter which method you opt for, there are some general best practices that you should adhere to in order to implement CRLs and OCSP in a secure and scalable way. It's important to plan your certificate lifecycle management and revocation policies carefully, and document them accurately. Utilizing CDPs and OCSP stapling can enhance the efficiency and reliability of CRLs and OCSP. Furthermore, it's essential to monitor and audit your CRLs and OCSP servers and responders on a regular basis, ensuring they are updated and synchronized. Additionally, it's crucial to protect your CRLs and OCSP responses with digital signatures and encryption, verifying them before using them. Moreover, you should implement caching and prefetching mechanisms to reduce the network traffic and latency of CRLs and OCSP. Lastly, you should test the functionality and performance of your CRLs and OCSP under different conditions and scenarios.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Implementing CRL and OCSP in a scalable and secure manner involves several best practices. For CRLs, regularly update them and use delta CRLs to reduce file size, implement caching mechanisms, distribute CRLs through multiple and geographically diverse locations, and ensure efficient revocation checking by applications. For OCSP, use OCSP stapling to reduce direct requests, deploy high availability and load-balanced OCSP responders, cache responses while maintaining freshness, secure OCSP communications, implement rate limiting and monitoring, and provide fallback mechanisms. General best practices include ensuring redundancy and fault tolerance, designing for scalability, protecting the infrastructure with security measures.

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What are the best practices for implementing CRL and OCSP in a scalable and secure way? (2024)

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3

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1 CRLs: pros and cons

2 OCSP: pros and cons

3 CRL distribution points

4 OCSP stapling

5 Hybrid solutions

6 Best practices

7 Here’s what else to consider

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