How do you design CRL policies in PKI to balance revocation and validation needs?

1 Understand the CRL lifecycle

A CRL is issued by a certificate authority (CA) and has a validity period and a signature. The CA updates the CRL periodically, adding new revoked certificates and removing expired ones. The CA also publishes the CRL to a repository, such as a web server or a directory service, where it can be accessed by certificate validators, such as browsers or applications. The validators download the CRL and check if the certificates they encounter are on the list. If they are, they reject them as invalid.

2 Choose the CRL format

There are two main formats for CRLs: base and delta. A base CRL contains all the revoked certificates issued by a CA, while a delta CRL contains only the changes since the last base CRL. Delta CRLs are smaller and faster to download, but they require validators to have the corresponding base CRL as well. You can use both formats in your PKI, depending on your revocation frequency and bandwidth requirements. For example, you can issue a base CRL once a day and a delta CRL every hour.

4 Configure the CRL caching and expiration

CRL caching is the process of storing a downloaded CRL locally, so validators do not have to fetch it every time they need it. CRL caching can improve the performance and reliability of your PKI, but it also introduces the risk of using stale or outdated CRLs. To mitigate this risk, you should configure the CRL expiration and refresh intervals, which determine how long a CRL is valid and how often it is checked for updates. You should balance these parameters to minimize the network traffic and the revocation latency.

5 Monitor and audit the CRL process

The CRL process involves several components and actors, such as the CA, the repository, the validator, and the revoker. You should monitor and audit the CRL process to ensure its correctness and effectiveness. You should track the CRL generation, publication, distribution, download, verification, and usage. You should also check for errors, anomalies, or attacks that may compromise the CRL process. You should use tools and logs to collect and analyze the CRL data and metrics.

6 Consider the alternatives to CRLs

CRLs are not the only way to implement revocation in PKI. There are other methods that may suit your needs better, depending on your scenario and constraints. For example, you can use online certificate status protocol (OCSP), which allows validators to query the CA directly for the status of a certificate, without downloading a CRL. You can also use OCSP stapling, which allows servers to attach an OCSP response to their certificates, reducing the load on the CA. You can also use short-lived certificates, which expire quickly and do not need revocation.