OCSP vs CRL: Which Certificate Revocation Method is Best?

Introduction

Security in online communication has become extremely important given the status of the world today. There and SSL/TLS certificates are extremely important for providing encrypted connections and protecting fragile information.

However, there are situations where these certificates might need to be revoked due to factors such as security violations, the loss or compromise of the key, or changes in the policies. This is where two crucial mechanisms come into play: the Online Certificate Status Protocol (OCSP) and Certificate Revocation Lists (CRLs).

Thus, knowledge of their differences is a fundamental requirement for delivering relevant and optimal certificate revocation practices in organizations.

What is OCSP?

The Online Certificate Status Protocol (OCSP) is one of the methods which enables creating real-time information regarding the revocation of a certificate. It functions through the client-server model, whereby an OCSP client (for example, a web browser or a server) makes a request to an OCSP responder that enquires about the revocation status of a given certificate.

Also Read: What is OCSP and OCSP Stapling?

OCSP responder, usually owned by the CA that provided the specific certificate, returns the revocation status. It can include data like Valid, revoked, or Unknown status, along with Revocation Causes and possible Validity from and to date.

Through OCSP, clients reap the benefits of up-to-date revocation information that assists them in knowing the credibility of the certificates that are in use. However, embodied in this nature, it will take a sequence of requester-responder turns for each validation information of the certificate, which may create more traffic and delay time for applicants.

What is a CRL?

A Certificate Revocation List (CRL) is a list that contains information on digital certificates that have been canceled, suspended, or whose authenticity has been otherwise questioned by the CA that issued them, although such certificates may not have expired.

Also Read: What is a Certificate Revocation List [CRL]?

These revocations can happen for any reason which include loss or theft of the private key, change of policies within an organization, or misuse of the certificates.

Also Read: How to Check the Certificate Revocation Status?

CRLs are also provided by CAs, and they are available for clients who want to download and store them locally. In cases where clients want to confirm if a submitted certificate is valid, they refer to the CRL to check whether the certificate has been revoked. CRLs contain only a binary revocation state in relation to the revoked and the non-revoked listed certificates.

As for CRLs, although they can provide a locally cached solution to make fewer network requests and provide cached information, there is certainly a possible time gap between the real-time revocation and the update of the CRL. If there are many certificates revoked, CRLs may contain numerous entries and thus can reach large sizes that complicate their administration and distribution.

CRL Distribution and Formats:

CRLs are available in one of the formats of distribution and of significant importance is the X.509 format that relates to the structure of the X. 509 standard.

The common data to be stored in CRLs include; the name of the issuing CA, the last update time, the next update time, and the revoked certificate serial numbers plus the date of revocation.

CAs can distribute CRLs through various channels, including:

1. HTTP/HTTPS: CRLs can be put up for download at the web servers so that clients can use valid HTTP/ HTTPS links to obtain the updated CRL.
2. LDAP: CRLs can also be stored and distributed through Lightweight Directory Access Protocol (LDAP) directories, and to get the latest CRL, clients would need to perform an LDAP search.
3. DNS: Certain CAs may opt to use the DNS to transmit CRLs; in this case, the clients can access the CRL through certain types of DNS queries.

The Need for CRL and OCSP:

In this regard, both CRLs and OCSP prove extremely important since they are responsible for maintaining the certificate’s integrity and its recipient’s confidence in the certificate’s authenticity.

Even if a certificate is valid and has not expired, it may need to be revoked due to various reasons, such as:

1. Compromised Private Key: It becomes necessary that if the private key of a certificate is lost or if it falls into the wrong hands, then the certificate could be revoked immediately for use as it can lead to compromise of security.
2. Change in Organizational Policies: Such policies may diametrically change or get supplemented, due to which existing certificates need to be withdrawn and, consequently, new ones need to be issued.
3. Certificate Misuse: This makes revocation necessary where a given certificate is being used in wrong practices or for the wrong reasons as designed.
4. Key Compromise or Loss: In the case where the private key of the certificate is lost, or even if it is believed to have been compromised, then the certificate must be revoked to avoid further use of the certificate in other unlawful activities.
5. Change in Certificate Information: If the given information placed into the certificate, for instance, the subject’s name or the organization that they represent, is not actual or changed, then, the certificate has to be recalled and reissued.

OCSP vs CRL: Comparison

While both OCSP and CRLs aim to provide certificate revocation information, they differ in several key aspects:

Real-time vs. Periodic Updates:

OCSP also allows revocation status checking in real-time because when the certificate is being checked, the OCSP responder is queried additionally.

CRLs are time to time published and downloaded so there can be a time lag between the revocation time and the availability of the CRL.

Bandwidth and Performance:

OCSP involves separate one-client-to-server protocol message exchange for each certificate validation at the cost of increased traffic and potential latency in a large number of validations.

CRLs can be downloaded and stored in the local vicinity which minimizes the centralized requests and overall performance for the program.

Privacy and Anonymity:

In OCSP, the browser sends the cert ID it wants to validate, and in return gets a response from the OCSP server about the given certificate which infringes the privacy of the user and aids traffic analysis.

The vulnerability of certificates encapsulated through CRLs can be downloaded and locally processed without revealing the individual details to the clients which in return protects the privacy of the users.

Scalability and Management:

OCSP responder shall perform a large number of requests, which is likely to result in more demands for infrastructure and resources when the number of validations rises.

CRLs can become very huge with time especially when they have many revoked certificates; it can be very tasking to manage or distribute them.

Revocation Granularity:

OCSP can offer further details like Revocation Reason Codes, Validity Time, and other general information.

CRLs provide for each listed certificate only a simple status, options of which include revoked or not revoked, therefore providing less detail.

Hybrid Approaches:

OCSP offers many benefits when used in conjunction with a CRL in today’s organizations because they both are powerful tools with the capability to provide strong certificate revocation.

This approach usually designates OCSP to be the primary mechanism of onboard checking for revocation, while CRLs remain as the backup or supplementary kind.

In this hybrid model, first, the clients try to verify certificates using the OCSP. In case the OCSP responder is not available or returns an error, the client can switch to CRLs or other kinds of failover ones depending on the soft-fail or hard-fail policies set and security level.

Conclusion:

OCSP and CRLs, respectively, are indispensable for ensuring the reliability and authenticity of certificates in the distributed environment. OCSP provides current revocation status and detailed information at the expense of the speed of examination, while CRLs offer locally cached data.

The decision between the two methods may depend on some needs, such as performance, security, scalability, and chosen granularity of revocation. Finally, the management in the long run of proper certificate revocation mechanisms has been critical to fostering a secure and credible online environment.

Frequently Asked Questions:

Can OCSP and CRLs be used together?

While implementing OCSP and CRLs, both the facilities can be used together so that the merits of the two methods will be utilized. This kind of hybrid approach is being used by various organizations with the intention of achieving both the revocation status on the one hand and the performance on the other hand.

How often are CRLs updated?

Regarding the frequency of update of Certificate Revocation Lists, it differs depending on the CA policies. However, the general trend of operations of most CAs is to release new CRLs periodically, and this tends to be done frequently as every few hours, daily, or even for a more extended period.

What happens if the OCSP Responder is unavailable?

Otherwise, if the OCSP responder is offline or won’t respond properly, the client may attempt to go back to CRLs or implement other fail-over strategies, such as soft fail or hard fail based on vulnerability and security levels.

Can CRLs be trusted if they are not frequently updated?

To retain its revocation data, a CRL can become old and in the worst case, it may not contain the necessary revocation information at the time when it was accessed. Effective methods for using CRLs include making sure the CRLs are updated from time to time and requesting them from reliable sources.

Are OCSP Requests and Responses encrypted?

Yes, OCSP requests and responses are encrypted using the same SSL/TLS protocols as the secure web communications and thus possess remote messages without compromising revocation data.

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