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The Need for OCSP

In order for applications to determine if a certificate has been revoked, the application examines the CRL Distribution Point (CDP) extension in the certificate. This extension will have information on locations where the CRL can be obtained. These locations are normally either HTTP or LDAP locations.

The application then can go to those locations to download the CRL. There are, however, some potential issues with this scenario. CRLs over time can get rather large depending on the number of certificates issued and revoked. If CRLs grow to a large size, and many clients have to download CRLs, this can have a negative impact on network performance. More importantly, by default Windows clients will timeout after 15 seconds while trying to download a CRL. Additionally, CRLs have information about every currently valid certificate that has been revoked, which is an excessive amount of data given the fact that an application may only need the revocation status for a few certificates. So, aside from downloading the CRL, the application or the OS has to parse the CRL and find a match for the serial number of the certificate that has been revoked.

With the above limitations, which mostly revolve around scalability, it is clear that there are some drawbacks to using CRLs. Hence, the introduction of Online Certificate Status Protocol (OCSP). OCSP reduces the overhead associated with CRLs. There are server/client components to OCSP: The OCSP responder, which is the server component, and the OCSP Client. The OCSP Responder accepts status requests from OCSP Clients. When the OCSP Responder receives the request from the client it then needs to determine the status of the certificate using the serial number presented by the client. First the OCSP Responder determines if it has any cached responses for the same request. If it does, it can then send that response to the client. If there is no cached response, the OCSP Responder then checks to see if it has the CRL issued by the CA cached locally on the OCSP. If it does, it can check the revocation status locally, and send a response to the client stating whether the certificate is valid or revoked. The response is signed by the OCSP Signing Certificate that is selected during installation. If the OCSP does not have the CRL cached locally, the OCSP Responder can retrieve the CRL from the CDP locations listed in the certificate. The OCSP Responder then can parse the CRL to determine the revocation status, and send the appropriate response to the client.


ADSS OCSP Server is an advanced x.509 certificate Validation Authority server, that fully conforms to the IETF RFC 6960 standard. It is also FIPS 201 Certified (APL #1411) and approved for use by US federal agencies for HSPD-12 implementations.

It has been designed to operate as a robust validation hub solution, capable of providing OCSP certificate validation services for multiple Certificate Authorities (CAs) concurrently! Simple or sophisticated validation policies are supported for each individual CA and ADSS OCSP Server provides a detailed historical record of all transactions together with an easy to use OCSP request & response viewer – essential for either billing and/or troubleshooting within managed service infrastructures or enterprise systems.


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