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An indirect CRLis one that is not necessarily issued by a CA, but instead by a third<br />

party. Such a CRLcan contain certificates from multiple CAs. The extension, then, is<br />

used to indicate which CA issued the certificate that has been revoked. Currently,<br />

indirect CRLs are not very common, particularly because CRLs in Version 2 format<br />

are not widely supported.<br />

Online Certificate Status Protocol<br />

The Online Certificate Status Protocol (OCSP), formally specified in RFC 2560, is a<br />

relatively new addition to PKI. Its primary aim is to address some of the distribution<br />

problems that have traditionally plagued CRLs.<br />

Using OCSP, an application makes a connection to an OCSP responder and requests<br />

the status of a certificate by passing the certificate’s serial number. The responder<br />

replies with one of these responses:<br />

Good<br />

Indicates that the certificate is valid, as far as the responder knows. This does<br />

not necessarily mean that the certificate was ever issued, just that it has not been<br />

revoked.<br />

Revoked<br />

Indicates that the certificate has indeed been issued and that it has also been<br />

revoked.<br />

Unknown<br />

Indicates that the responder does not know anything about the certificate. A typical<br />

reason for this response could be that a CA unknown to the responder<br />

issued the certificate.<br />

An OCSP responder is typically operated by a CA or by a trusted third party that is<br />

authorized by the CAs for which it provides information. The client must trust the<br />

OCSP responder in a manner similar to a root CA. More importantly, there is only<br />

one way to revoke an OCSP’s trusted status, and it is not pretty. If an OCSP<br />

responder is compromised, every client that makes use of that responder must be<br />

manually reconfigured either to not trust it or to use a new certificate that can be<br />

trusted. While it is theoretically possible to revoke an OCSP responder’s certificate, it<br />

is essentially impossible to do so in practice.<br />

A client’s request includes information about the issuer of the certificate for which it<br />

is requesting status information, so it is possible for a single OCSP responder to provide<br />

certificate revocation information for more than a single CA. Unfortunately, one<br />

of the problems of OCSP responders when run by a third party is that the information<br />

they are serving can become stale. At the very least, a delay often occurs between<br />

the time that a CA revokes a certificate and the time the responder receives the information<br />

from the CA, particularly if the responder is relying on CRLs published by its<br />

serviceable CAs to supply its information.<br />

510 | Chapter 10: Public Key Infrastructure<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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