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Front cover - IBM Redbooks

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For example, when establishing a secure HTTP connection via SSL, mainstream<br />

Web browsers have a list of several trustworthy CA certificates (generally<br />

referred to as “Trusted Roots”, or “Trusted CAs”) already incorporated when they<br />

ship, such as, but not limited to: VeriSign, Entrust, Thawte, Baltimore, <strong>IBM</strong> World<br />

Registry, and so forth. If a Web server uses a certificate that is signed by such a<br />

trusted CA, they will implicitly trust the server, unless the user intentionally<br />

deletes the signer CA certificate from the list of Trusted CAs.<br />

A CA is able to issue a number of different types of certificates, such as:<br />

► User certificates: These may be issued to an ordinary user or another type<br />

of entity, such as a server or an application. These will then be, with the user<br />

certificate, trusted end-entities for the CA. If an RA is part of the infrastructure,<br />

it should also have this certificate. A user certificate may be limited to specific<br />

uses and purposes (such as secure e-mail, secure access to servers, and so<br />

forth).<br />

► CA certificates: When a CA issues a certificate for itself, it is called a<br />

self-signed certificate, or root certificate for that CA. If a CA issues a<br />

certificate for a subordinate CA, the certificate is also called a CA certificate.<br />

► Cross certificates: These are used for cross-certification, which is an<br />

authentication process across security domains.<br />

Certificate renewal<br />

Every certificate has a validity period with an expiration date associated with it.<br />

When a certificate expires, a renewal process may be initiated and, once<br />

approved, a new certificate will be issued to the End-Entity.<br />

Certificate revocation<br />

The maximum lifetime of a certificate is its expiration date. In some cases,<br />

however, a certificates needs to be revoked before its expiration date. When this<br />

happens, the CA posts the certificate to a Certificate Revocation List (CRL).<br />

Actually, to be more precise, the CA posts the certificate’s serial number, along<br />

with some other information, to the CRL. Clients that need to know the validity of<br />

a certificate can search the CRL for any revocation notice.<br />

The Certificate Repository (CR)<br />

The Certificate Repository is a store of issued certificates and revoked<br />

certificates in a CRL. Although a Certificate Repository is not a required<br />

component in a public key infrastructure, it significantly contributes to the<br />

availability and manageability of the PKI.<br />

Because the X.509 certificate format is a natural fit to an X.500 Directory, a CR is<br />

thus best implemented as a Directory, which can then be access by the most<br />

Chapter 6. Public key infrastructures 235

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