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Certification<br />

Authority<br />

Figure 10-1. Client-server key exchange<br />

For example, suppose you were to receive through an insecure medium a certificate<br />

purporting to belong to Microsoft. If that certificate is signed by VeriSign (the most<br />

popular trusted third party), and if you have previously obtained VeriSign’s public<br />

key in a secure manner, you can determine whether the certificate really does belong<br />

to Microsoft. PKI allows you to make many secure connections by exchanging keys<br />

over an insecure medium after receiving a single key over a secure medium.<br />

Certificates<br />

Certificates contain a wealth of information that can be used to tie the public key<br />

inside the certificate to an entity (see Figure 10-2), either an individual or an organization.<br />

Certificates have the name of the entity, called the distinguished name in the<br />

PKI world. Server-side certificates also usually contain the fully qualified domain<br />

name of the server. They have an expiration date, which means you will have to go<br />

back and get a new certificate periodically (actually, another reason is to minimize<br />

windows of vulnerability).<br />

Figure 10-2. Contents of a certificate<br />

Ask for key<br />

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

Send key<br />

Client Server<br />

Certificate<br />

Contents<br />

Attachments<br />

Version<br />

Serial number<br />

Algorithm information<br />

Issuer information<br />

Start and expiration dates<br />

Owner (subject) information<br />

Options<br />

Extensions<br />

Copy of algorithm information<br />

Signature<br />

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

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

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