ZyXEL ISG50-PSTN User Guide - Page 643

CHAPTER 50 Certificates 50.1 Overview The ISG50 can use certificates (also called digital IDs) to authenticate users. Certificates are based on public-private key pairs. A certificate contains the certificate owner's identity and public key. Certificates provide a way to exchange public keys for use in authentication. 50.1.1 What You Can Do in this Chapter • Use the My Certificate screens (see Section 50.2 on page 646 to Section 50.2.3 on page 653) to generate and export self-signed certificates or certification requests and import the ISG50's CAsigned certificates. • Use the Trusted Certificates screens (see Section 50.3 on page 654 to Section 50.3.2 on page 659) to save CA certificates and trusted remote host certificates to the ISG50. The ISG50 trusts any valid certificate that you have imported as a trusted certificate. It also trusts any valid certificate signed by any of the certificates that you have imported as a trusted certificate. 50.1.2 What You Need to Know When using public-key cryptology for authentication, each host has two keys. One key is public and can be made openly available. The other key is private and must be kept secure. These keys work like a handwritten signature (in fact, certificates are often referred to as "digital signatures"). Only you can write your signature exactly as it should look. When people know what your signature looks like, they can verify whether something was signed by you, or by someone else. In the same way, your private key "writes" your digital signature and your public key allows people to verify whether data was signed by you, or by someone else. This process works as follows. 1 Tim wants to send a message to Jenny. He needs her to be sure that it comes from him, and that the message content has not been altered by anyone else along the way. Tim generates a public key pair (one public key and one private key). 2 Tim keeps the private key and makes the public key openly available. This means that anyone who receives a message seeming to come from Tim can read it and verify whether it is really from him or not. 3 Tim uses his private key to sign the message and sends it to Jenny. 4 Jenny receives the message and uses Tim's public key to verify it. Jenny knows that the message is from Tim, and that although other people may have been able to read the message, no-one can have altered it (because they cannot re-sign the message with Tim's private key). ISG50 User's Guide 643