Chapter 15 Certificates; Certificates Overview - ZyXEL Communications ZyWall 35 User Manual

This chapter gives background information about public-key certificates and explains how to
use them.

15.1 Certificates Overview

The ZyWALL 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.
A Certification Authority (CA) issues certificates and guarantees the identity of each
certificate owner. There are commercial certification authorities like CyberTrust or VeriSign
and government certification authorities. You can use the ZyWALL to generate certification
requests that contain identifying information and public keys and then send the certification
requests to a certification authority.
In public-key encryption and decryption, 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. Public-key encryption
in general works as follows.
1 Tim wants to send a private message to Jenny. Tim generates a public key pair. What is
encrypted with one key can only be decrypted using the other.
2 Tim keeps the private key and makes the public key openly available.
3 Tim uses his private key to encrypt the message and sends it to Jenny.
4 Jenny receives the message and uses Tim's public key to decrypt it.
5 Additionally, Jenny uses her own private key to encrypt a message and Tim uses Jenny's
public key to decrypt the message.
The ZyWALL uses certificates based on public-key cryptology to authenticate users
attempting to establish a connection, not to encrypt the data that you send after establishing a
connection. The method used to secure the data that you send through an established
connection depends on the type of connection. For example, a VPN tunnel might use the triple
DES encryption algorithm.
The certification authority uses its private key to sign certificates. Anyone can then use the
certification authority's public key to verify the certificates.
A certification path is the hierarchy of certification authority certificates that validate a
certificate. The ZyWALL does not trust a certificate if any certificate on its path has expired or
been revoked.
Certification authorities maintain directory servers with databases of valid and revoked
certificates. A directory of certificates that have been revoked before the scheduled expiration
is called a CRL (Certificate Revocation List). The ZyWALL can check a peer's certificate
against a directory server's list of revoked certificates. The framework of servers, software,
procedures and policies that handles keys is called PKI (public-key infrastructure).

Chapter 15 Certificates

C
H A P T E R
Certificates
ZyWALL 35 User's Guide
1 5
267