User's Manual

ISG50 User’s Guide 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 CA-
signed 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).