ZyXEL Communications MAX-306M1, User Manual

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Chapter 15 The Certificates Screens
User’s Guide
184
15.4.1 Certificate Authorities
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
ought to look. When people know what your signature ought to look 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 she knows 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).
5
Additionally, Jenny uses her own private key to sign a message and Tim uses
Jenny’s public key to verify the message.
The WiMAX Modem 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 WiMAX Modem does not trust a certificate if any
certificate on its path has expired or been revoked.