ZyXEL Communications ZyWALL USG 200 Series User Manual page 772

Chapter 46 Certificates
Tim keeps the private key and makes the public key openly available. This means
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that anyone who receives a message seeming to come from Tim can read it and
verify whether it is really from him or not.
Tim uses his private key to sign the message and sends it to Jenny.
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Jenny receives the message and uses Tim's public key to verify it. Jenny knows
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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).
Additionally, Jenny uses her own private key to sign a message and Tim uses
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Jenny's public key to verify 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its path has expired or been revoked.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 A directory of certificates that have been revoked before the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).
Advantages of Certificates
Certificates offer the following benefits.
• The ZyWALL only has to store the certificates of the certification authorities that
you decide to trust, no matter how many devices you need to authenticate.
• Key distribution is simple and very secure since you can freely distribute public
keys and you never need to transmit private keys.
Self-signed Certificates
You can have the ZyWALL act as a certification authority and sign its own
certificates.
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ZyWALL USG 100/200 Series User's Guide