ZyXEL Communications 200 Series User Manual page 640

Chapter 41 Certificates
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 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been revoked.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 A directory of certificates that have been revoked before the scheduled expiration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.
Factory Default Certificate
The ZyWALL generates its own unique self-signed certificate when you first turn it on. This
certificate is referred to in the GUI as the factory default certificate.
Certificate File Formats
Any certificate that you want to import has to be in one of these file formats:
• Binary X.509: This is an ITU-T recommendation that defines the formats for X.509
certificates.
• PEM (Base-64) encoded X.509: This Privacy Enhanced Mail format uses lowercase
letters, uppercase letters and numerals to convert a binary X.509 certificate into a printable
form.
• Binary PKCS#7: This is a standard that defines the general syntax for data (including
digital signatures) that may be encrypted. A PKCS #7 file is used to transfer a public key
certificate. The private key is not included. The ZyWALL currently allows the importation
of a PKS#7 file that contains a single certificate.
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ZyWALL USG 100/200 Series User's Guide