User's Manual

Chapter 16 Certificates 174
5 Additionally, Jenny uses her own private key to encrypt a message and Tim uses Jenny’s public key
to decrypt the message.
The Device uses certificates based on public-key cryptology to authenticate users attempting to
establish 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.
Certification Path
A certification path is the hierarchy of certification authority certificates that validate a certificate.
The Device does not trust a certificate if any certificate on its path has expired or been revoked.
Certificate Directory Servers
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A directory of certificates that have been revoked before the scheduled expiration is called a CRL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 Device 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 Device 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.
Certificate File Format
The certification authority certificate that you want to import has to be in PEM (Base-64) encoded
X.509 file format. This Privacy Enhanced Mail format uses 64 ASCII characters to convert a binary
X.509 certificate into a printable form.
16.1.3 Verifying a Certificate
Before you import a trusted CA or trusted remote host certificate into the Device, you should verify
that you have the actual certificate. This is especially true of trusted CA certificates since the Device
also trusts any valid certificate signed by any of the imported trusted CA certificates.
You can use a certificate’s fingerprint to verify it. A certificate’s fingerprint is a message digest
calculated using the MD5 or SHA1 algorithms. The following procedure describes how to check a
certificate’s fingerprint to verify that you have the actual certificate.