Certificate-based authentication

An RSA X.509 server certificate is a small file issued by a Certificate Authority (CA) that is installed on a computer or FortiGate unit to authenticate itself to other devices on the network. When one party on a network presents the certificate as authentication, the other party can validate that the certificate was issued by the CA. The identification is therefore as trustworthy as the Certificate Authority (CA) that issued the certificate.

To protect against compromised or misused certificates, CAs can revoke any certificate by adding it to a Certificate Revocation List (CRL). Certificate status can also be checked online using Online Certificate Status Protocol (OCSP).

RSA X.509 certificates are based on public-key cryptography, in which there are two keys: the private key and the public key. Data encrypted with the private key can be decrypted only with the public key and vice versa. As the names suggest, the private key is never revealed to anyone and the public key can be freely distributed. Encryption with the recipient’s public key creates a message that only the intended recipient can read. Encryption with the sender’s private key creates a message whose authenticity is proven because it can be decrypted only with the sender’s public key.

Server certificates contain a signature string encrypted with the CA’s private key. The CA’s public key is contained in a CA root certificate. If the signature string can be decrypted with the CA’s public key, the certificate is genuine.

Certificate authorities

A certificate authority can be:

• an organization, such as VeriSign Inc., that provides certificate services
• a software application, such as Microsoft Certificate Services or OpenSSH

For a company web portal or customer-facing SSL VPN, a third-party certificate service has some advantages. The CA certificates are already included in popular web browsers and customers trust the third-party. On the other hand, third-party services have a cost.

For administrators and for employee VPN users, the local CA based on a software application provides the required security at low cost. You can generate and distribute certificates as needed. If an employee leaves the organization, you can simply revoke their certificate.

Certificates for users

FortiGate unit administrators and SSL VPN users can install certificates in their web browsers to authenticate themselves. If the FortiGate unit uses a CA-issued certificate to authenticate itself to the clients, the browser will also need the appropriate CA certificate.

FortiGate IPsec VPN users can install server and CA certificates according to the instructions for their IPsec VPN client software. The FortiClient Endpoint Security application, for example, can import and store the certificates required by VPN connections.

FortiGate units are also compatible with some Public Key Infrastructure systems. For an example of this type of system, see RSA ACE (SecurID) servers on page 469.