Chapter 1 What’s New for FortiOS 5.2.1 : Authentication : Certificates for Policy Authentication
  
Certificates for Policy Authentication
The CA certificate used for policy authentication can now be configured, instead of being restricted to the built-in Fortinet certificate. By doing this, the authenticated user can be presented with a certificate that is already trusted by their browser and certificate errors can be avoided.
Syntax
config user setting
set auth-ca-cert <name>
end