20.12. Certificate Authentication
  This authentication method uses SSL client certificates to perform
    authentication. It is therefore only available for SSL connections.
    When using this authentication method, the server will require that
    the client provide a valid, trusted certificate.  No password prompt
    will be sent to the client.  The
  
   cn
  
  (Common Name)
    attribute of the certificate
    will be compared to the requested database user name, and if they match
    the login will be allowed.  User name mapping can be used to allow
  
   cn
  
  to be different from the database user name.
 
The following configuration options are supported for SSL certificate authentication:
- 
    
     map
- 
    Allows for mapping between system and database user names. See Section 20.2 for details. 
  It is redundant to use the
  
   clientcert
  
  option with
  
   cert
  
  authentication because
  
   cert
  
  authentication is effectively
  
   trust
  
  authentication
    with
  
   clientcert=verify-full
  
  .