Full SSL inspection - Certificate requirements:
FortiGate is acting as a proxy web server. In order for FortiGate to act in these roles, its CA certificate must have the basic constraints extension set to cA=True and the value of the keyUsage extension set to keyCertSign.
The CA=True value identifies the certificate as a CA certificate. The KryUsage =KeyCertSign value indicates that the certificate corresponding private key is permitted to sign certificates. see RFC 5280 section 4.2.1.9 basic Constraints.
Although it appears as though the user browser is connected to the web server, the browser is connected to FortiGate. FortiGate is acting as a proxy web server. In order for FortiGate to act in these roles, its CA certificate must have the basic constraints extension set to cA=True and the value of the keyUsage extension set to keyCertSign.