Certificates are associated with security mechanisms to control their use and are based on different layers of protection: something the person knows or inherent to the user. These identification procedures are known as authentication factors. To be considered robust authentication at least two authentication factors are required: Something the user ''knows'' (password). Something the user may ''possess'' (key card, SMS Token, OTP Token). Something the user ''makes or is'' (signature, voice-over, fingerprint, iris or any other biometric factors).