let context : LAContext = LAContext()
            var error: NSError?

            if context.canEvaluatePolicy(LAPolicy.deviceOwnerAuthentication, error: &error) {
                context.evaluatePolicy(LAPolicy.deviceOwnerAuthentication,
                                        localizedReason: messageString,


                                      reply: { (success: Bool, evalPolicyError: Error?) -> Void in

                    if success {
                        //... perform login
                        }
                    }
                    else {

                    switch evalPolicyError!._code {

                    case LAError.systemCancel.rawValue:
                        print("Authentication was cancelled by the system")

                    case LAError.userCancel.rawValue:
                        print("Authentication was cancelled by the user")

                    case LAError.userFallback.rawValue:
                        print("User selected to enter custom password")

                    default:
                        print("Authentication failed")
                        OperationQueue.main.addOperation({ () -> Void in
                            print("Error input")
                        })
                    }
                }

            })
        }
        else {
            // If the security policy cannot be evaluated then show a short message depending on the error.
            switch error!.code{

            case LAError.touchIDNotEnrolled.rawValue:
            print("TouchID is not enrolled")

            case LAError.passcodeNotSet.rawValue:
            print("A passcode has not been set")

            default:
            // The LAError.TouchIDNotAvailable case.
            print("TouchID not available")
            }
        }