The efs has to be applied to a chart object, although you can code it to not show any study data on the chart itself.
Anyone running a backtest of the efs, if it includes entry/exit logic, will be able to determine the prices at which the system would have executed, as well as if they were market, limit, stop orders, but that's not enough to reverse-engineer a system.
As far as strength of the encryption? No idea.