What info (if any) should be kept secret about new startup ideas?

Many experts, including Brad Feld in his awesome book Venture Deals, have said that asking VCs or others to sign an NDA for a basic pitch is an obvious sign of an amateur and totally not necessary. That makes a lot of sense to me. But very recently I came up with a concept for a netsec startup that I think I can whip up a MVP for in less than a month and start generating significant and very scalable income immediately there after. So in other words, with minimal time and resources and expertise, this idea can generate a lot of cash.

Now given time, I think I will be able to build up increasing complex algorithms to improve upon the process to such a degree that I will have a good amount of defensibility for the longer-term.

At this early stage of the game, I would like to get as much feedback about my idea as possible (I've already told about 7 people and almost all thought it was a good-to-great idea, many of them have already given me token investments as a sign of confidence) but if my idea is even half as clever as I think it is, if enough people hear about it someone is likely to give it a try for themselves because it is such a simple money-maker.

So what do you think? Should I stop being paranoid and just tell the world or if not, which elements (generally speaking) should I keep to myself?