How to have confidence in my research? I'm a PhD student at a reputed institution and in my 4th year now. I'm researching in the domain of information security &amp; privacy. 
I've got a number of good conference publications (4 conferences) but not in top conferences.
I attempted for the top conference (S&amp;P or ACM CCS - their workshops as well) but my papers got rejected. 
I'm writing my dissertation now, however, I still have this unsatisfied feeling that my work is not really good, it may have been the imposter syndrome. I feel that my work doesn't look as good as other people's (PhD students) work who are working in the same domain. I've got a few questions maybe some experienced researchers who have gone through can answer. 


How do you know your PhD work is strong work?
How to gain confidence in your PhD work?
How to know if the main idea of PhD work is a good one? (How to
get rid of this feeling?)


Thank you for your help! :)
How do you know your PhD work is strong work?
How to know if the main idea of PhD work is a good one?

It is strong work if it makes an impact. This could be in terms of highlighting new techniques that get widely adopted, by winning awards, by getting published or by setting up a startup and making millions. A lot of people define success in many different ways. How do you define it? If your metric is publishing in good venues - then I suppose that you can answer your own question.
Your ideas are as good as what you make of them, and as good as how others perceive them.

How to gain confidence in your PhD work?

You need to ask yourself why you're doing the PhD and have a realistic outlook. It is likely that without a good publication record you won't be able to land a strong academic position, but that's hardly the end of the world. There is massive demand out there for CS researchers in all sorts of positions.
What does your advisor say? Do they think your work is good and deserves publication in top venues, and it was just bad luck that plagued you? What do other figures of authority in the field think of your work?
As a general comment - you are getting the highest possible academic university qualification from a top school, in a highly competitive and popular field. And you feel inadequate? Say that last sentence out loud to yourself. You're anything but!
Good luck!