The positive:
Straight forward design, not much to add to that, other than maybe some visual "gimmicks" which, in the end, would distract from the main purpose of the site. So, i think you're head on with the looks.
The menu on the side is also well placed, straight forward and simple.
The explanation about the use and purpose of the site are also easy to follow and understand. :tu:
If you find users, you already have the return user content by the recurrent use of your service.
The negative:
I don't quite see your possible target group for this site. I'm somewhat at a loss here. Who would need a service like that and for what purpose? If i needed something done in confidence, i'll simply pick up the phone and place a call, or if a company needs secure services, the service and/or company are most likely in the position to afford secure servers and encryption.
I am certain that i'm missing the idea here, enlighten me please.
Total overview:
Simple & Straight forward, easy to read and to follow the instructions. Questionable target group.