Dropbox isn't really meant for this and you can't use Dropbox in the natural manner (as a file system) and actually maintain that security. Safemonk tries to interrupt sync and do some juggling to actually implement itself, and I wouldn't trust that with anything I care about.
Give SpiderOak a few bucks. It actually does what you want. (That goes for pretty much everyone, honestly. Dropbox is nothing special, SpiderOak encrypts on-device and has a similar price model.)