Is using a service like privnote a plausible way to communicate privately? According to the privacy policy, at no time is any note held in any readable format state on their servers. Also each message 'self destructs' after being opened one time.
Always assume "No" unless you have a bunch of established crypto researchers who've looked at the system and who are prepared to use it themselves.
People might say that "there are no posted attacks for privnote", but that just means the bad guys haven't told you they can crack your encryption. Since bad guys will never tell you that they can crack your encryption you must not assume lack of evidence of cracks is the same as lack of cracks.
