RE: Private Messaging on Steem and eSteem...
I had an idea way back that i posted. If we had a convention for posting PGP public keys and messages, it would work just to stick them on the blockchain as posts. In fact, i suppose you could store a pgp public key in JSON metadata
It seems like it would be easy to write something that would look for a pgp public key in your user metadata, then post a pgp encrypted post to a special tag that isnt on global feeds
here is the post from five months ago... of course, you get the problem that this method would increase traffic, Especially if everyone had their own post thread for an inbox, with comments as messages, it seems like it could work.
So, for example,if everyone had a key in steemit.com/pgp/@xxx/pgp (where XXX is their username) and had a post on steemit.com/inbox/@xxx/inbox where messages were posted as replies, it would be fairly easy to send and recieve encrypted messages.
Of course, this would require everyone to use PGP... but they should be doing that anyway, and there are plugins that make it relatively painless, at least for chrome.
And it would require everyone to get on board with the basic tagging conventions.
Incidentally, another advantage of this is that once users post their pgp public key, as long as they have access to their keypair, they can demonstrate their steem identity on any other platform, without using their steem credentials