A point I wanted to add before I forget.
message system as db is fine, be sure to add in encryption key change every day/month/week or something.
Imagine this scenario.
user1 gets a visit from LE, they spill their nxt password and LE logs in. They now have access to transaction history of account and can form all the relationships with user2, 3, 4, 5 needed to move forward in the bully way.
If the encryption keys changed in a deterministic fashion you could only have to worry about 1 month or so of history on each account.