* Add timings, so logs can be replayed.
* potential DOS where developer sends Activity Entered that all
  refer back to the first Activity Seen. This requires the user
  side to keep a Backlog containing all later Activity Seen, so uses
  up a lot of memory.

  For this to work, the developer would need to include
  in their Activity Entered echoData anticipating all the Activity Seen
  that they have generated. This is doable; for example the developer
  can send a stream of A's, and then needs to only include the right
  amount in the echoData.

  Should probably cap the BackLog to some maximum size to prevent
  this kind of DOS. Dropping old items from the BackLog after some
  amount of time would also work; after eg 10 seconds it's very
  unlikely that a Activity Entered will legitimately refer to an
  old backlog item.
* Encryption!
* Add random nonce to start message, to avoid replay issues.
  (Or perhaps the encryption derives a RSA key in a way that avoids
  replay..)
* Network!
* Server!
* gpg key downloading, web of trust checking, prompting