XmppAxolotlMessage is now entirely responsible for handling encryption
and decryption of messages, only leveraging XmppAxolotlSession as a
packing/unpacking primitive for payload keys.
Removed pseudo-dead session generation code step from prepareMessage
function, as sessions have been created by invoking the
TrustKeysActivity for a while now.
Added prepareKeyTransportMessage function, which creates a message with
no payload. The key that is packed into the header keyElements can then
be used for other purposes (e.g. encrypted file transfer).
Messages sent from another device of the own account are now explicitly
tagged as carboned message. The session detection logic now uses this
tag to find "session borders".
* change chat states to type=chat and chat markers to type=chat
* use same type as requesting stanza for delivery receipts (which should make them type=chat most of the time)
Messages are now tagged with the IdentityKey fingerprint of the
originating session. IdentityKeys have one of three trust states:
undecided (default), trusted, and untrusted/not yet trusted.
No longer store own device ID (so that we don't encrypt messages for
ourselves), verify that own device ID is present in update list
(otherwise republish), reflect update in UI.
Previously, the sender was assumed to be the conversation counterpart.
This broke carboned own-device messages. We now track the sender
properly, and also set the status (sent by one of the own devices vs
received from the counterpart) accordingly.
Bundles are now fetched on demand when a session needs to be
established. This should lessen the chance of changes to the bundles
occuring before they're used, as well as lessen the load of fetching
bundles.
Also, the message encryption is now done in a background thread, as this
can be somewhat costly if many sessions are present. This is probably
not going to be an issue in real use, but it's good practice anyway.
