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+A sync client to establish keys for newly added transports.
+
+It establishes transport keys with each contact for any transports that were added more recently than the contact was added.
+
+The `ACTIVATED` state lets us know that we already completed a session, so we don't create a new session if we receive another key message from the contact. Otherwise a malicious contact could cause us to create an unlimited number of key sets and run out of memory.
+
+TODO document the rest including state machine diagram
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