C1 (Android-only, critical): building a session from a prekey envelope
accepted whatever identity key it offered — no pin check, where iOS has
always had one. Anyone could hand us a handshake with their own key
while claiming to be a pinned contact, and every later message from that
session rendered as that contact. The same gate the send path uses now
runs on receive.
C2 (both platforms): a device binds first-use-wins to its user, the
claim is checked against that binding on every decrypt, and control
traffic (delivery key, avatar) is refused from an unproven sender — a
poisoned delivery key forces the next send UNSEALED and deanonymizes it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
