fix(e2ee): pass caller_key as shared_key at connect time

Per-participant set_key() for remote identities doesn't work for
incoming decryption in this Rust FFI build (set_shared_key() after
connect is also ignored in per-participant mode).

Solution: initialize with caller_key as shared_key (true shared-key
mode) so the Rust FFI uses it for incoming decryption. Then override
outgoing encryption via set_key(bot_identity, bot_key) after connect.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

voice.py

@@ -69,14 +69,16 @@ def _generate_lk_jwt(room_id, user_id, device_id):
 KDF_HKDF = 1
 
 
-def _build_e2ee_options() -> rtc.E2EEOptions:
+def _build_e2ee_options(caller_key: bytes = b"") -> rtc.E2EEOptions:
     """Build HKDF E2EE options matching Element Call's key derivation.
 
-    No shared_key — initializes in per-participant mode so set_key() works.
+    Pass caller_key as shared_key to initialize in true shared-key mode.
+    This ensures the Rust FFI decrypts incoming frames using caller's key.
+    Outgoing encryption is overridden via set_key(bot_identity, bot_key) after connect.
     Element Call uses: ratchetWindowSize=16, keyringSize=256, salt="LKFrameEncryptionKey"
     """
     key_opts = rtc.KeyProviderOptions(
-        shared_key=b"",
+        shared_key=caller_key,
         ratchet_window_size=16,
         ratchet_salt=b"LKFrameEncryptionKey",
         failure_tolerance=-1,
@@ -223,9 +225,10 @@ class VoiceSession:
                         break
                     await asyncio.sleep(0.1)
 
-            # Connect with E2EE in per-participant mode (no shared_key)
-            # so set_key() calls work correctly for both directions
-            e2ee_opts = _build_e2ee_options()
+            # Connect with caller_key as shared_key so Rust FFI decrypts
+            # incoming audio in true shared-key mode. Outgoing encryption
+            # is overridden to bot_key via set_key(bot_identity) after connect.
+            e2ee_opts = _build_e2ee_options(self._caller_key or b"")
             room_opts = rtc.RoomOptions(e2ee=e2ee_opts)
             self.lk_room = rtc.Room()
 
@@ -267,25 +270,12 @@ class VoiceSession:
                     if remote_identity:
                         break
 
-            # Set caller's key(s) — decrypts incoming audio
-            # Use all collected keys with their correct indices (Element Call may rotate)
-            # Also set as shared key fallback: Rust FFI may not use per-participant
-            # key for remote participants in all code paths.
+            # Caller key was passed as shared_key at connect time — no further
+            # per-participant set_key needed for decryption.
             if self._caller_key:
-                caller_id = remote_identity or self._caller_identity
-                if caller_id:
-                    keys_to_set = self._caller_all_keys or {0: self._caller_key}
-                    for idx, key in keys_to_set.items():
-                        kp.set_key(caller_id, key, idx)
-                        logger.info("Set caller key[%d] for %s (%d bytes)", idx, caller_id, len(key))
-                    # Shared key fallback: use highest-index caller key
-                    max_idx = max(keys_to_set.keys())
-                    kp.set_shared_key(keys_to_set[max_idx], max_idx)
-                    logger.info("Set shared key fallback[%d] (%d bytes)", max_idx, len(keys_to_set[max_idx]))
-                else:
-                    logger.warning("Have caller key but no caller identity")
+                logger.info("Caller key active as shared_key (%d bytes, index 0)", len(self._caller_key))
             else:
-                logger.warning("No caller E2EE key available")
+                logger.warning("No caller E2EE key — incoming audio will be silence")
 
             if remote_identity:
                 logger.info("Linking to remote participant: %s", remote_identity)