participant_step2: add unfinalized session warning

Author: jonasnick

README.md

@@ -833,6 +833,18 @@ def participant_step2(hostseckey: bytes, state1: ParticipantState1, cmsg1: Coord
 
 Perform a participant's second step of a ChillDKG session.
 
+*Warning:*
+After sending the returned message to the coordinator, this participant
+**must not** erase the hostseckey, even if this participant does not receive
+the coordinator reply needed for the `participant_finalize` call. The
+underlying reason is that some other participant may receive the coordinator
+reply, deem the DKG session successful and use the resulting threshold
+public key (e.g., by sending funds to it). If the coordinator reply remains
+missing, that other participant can, at any point in the future, convince
+this participant of the success of the DKG session by presenting recovery
+data, from which this participant can recover the DKG output using the
+`recover` function.
+
 *Arguments*:
 
 - `hostseckey` - Participant's long-term host secret key (32 bytes).

python/chilldkg_ref/chilldkg.py

@@ -490,6 +490,18 @@ def participant_step2(
 ) -> Tuple[ParticipantState2, ParticipantMsg2]:
     """Perform a participant's second step of a ChillDKG session.
 
+    **Warning:**
+    After sending the returned message to the coordinator, this participant
+    **must not** erase the hostseckey, even if this participant does not receive
+    the coordinator reply needed for the `participant_finalize` call. The
+    underlying reason is that some other participant may receive the coordinator
+    reply, deem the DKG session successful and use the resulting threshold
+    public key (e.g., by sending funds to it). If the coordinator reply remains
+    missing, that other participant can, at any point in the future, convince
+    this participant of the success of the DKG session by presenting recovery
+    data, from which this participant can recover the DKG output using the
+    `recover` function.
+
     Arguments:
         hostseckey: Participant's long-term host secret key (32 bytes).
         state1: The participant's session state as output by