fix(tss): correct threshold signing to support t-of-n properly

Previously, signing incorrectly required all n parties from keygen to participate. For 2-of-3 threshold, it required all 3 parties instead of just 2.

Root cause: tss.NewParameters was using len(currentSigners) instead of the original n from keygen.

Changes:
- Added TotalParties field to SigningConfig to store original n from keygen
- Modified participate_signing.go to read threshold_n from database
- Updated tss.NewParameters to use TotalParties instead of current signer count
- Added logging to show t, n, and current_signers

For 2-of-3: threshold_t=2, threshold_n=3, any 2 parties can now sign.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/mpc-system/pkg/tss/signing.go

@@ -43,8 +43,9 @@ type SigningParty struct {
 
 // SigningConfig contains configuration for signing
 type SigningConfig struct {
-	Threshold    int           // t in t-of-n (number of signers required)
-	TotalSigners int           // Number of parties participating in this signing
+	Threshold    int           // t in t-of-n threshold value from keygen
+	TotalParties int           // n in t-of-n - total parties from keygen (NOT current signers)
+	TotalSigners int           // Number of parties participating in this signing session
 	Timeout      time.Duration // Signing timeout
 }
 
@@ -112,8 +113,10 @@ func NewSigningSession(
 	sortedPartyIDs := tss.SortPartyIDs(tssPartyIDs)
 
 	// Create peer context and parameters
+	// IMPORTANT: Use TotalParties from keygen, not len(sortedPartyIDs) which is current signers
+	// For 2-of-3: threshold=2, TotalParties=3, but only 2 parties might participate in signing
 	peerCtx := tss.NewPeerContext(sortedPartyIDs)
-	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, len(sortedPartyIDs), config.Threshold)
+	params := tss.NewParameters(tss.S256(), peerCtx, selfTSSID, config.TotalParties, config.Threshold)
 
 	// Convert message hash to big.Int
 	msgHash := new(big.Int).SetBytes(messageHash)

backend/mpc-system/services/server-party/application/use_cases/participate_signing.go

@@ -81,6 +81,7 @@ func (uc *ParticipateSigningUseCase) Execute(
 	// 2. Get share data - either from user input (delegate) or from database (persistent)
 	var shareData []byte
 	var keyShareForUpdate *entities.PartyKeyShare
+	var originalThresholdN int // Original total parties from keygen
 
 	if len(input.UserShareData) > 0 {
 		// Delegate party: use share provided by user
@@ -88,6 +89,8 @@ func (uc *ParticipateSigningUseCase) Execute(
 		if err != nil {
 			return nil, err
 		}
+		// For delegate party, get threshold info from session
+		originalThresholdN = sessionInfo.ThresholdN
 		logger.Info("Using user-provided share (delegate party)",
 			zap.String("party_id", input.PartyID),
 			zap.String("session_id", input.SessionID.String()))
@@ -101,6 +104,9 @@ func (uc *ParticipateSigningUseCase) Execute(
 		// Use the most recent key share (in production, would match by public key or session reference)
 		keyShareForUpdate = keyShares[len(keyShares)-1]
 
+		// Get original threshold_n from keygen
+		originalThresholdN = keyShareForUpdate.ThresholdN
+
 		// Decrypt share data
 		shareData, err = uc.cryptoService.DecryptShare(keyShareForUpdate.ShareData, input.PartyID)
 		if err != nil {
@@ -108,7 +114,9 @@ func (uc *ParticipateSigningUseCase) Execute(
 		}
 		logger.Info("Using database share (persistent party)",
 			zap.String("party_id", input.PartyID),
-			zap.String("session_id", input.SessionID.String()))
+			zap.String("session_id", input.SessionID.String()),
+			zap.Int("original_threshold_n", originalThresholdN),
+			zap.Int("threshold_t", keyShareForUpdate.ThresholdT))
 	}
 
 	// 4. Find self in participants and build party index map
@@ -141,6 +149,7 @@ func (uc *ParticipateSigningUseCase) Execute(
 		selfIndex,
 		sessionInfo.Participants,
 		sessionInfo.ThresholdT,
+		originalThresholdN,
 		shareData,
 		messageHash,
 		msgChan,
@@ -179,6 +188,7 @@ func (uc *ParticipateSigningUseCase) runSigningProtocol(
 	selfIndex int,
 	participants []ParticipantInfo,
 	t int,
+	n int, // Original total parties from keygen
 	shareData []byte,
 	messageHash []byte,
 	msgChan <-chan *MPCMessage,
@@ -189,6 +199,8 @@ func (uc *ParticipateSigningUseCase) runSigningProtocol(
 		zap.String("party_id", partyID),
 		zap.Int("self_index", selfIndex),
 		zap.Int("t", t),
+		zap.Int("n", n),
+		zap.Int("current_signers", len(participants)),
 		zap.Int("message_hash_len", len(messageHash)))
 
 	// Create message handler adapter
@@ -204,8 +216,11 @@ func (uc *ParticipateSigningUseCase) runSigningProtocol(
 	go msgHandler.convertMessages(ctx, msgChan)
 
 	// Create signing config
+	// IMPORTANT: TotalParties must be the original n from keygen, not current signers
+	// For 2-of-3: t=2, n=3, but only 2 parties participate in signing
 	config := tss.SigningConfig{
 		Threshold:    t,
+		TotalParties: n, // Original total from keygen
 		TotalSigners: len(participants),
 		Timeout:      5 * time.Minute,
 	}