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

package use_cases

import (
	"context"
	"errors"
	"math/big"
	"time"

	"github.com/google/uuid"
	"github.com/rwadurian/mpc-system/pkg/crypto"
	"github.com/rwadurian/mpc-system/pkg/logger"
	"github.com/rwadurian/mpc-system/pkg/tss"
	"github.com/rwadurian/mpc-system/services/server-party/domain/repositories"
	"go.uber.org/zap"
)

var (
	ErrSigningFailed      = errors.New("signing failed")
	ErrSigningTimeout     = errors.New("signing timeout")
	ErrKeyShareNotFound   = errors.New("key share not found")
	ErrInvalidSignSession = errors.New("invalid sign session")
)

// ParticipateSigningInput contains input for signing participation
type ParticipateSigningInput struct {
	SessionID   uuid.UUID
	PartyID     string
	JoinToken   string
	MessageHash []byte
}

// ParticipateSigningOutput contains output from signing participation
type ParticipateSigningOutput struct {
	Success   bool
	Signature []byte
	R         *big.Int
	S         *big.Int
}

// ParticipateSigningUseCase handles signing participation
type ParticipateSigningUseCase struct {
	keyShareRepo  repositories.KeyShareRepository
	sessionClient SessionCoordinatorClient
	messageRouter MessageRouterClient
	cryptoService *crypto.CryptoService
}

// NewParticipateSigningUseCase creates a new participate signing use case
func NewParticipateSigningUseCase(
	keyShareRepo repositories.KeyShareRepository,
	sessionClient SessionCoordinatorClient,
	messageRouter MessageRouterClient,
	cryptoService *crypto.CryptoService,
) *ParticipateSigningUseCase {
	return &ParticipateSigningUseCase{
		keyShareRepo:  keyShareRepo,
		sessionClient: sessionClient,
		messageRouter: messageRouter,
		cryptoService: cryptoService,
	}
}

// Execute participates in a signing session using real TSS protocol
func (uc *ParticipateSigningUseCase) Execute(
	ctx context.Context,
	input ParticipateSigningInput,
) (*ParticipateSigningOutput, error) {
	// 1. Join session via coordinator
	sessionInfo, err := uc.sessionClient.JoinSession(ctx, input.SessionID, input.PartyID, input.JoinToken)
	if err != nil {
		return nil, err
	}

	if sessionInfo.SessionType != "sign" {
		return nil, ErrInvalidSignSession
	}

	// 2. Load key share for this party
	keyShares, err := uc.keyShareRepo.ListByParty(ctx, input.PartyID)
	if err != nil || len(keyShares) == 0 {
		return nil, ErrKeyShareNotFound
	}

	// Use the most recent key share (in production, would match by public key or session reference)
	keyShare := keyShares[len(keyShares)-1]

	// 3. Decrypt share data
	shareData, err := uc.cryptoService.DecryptShare(keyShare.ShareData, input.PartyID)
	if err != nil {
		return nil, err
	}

	// 4. Find self in participants and build party index map
	var selfIndex int
	partyIndexMap := make(map[string]int)
	for _, p := range sessionInfo.Participants {
		partyIndexMap[p.PartyID] = p.PartyIndex
		if p.PartyID == input.PartyID {
			selfIndex = p.PartyIndex
		}
	}

	// 5. Subscribe to messages
	msgChan, err := uc.messageRouter.SubscribeMessages(ctx, input.SessionID, input.PartyID)
	if err != nil {
		return nil, err
	}

	// Use message hash from session if not provided
	messageHash := input.MessageHash
	if len(messageHash) == 0 {
		messageHash = sessionInfo.MessageHash
	}

	// 6. Run TSS Signing protocol
	signature, r, s, err := uc.runSigningProtocol(
		ctx,
		input.SessionID,
		input.PartyID,
		selfIndex,
		sessionInfo.Participants,
		sessionInfo.ThresholdT,
		shareData,
		messageHash,
		msgChan,
		partyIndexMap,
	)
	if err != nil {
		return nil, err
	}

	// 7. Update key share last used
	keyShare.MarkUsed()
	if err := uc.keyShareRepo.Update(ctx, keyShare); err != nil {
		logger.Warn("failed to update key share last used", zap.Error(err))
	}

	// 8. Report completion to coordinator
	if err := uc.sessionClient.ReportCompletion(ctx, input.SessionID, input.PartyID, signature); err != nil {
		logger.Error("failed to report signing completion", zap.Error(err))
	}

	return &ParticipateSigningOutput{
		Success:   true,
		Signature: signature,
		R:         r,
		S:         s,
	}, nil
}

// runSigningProtocol runs the TSS signing protocol using tss-lib
func (uc *ParticipateSigningUseCase) runSigningProtocol(
	ctx context.Context,
	sessionID uuid.UUID,
	partyID string,
	selfIndex int,
	participants []ParticipantInfo,
	t int,
	shareData []byte,
	messageHash []byte,
	msgChan <-chan *MPCMessage,
	partyIndexMap map[string]int,
) ([]byte, *big.Int, *big.Int, error) {
	logger.Info("Running signing protocol",
		zap.String("session_id", sessionID.String()),
		zap.String("party_id", partyID),
		zap.Int("self_index", selfIndex),
		zap.Int("t", t),
		zap.Int("message_hash_len", len(messageHash)))

	// Create message handler adapter
	msgHandler := &signingMessageHandler{
		sessionID:     sessionID,
		partyID:       partyID,
		messageRouter: uc.messageRouter,
		msgChan:       make(chan *tss.ReceivedMessage, 100),
		partyIndexMap: partyIndexMap,
	}

	// Start message conversion goroutine
	go msgHandler.convertMessages(ctx, msgChan)

	// Create signing config
	config := tss.SigningConfig{
		Threshold:    t,
		TotalSigners: len(participants),
		Timeout:      5 * time.Minute,
	}

	// Create party list
	allParties := make([]tss.SigningParty, len(participants))
	for i, p := range participants {
		allParties[i] = tss.SigningParty{
			PartyID:    p.PartyID,
			PartyIndex: p.PartyIndex,
		}
	}

	selfParty := tss.SigningParty{
		PartyID:    partyID,
		PartyIndex: selfIndex,
	}

	// Create signing session
	session, err := tss.NewSigningSession(config, selfParty, allParties, messageHash, shareData, msgHandler)
	if err != nil {
		return nil, nil, nil, err
	}

	// Run signing
	result, err := session.Start(ctx)
	if err != nil {
		return nil, nil, nil, err
	}

	logger.Info("Signing completed successfully",
		zap.String("session_id", sessionID.String()),
		zap.String("party_id", partyID))

	return result.Signature, result.R, result.S, nil
}

// signingMessageHandler adapts MPCMessage channel to tss.MessageHandler
type signingMessageHandler struct {
	sessionID     uuid.UUID
	partyID       string
	messageRouter MessageRouterClient
	msgChan       chan *tss.ReceivedMessage
	partyIndexMap map[string]int
}

func (h *signingMessageHandler) SendMessage(ctx context.Context, isBroadcast bool, toParties []string, msgBytes []byte) error {
	return h.messageRouter.RouteMessage(ctx, h.sessionID, h.partyID, toParties, 0, msgBytes)
}

func (h *signingMessageHandler) ReceiveMessages() <-chan *tss.ReceivedMessage {
	return h.msgChan
}

func (h *signingMessageHandler) convertMessages(ctx context.Context, inChan <-chan *MPCMessage) {
	for {
		select {
		case <-ctx.Done():
			close(h.msgChan)
			return
		case msg, ok := <-inChan:
			if !ok {
				close(h.msgChan)
				return
			}

			fromIndex, exists := h.partyIndexMap[msg.FromParty]
			if !exists {
				continue
			}

			tssMsg := &tss.ReceivedMessage{
				FromPartyIndex: fromIndex,
				IsBroadcast:    msg.IsBroadcast,
				MsgBytes:       msg.Payload,
			}

			select {
			case h.msgChan <- tssMsg:
			case <-ctx.Done():
				return
			}
		}
	}
}