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/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
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
	// In a real implementation, we'd need to identify which keygen session this signing session relates to
	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
	var selfIndex int
	for _, p := range sessionInfo.Participants {
		if p.PartyID == input.PartyID {
			selfIndex = p.PartyIndex
			break
		}
	}

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

	// 6. Run TSS Signing protocol
	signature, r, s, err := uc.runSigningProtocol(
		ctx,
		input.SessionID,
		input.PartyID,
		selfIndex,
		sessionInfo.Participants,
		sessionInfo.ThresholdT,
		shareData,
		input.MessageHash,
		msgChan,
	)
	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
// This is a placeholder implementation
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,
) ([]byte, *big.Int, *big.Int, error) {
	/*
	Real implementation would:
	1. Deserialize LocalPartySaveData from shareData
	2. Create tss.PartyID list
	3. Create tss.Parameters
	4. Create signing.LocalParty with message hash
	5. Handle outgoing messages via messageRouter
	6. Handle incoming messages from msgChan
	7. Wait for signing completion
	8. Return signature (R, S)

	Example with tss-lib:

	var saveData keygen.LocalPartySaveData
	saveData.UnmarshalBinary(shareData)

	parties := make([]*tss.PartyID, len(participants))
	for i, p := range participants {
		parties[i] = tss.NewPartyID(p.PartyID, p.PartyID, big.NewInt(int64(p.PartyIndex)))
	}

	selfPartyID := parties[selfIndex]
	tssCtx := tss.NewPeerContext(parties)
	params := tss.NewParameters(tss.S256(), tssCtx, selfPartyID, len(participants), t)

	outCh := make(chan tss.Message, len(participants)*10)
	endCh := make(chan *common.SignatureData, 1)

	msgHash := new(big.Int).SetBytes(messageHash)
	party := signing.NewLocalParty(msgHash, params, saveData, outCh, endCh)

	go handleOutgoingMessages(ctx, sessionID, partyID, outCh)
	go handleIncomingMessages(ctx, party, msgChan)

	party.Start()

	select {
	case signData := <-endCh:
		signature := append(signData.R, signData.S...)
		return signature, signData.R, signData.S, nil
	case <-time.After(5*time.Minute):
		return nil, nil, nil, ErrSigningTimeout
	}
	*/

	// Placeholder: Generate mock signature for demonstration
	logger.Info("Running signing protocol (placeholder)",
		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)))

	// Simulate signing delay
	select {
	case <-ctx.Done():
		return nil, nil, nil, ctx.Err()
	case <-time.After(1 * time.Second):
	}

	// Generate placeholder signature (R || S, each 32 bytes)
	r := new(big.Int).SetBytes(messageHash[:16])
	s := new(big.Int).SetBytes(messageHash[16:])

	signature := make([]byte, 64)
	rBytes := r.Bytes()
	sBytes := s.Bytes()

	// Pad to 32 bytes each
	copy(signature[32-len(rBytes):32], rBytes)
	copy(signature[64-len(sBytes):64], sBytes)

	return signature, r, s, nil
}