watchtower/wtdb/session_info: compute rewards outputs

watchtower/wtdb/session_info.go

@@ -3,6 +3,7 @@ package wtdb
 import (
 	"errors"
 
+	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/lnwallet"
 )
 
@@ -34,6 +35,11 @@ var (
 	// number larger than the session's max number of updates.
 	ErrSessionConsumed = errors.New("all session updates have been " +
 		"consumed")
+
+	// ErrFeeExceedsInputs signals that the total input value of breaching
+	// commitment txn is insufficient to cover the fees required to sweep
+	// it.
+	ErrFeeExceedsInputs = errors.New("sweep fee exceeds input values")
 )
 
 // SessionInfo holds the negotiated session parameters for single session id,
@@ -103,3 +109,58 @@ func (s *SessionInfo) AcceptUpdateSequence(seqNum, lastApplied uint16) error {
 
 	return nil
 }
+
+// ComputeSweepOutputs splits the total funds in a breaching commitment
+// transaction between the victim and the tower, according to the sweep fee rate
+// and reward rate. The fees are first subtracted from the overall total, before
+// splitting the remaining balance amongst the victim and tower.
+func (s *SessionInfo) ComputeSweepOutputs(totalAmt btcutil.Amount,
+	txVSize int64) (btcutil.Amount, btcutil.Amount, error) {
+
+	txFee := s.SweepFeeRate.FeeForWeight(txVSize)
+	if txFee > totalAmt {
+		return 0, 0, ErrFeeExceedsInputs
+	}
+
+	totalAmt -= txFee
+
+	// Apply the reward rate to the remaining total, specified in millionths
+	// of the available balance.
+	rewardAmt := (totalAmt*btcutil.Amount(s.RewardRate) + 999999) / 1000000
+	sweepAmt := totalAmt - rewardAmt
+
+	// TODO(conner): check dustiness
+
+	return sweepAmt, rewardAmt, nil
+}
+
+// Match is returned in response to a database query for a breach hints
+// contained in a particular block. The match encapsulates all data required to
+// properly decrypt a client's encrypted blob, and pursue action on behalf of
+// the victim by reconstructing the justice transaction and broadcasting it to
+// the network.
+//
+// NOTE: It is possible for a match to cause a false positive, since they are
+// matched on a prefix of the txid. In such an event, the likely behavior is
+// that the payload will fail to decrypt.
+type Match struct {
+	// ID is the session id of the client who uploaded the state update.
+	ID SessionID
+
+	// SeqNum is the session sequence number occupied by the client's state
+	// update. Together with ID, this allows the tower to derive the
+	// appropriate nonce for decryption.
+	SeqNum uint16
+
+	// Hint is the breach hint that triggered the match.
+	Hint BreachHint
+
+	// EncryptedBlob is the encrypted payload containing the justice kit
+	// uploaded by the client.
+	EncryptedBlob []byte
+
+	// SessionInfo is the contract negotiated between tower and client, that
+	// provides input parameters such as fee rate, reward rate, and reward
+	// address when attempting to reconstruct the justice transaction.
+	SessionInfo *SessionInfo
+}