lnd.xprv/watchtower/wtpolicy/policy.go
2019-05-04 15:35:37 -07:00

244 lines
8.6 KiB
Go

package wtpolicy
import (
"errors"
"fmt"
"github.com/btcsuite/btcd/wire"
"github.com/btcsuite/btcutil"
"github.com/lightningnetwork/lnd/lnwallet"
"github.com/lightningnetwork/lnd/watchtower/blob"
)
const (
// RewardScale is the denominator applied when computing the
// proportional component for a tower's reward output. The current scale
// is in millionths.
RewardScale = 1000000
// DefaultMaxUpdates specifies the number of encrypted blobs a client
// can send to the tower in a single session.
DefaultMaxUpdates = 1024
// DefaultRewardRate specifies the fraction of the channel that the
// tower takes if it successfully sweeps a breach. The value is
// expressed in millionths of the channel capacity.
DefaultRewardRate = 10000
// DefaultSweepFeeRate specifies the fee rate used to construct justice
// transactions. The value is expressed in satoshis per kilo-weight.
DefaultSweepFeeRate = 3000
)
var (
// 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 value")
// ErrRewardExceedsInputs signals that the reward given to the tower (in
// addition to the transaction fees) is more than the input amount.
ErrRewardExceedsInputs = errors.New("reward amount exceeds input value")
// ErrCreatesDust signals that the session's policy would create a dust
// output for the victim.
ErrCreatesDust = errors.New("justice transaction creates dust at fee rate")
)
// DefaultPolicy returns a Policy containing the default parameters that can be
// used by clients or servers.
func DefaultPolicy() Policy {
return Policy{
BlobType: blob.TypeDefault,
MaxUpdates: DefaultMaxUpdates,
RewardRate: DefaultRewardRate,
SweepFeeRate: lnwallet.SatPerKWeight(
DefaultSweepFeeRate,
),
}
}
// Policy defines the negotiated parameters for a session between a client and
// server. The parameters specify the format of encrypted blobs sent to the
// tower, the reward schedule for the tower, and the number of encrypted blobs a
// client can send in one session.
type Policy struct {
// BlobType specifies the blob format that must be used by all updates sent
// under the session key used to negotiate this session.
BlobType blob.Type
// MaxUpdates is the maximum number of updates the watchtower will honor
// for this session.
MaxUpdates uint16
// RewardBase is the fixed amount allocated to the tower when the
// policy's blob type specifies a reward for the tower. This is taken
// before adding the proportional reward.
RewardBase uint32
// RewardRate is the fraction of the total balance of the revoked
// commitment that the watchtower is entitled to. This value is
// expressed in millionths of the total balance.
RewardRate uint32
// SweepFeeRate expresses the intended fee rate to be used when
// constructing the justice transaction. All sweep transactions created
// for this session must use this value during construction, and the
// signatures must implicitly commit to the resulting output values.
SweepFeeRate lnwallet.SatPerKWeight
}
// String returns a human-readable description of the current policy.
func (p Policy) String() string {
return fmt.Sprintf("(blob-type=%b max-updates=%d reward-rate=%d "+
"sweep-fee-rate=%d)", p.BlobType, p.MaxUpdates, p.RewardRate,
p.SweepFeeRate)
}
// ComputeAltruistOutput computes the lone output value of a justice transaction
// that pays no reward to the tower. The value is computed using the weight of
// of the justice transaction and subtracting an amount that satisfies the
// policy's fee rate.
func (p *Policy) ComputeAltruistOutput(totalAmt btcutil.Amount,
txWeight int64) (btcutil.Amount, error) {
txFee := p.SweepFeeRate.FeeForWeight(txWeight)
if txFee > totalAmt {
return 0, ErrFeeExceedsInputs
}
sweepAmt := totalAmt - txFee
// TODO(conner): replace w/ configurable dust limit
dustLimit := lnwallet.DefaultDustLimit()
// Check that the created outputs won't be dusty.
if sweepAmt <= dustLimit {
return 0, ErrCreatesDust
}
return sweepAmt, nil
}
// ComputeRewardOutputs 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 reward to he tower is subtracted first, before
// splitting the remaining balance amongst the victim and fees.
func (p *Policy) ComputeRewardOutputs(totalAmt btcutil.Amount,
txWeight int64) (btcutil.Amount, btcutil.Amount, error) {
txFee := p.SweepFeeRate.FeeForWeight(txWeight)
if txFee > totalAmt {
return 0, 0, ErrFeeExceedsInputs
}
// Apply the reward rate to the remaining total, specified in millionths
// of the available balance.
rewardAmt := ComputeRewardAmount(totalAmt, p.RewardBase, p.RewardRate)
if rewardAmt+txFee > totalAmt {
return 0, 0, ErrRewardExceedsInputs
}
// The sweep amount for the victim constitutes the remainder of the
// input value.
sweepAmt := totalAmt - rewardAmt - txFee
// TODO(conner): replace w/ configurable dust limit
dustLimit := lnwallet.DefaultDustLimit()
// Check that the created outputs won't be dusty.
if sweepAmt <= dustLimit {
return 0, 0, ErrCreatesDust
}
return sweepAmt, rewardAmt, nil
}
// ComputeRewardAmount computes the amount rewarded to the tower using the
// proportional rate expressed in millionths, e.g. one million is equivalent to
// one hundred percent of the total amount. The amount is rounded up to the
// nearest whole satoshi.
func ComputeRewardAmount(total btcutil.Amount, base, rate uint32) btcutil.Amount {
rewardBase := btcutil.Amount(base)
rewardRate := btcutil.Amount(rate)
// If the base reward exceeds the total, there is no more funds left
// from which to derive the proportional fee. We simply return the base,
// the caller should detect that this exceeds the total amount input.
if rewardBase > total {
return rewardBase
}
// Otherwise, subtract the base from the total and compute the
// proportional reward from the remaining total.
afterBase := total - rewardBase
proportional := (afterBase*rewardRate + RewardScale - 1) / RewardScale
return rewardBase + proportional
}
// ComputeJusticeTxOuts constructs the justice transaction outputs for the given
// policy. If the policy specifies a reward for the tower, there will be two
// outputs paying to the victim and the tower. Otherwise there will be a single
// output sweeping funds back to the victim. The totalAmt should be the sum of
// any inputs used in the transaction. The passed txWeight should include the
// weight of the outputs for the justice transaction, which is dependent on
// whether the justice transaction has a reward. The sweepPkScript should be the
// pkScript of the victim to which funds will be recovered. The rewardPkScript
// is the pkScript of the tower where its reward will be deposited, and will be
// ignored if the blob type does not specify a reward.
func (p *Policy) ComputeJusticeTxOuts(totalAmt btcutil.Amount, txWeight int64,
sweepPkScript, rewardPkScript []byte) ([]*wire.TxOut, error) {
var outputs []*wire.TxOut
// If the policy specifies a reward for the tower, compute a split of
// the funds based on the policy's parameters. Otherwise, we will use an
// the altruist output computation and sweep as much of the funds back
// to the victim as possible.
if p.BlobType.Has(blob.FlagReward) {
// Using the total input amount and the transaction's weight,
// compute the sweep and reward amounts. This corresponds to the
// amount returned to the victim and the amount paid to the
// tower, respectively. To do so, the required transaction fee
// is subtracted from the total, and the remaining amount is
// divided according to the prenegotiated reward rate from the
// client's session info.
sweepAmt, rewardAmt, err := p.ComputeRewardOutputs(
totalAmt, txWeight,
)
if err != nil {
return nil, err
}
// Add the sweep and reward outputs to the list of txouts.
outputs = append(outputs, &wire.TxOut{
PkScript: sweepPkScript,
Value: int64(sweepAmt),
})
outputs = append(outputs, &wire.TxOut{
PkScript: rewardPkScript,
Value: int64(rewardAmt),
})
} else {
// Using the total input amount and the transaction's weight,
// compute the sweep amount, which corresponds to the amount
// returned to the victim. To do so, the required transaction
// fee is subtracted from the total input amount.
sweepAmt, err := p.ComputeAltruistOutput(
totalAmt, txWeight,
)
if err != nil {
return nil, err
}
// Add the sweep output to the list of txouts.
outputs = append(outputs, &wire.TxOut{
PkScript: sweepPkScript,
Value: int64(sweepAmt),
})
}
return outputs, nil
}