package wtclient import ( "github.com/btcsuite/btcd/blockchain" "github.com/btcsuite/btcd/btcec" "github.com/btcsuite/btcd/txscript" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/btcsuite/btcutil/txsort" "github.com/lightningnetwork/lnd/input" "github.com/lightningnetwork/lnd/lnwallet" "github.com/lightningnetwork/lnd/lnwire" "github.com/lightningnetwork/lnd/watchtower/blob" "github.com/lightningnetwork/lnd/watchtower/wtdb" ) // backupTask is an internal struct for computing the justice transaction for a // particular revoked state. A backupTask functions as a scratch pad for storing // computing values of the transaction itself, such as the final split in // balance if the justice transaction will give a reward to the tower. The // backup task has three primary phases: // 1. Init: Determines which inputs from the breach transaction will be spent, // and the total amount contained in the inputs. // 2. Bind: Asserts that the revoked state is eligible under a given session's // parameters. Certain states may be ineligible due to fee rates, too little // input amount, etc. Backup of these states can be deferred to a later time // or session with more favorable parameters. If the session is bound // successfully, the final session-dependent values to the justice // transaction are solidified. // 3. Send: Once the task is bound, it will be queued to send to a specific // tower corresponding to the session in which it was bound. The justice // transaction will be assembled by examining the parameters left as a // result of the binding. After the justice transaction is signed, the // necessary components are stripped out and encrypted before being sent to // the tower in a StateUpdate. type backupTask struct { id wtdb.BackupID breachInfo *lnwallet.BreachRetribution // state-dependent variables toLocalInput input.Input toRemoteInput input.Input totalAmt btcutil.Amount sweepPkScript []byte // session-dependent variables blobType blob.Type outputs []*wire.TxOut } // newBackupTask initializes a new backupTask and populates all state-dependent // variables. func newBackupTask(chanID *lnwire.ChannelID, breachInfo *lnwallet.BreachRetribution, sweepPkScript []byte, isTweakless bool) *backupTask { // Parse the non-dust outputs from the breach transaction, // simultaneously computing the total amount contained in the inputs // present. We can't compute the exact output values at this time // since the task has not been assigned to a session, at which point // parameters such as fee rate, number of outputs, and reward rate will // be finalized. var ( totalAmt int64 toLocalInput input.Input toRemoteInput input.Input ) // Add the sign descriptors and outputs corresponding to the to-local // and to-remote outputs, respectively, if either input amount is // non-dust. Note that the naming here seems reversed, but both are // correct. For example, the to-remote output on the remote party's // commitment is an output that pays to us. Hence the retribution refers // to that output as local, though relative to their commitment, it is // paying to-the-remote party (which is us). if breachInfo.RemoteOutputSignDesc != nil { toLocalInput = input.NewBaseInput( &breachInfo.RemoteOutpoint, input.CommitmentRevoke, breachInfo.RemoteOutputSignDesc, 0, ) totalAmt += breachInfo.RemoteOutputSignDesc.Output.Value } if breachInfo.LocalOutputSignDesc != nil { witnessType := input.CommitmentNoDelay if isTweakless { witnessType = input.CommitSpendNoDelayTweakless } toRemoteInput = input.NewBaseInput( &breachInfo.LocalOutpoint, witnessType, breachInfo.LocalOutputSignDesc, 0, ) totalAmt += breachInfo.LocalOutputSignDesc.Output.Value } return &backupTask{ id: wtdb.BackupID{ ChanID: *chanID, CommitHeight: breachInfo.RevokedStateNum, }, breachInfo: breachInfo, toLocalInput: toLocalInput, toRemoteInput: toRemoteInput, totalAmt: btcutil.Amount(totalAmt), sweepPkScript: sweepPkScript, } } // inputs returns all non-dust inputs that we will attempt to spend from. // // NOTE: Ordering of the inputs is not critical as we sort the transaction with // BIP69. func (t *backupTask) inputs() map[wire.OutPoint]input.Input { inputs := make(map[wire.OutPoint]input.Input) if t.toLocalInput != nil { inputs[*t.toLocalInput.OutPoint()] = t.toLocalInput } if t.toRemoteInput != nil { inputs[*t.toRemoteInput.OutPoint()] = t.toRemoteInput } return inputs } // bindSession determines if the backupTask is compatible with the passed // SessionInfo's policy. If no error is returned, the task has been bound to the // session and can be queued to upload to the tower. Otherwise, the bind failed // and should be rescheduled with a different session. func (t *backupTask) bindSession(session *wtdb.ClientSessionBody) error { // First we'll begin by deriving a weight estimate for the justice // transaction. The final weight can be different depending on whether // the watchtower is taking a reward. var weightEstimate input.TxWeightEstimator // Next, add the contribution from the inputs that are present on this // breach transaction. if t.toLocalInput != nil { // An older ToLocalPenaltyWitnessSize constant used to // underestimate the size by one byte. The diferrence in weight // can cause different output values on the sweep transaction, // so we mimic the original bug and create signatures using the // original weight estimate. weightEstimate.AddWitnessInput( input.ToLocalPenaltyWitnessSize - 1, ) } if t.toRemoteInput != nil { weightEstimate.AddWitnessInput(input.P2WKHWitnessSize) } // All justice transactions have a p2wkh output paying to the victim. weightEstimate.AddP2WKHOutput() // If the justice transaction has a reward output, add the output's // contribution to the weight estimate. if session.Policy.BlobType.Has(blob.FlagReward) { weightEstimate.AddP2WKHOutput() } // Now, compute the output values depending on whether FlagReward is set // in the current session's policy. outputs, err := session.Policy.ComputeJusticeTxOuts( t.totalAmt, int64(weightEstimate.Weight()), t.sweepPkScript, session.RewardPkScript, ) if err != nil { return err } t.blobType = session.Policy.BlobType t.outputs = outputs return nil } // craftSessionPayload is the final stage for a backupTask, and generates the // encrypted payload and breach hint that should be sent to the tower. This // method computes the final justice transaction using the bound // session-dependent variables, and signs the resulting transaction. The // required pieces from signatures, witness scripts, etc are then packaged into // a JusticeKit and encrypted using the breach transaction's key. func (t *backupTask) craftSessionPayload( signer input.Signer) (blob.BreachHint, []byte, error) { var hint blob.BreachHint // First, copy over the sweep pkscript, the pubkeys used to derive the // to-local script, and the remote CSV delay. keyRing := t.breachInfo.KeyRing justiceKit := &blob.JusticeKit{ BlobType: t.blobType, SweepAddress: t.sweepPkScript, RevocationPubKey: toBlobPubKey(keyRing.RevocationKey), LocalDelayPubKey: toBlobPubKey(keyRing.ToLocalKey), CSVDelay: t.breachInfo.RemoteDelay, } // If this commitment has an output that pays to us, copy the to-remote // pubkey into the justice kit. This serves as the indicator to the // tower that we expect the breaching transaction to have a non-dust // output to spend from. if t.toRemoteInput != nil { justiceKit.CommitToRemotePubKey = toBlobPubKey( keyRing.ToRemoteKey, ) } // Now, begin construction of the justice transaction. We'll start with // a version 2 transaction. justiceTxn := wire.NewMsgTx(2) // Next, add the non-dust inputs that were derived from the breach // information. This will either be contain both the to-local and // to-remote outputs, or only be the to-local output. inputs := t.inputs() for prevOutPoint := range inputs { justiceTxn.AddTxIn(&wire.TxIn{ PreviousOutPoint: prevOutPoint, }) } // Add the sweep output paying directly to the user and possibly a // reward output, using the outputs computed when the task was bound. justiceTxn.TxOut = t.outputs // Sort the justice transaction according to BIP69. txsort.InPlaceSort(justiceTxn) // Check that the justice transaction meets basic validity requirements // before attempting to attach the witnesses. btx := btcutil.NewTx(justiceTxn) if err := blockchain.CheckTransactionSanity(btx); err != nil { return hint, nil, err } // Construct a sighash cache to improve signing performance. hashCache := txscript.NewTxSigHashes(justiceTxn) // Since the transaction inputs could have been reordered as a result of // the BIP69 sort, create an index mapping each prevout to it's new // index. inputIndex := make(map[wire.OutPoint]int) for i, txIn := range justiceTxn.TxIn { inputIndex[txIn.PreviousOutPoint] = i } // Now, iterate through the list of inputs that were initially added to // the transaction and store the computed witness within the justice // kit. for _, inp := range inputs { // Lookup the input's new post-sort position. i := inputIndex[*inp.OutPoint()] // Construct the full witness required to spend this input. inputScript, err := inp.CraftInputScript( signer, justiceTxn, hashCache, i, ) if err != nil { return hint, nil, err } // Parse the DER-encoded signature from the first position of // the resulting witness. We trim an extra byte to remove the // sighash flag. witness := inputScript.Witness rawSignature := witness[0][:len(witness[0])-1] // Reencode the DER signature into a fixed-size 64 byte // signature. signature, err := lnwire.NewSigFromRawSignature(rawSignature) if err != nil { return hint, nil, err } // Finally, copy the serialized signature into the justice kit, // using the input's witness type to select the appropriate // field. switch inp.WitnessType() { case input.CommitmentRevoke: copy(justiceKit.CommitToLocalSig[:], signature[:]) case input.CommitSpendNoDelayTweakless: fallthrough case input.CommitmentNoDelay: copy(justiceKit.CommitToRemoteSig[:], signature[:]) } } breachTxID := t.breachInfo.BreachTransaction.TxHash() // Compute the breach key as SHA256(txid). hint, key := blob.NewBreachHintAndKeyFromHash(&breachTxID) // Then, we'll encrypt the computed justice kit using the full breach // transaction id, which will allow the tower to recover the contents // after the transaction is seen in the chain or mempool. encBlob, err := justiceKit.Encrypt(key) if err != nil { return hint, nil, err } return hint, encBlob, nil } // toBlobPubKey serializes the given pubkey into a blob.PubKey that can be set // as a field on a blob.JusticeKit. func toBlobPubKey(pubKey *btcec.PublicKey) blob.PubKey { var blobPubKey blob.PubKey copy(blobPubKey[:], pubKey.SerializeCompressed()) return blobPubKey }