package chanfunding import ( "crypto/sha256" "errors" "fmt" "sync" "github.com/btcsuite/btcd/btcec" "github.com/btcsuite/btcd/chaincfg" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/btcsuite/btcutil/psbt" "github.com/lightningnetwork/lnd/input" "github.com/lightningnetwork/lnd/keychain" ) // PsbtState is a type for the state of the PSBT intent state machine. type PsbtState uint8 const ( // PsbtShimRegistered denotes a channel funding process has started with // a PSBT shim attached. This is the default state for a PsbtIntent. We // don't use iota here because the values have to be in sync with the // RPC constants. PsbtShimRegistered PsbtState = 1 // PsbtOutputKnown denotes that the local and remote peer have // negotiated the multisig keys to be used as the channel funding output // and therefore the PSBT funding process can now start. PsbtOutputKnown PsbtState = 2 // PsbtVerified denotes that a potential PSBT has been presented to the // intent and passed all checks. The verified PSBT can be given to a/the // signer(s). PsbtVerified PsbtState = 3 // PsbtFinalized denotes that a fully signed PSBT has been given to the // intent that looks identical to the previously verified transaction // but has all witness data added and is therefore completely signed. PsbtFinalized PsbtState = 4 // PsbtFundingTxCompiled denotes that the PSBT processed by this intent // has been successfully converted into a protocol transaction. It is // not yet completely certain that the resulting transaction will be // published because the commitment transactions between the channel // peers first need to be counter signed. But the job of the intent is // hereby completed. PsbtFundingTxCompiled PsbtState = 5 // PsbtInitiatorCanceled denotes that the user has canceled the intent. PsbtInitiatorCanceled PsbtState = 6 // PsbtResponderCanceled denotes that the remote peer has canceled the // funding, likely due to a timeout. PsbtResponderCanceled PsbtState = 7 ) // String returns a string representation of the PsbtState. func (s PsbtState) String() string { switch s { case PsbtShimRegistered: return "shim_registered" case PsbtOutputKnown: return "output_known" case PsbtVerified: return "verified" case PsbtFinalized: return "finalized" case PsbtFundingTxCompiled: return "funding_tx_compiled" case PsbtInitiatorCanceled: return "user_canceled" case PsbtResponderCanceled: return "remote_canceled" default: return fmt.Sprintf("", s) } } var ( // ErrRemoteCanceled is the error that is returned to the user if the // funding flow was canceled by the remote peer. ErrRemoteCanceled = errors.New("remote canceled funding, possibly " + "timed out") // ErrUserCanceled is the error that is returned through the PsbtReady // channel if the user canceled the funding flow. ErrUserCanceled = errors.New("user canceled funding") ) // PsbtIntent is an intent created by the PsbtAssembler which represents a // funding output to be created by a PSBT. This might be used when a hardware // wallet, or a channel factory is the entity crafting the funding transaction, // and not lnd. type PsbtIntent struct { // ShimIntent is the wrapped basic intent that contains common fields // we also use in the PSBT funding case. ShimIntent // State is the current state the intent state machine is in. State PsbtState // BasePsbt is the user-supplied base PSBT the channel output should be // added to. If this is nil we will create a new, empty PSBT as the base // for the funding transaction. BasePsbt *psbt.Packet // PendingPsbt is the parsed version of the current PSBT. This can be // in two stages: If the user has not yet provided any PSBT, this is // nil. Once the user sends us an unsigned funded PSBT, we verify that // we have a valid transaction that sends to the channel output PK // script and has an input large enough to pay for it. We keep this // verified but not yet signed version around until the fully signed // transaction is submitted by the user. At that point we make sure the // inputs and outputs haven't changed to what was previously verified. // Only witness data should be added after the verification process. PendingPsbt *psbt.Packet // FinalTX is the final, signed and ready to be published wire format // transaction. This is only set after the PsbtFinalize step was // completed successfully. FinalTX *wire.MsgTx // PsbtReady is an error channel the funding manager will listen for // a signal about the PSBT being ready to continue the funding flow. In // the normal, happy flow, this channel is only ever closed. If a // non-nil error is sent through the channel, the funding flow will be // canceled. // // NOTE: This channel must always be buffered. PsbtReady chan error // signalPsbtReady is a Once guard to make sure the PsbtReady channel is // only closed exactly once. signalPsbtReady sync.Once // netParams are the network parameters used to encode the P2WSH funding // address. netParams *chaincfg.Params } // BindKeys sets both the remote and local node's keys that will be used for the // channel funding multisig output. func (i *PsbtIntent) BindKeys(localKey *keychain.KeyDescriptor, remoteKey *btcec.PublicKey) { i.localKey = localKey i.remoteKey = remoteKey i.State = PsbtOutputKnown } // FundingParams returns the parameters that are necessary to start funding the // channel output this intent was created for. It returns the P2WSH funding // address, the exact funding amount and a PSBT packet that contains exactly one // output that encodes the previous two parameters. func (i *PsbtIntent) FundingParams() (btcutil.Address, int64, *psbt.Packet, error) { if i.State != PsbtOutputKnown { return nil, 0, nil, fmt.Errorf("invalid state, got %v "+ "expected %v", i.State, PsbtOutputKnown) } // The funding output needs to be known already at this point, which // means we need to have the local and remote multisig keys bound // already. witnessScript, out, err := i.FundingOutput() if err != nil { return nil, 0, nil, fmt.Errorf("unable to create funding "+ "output: %v", err) } witnessScriptHash := sha256.Sum256(witnessScript) // Encode the address in the human readable bech32 format. addr, err := btcutil.NewAddressWitnessScriptHash( witnessScriptHash[:], i.netParams, ) if err != nil { return nil, 0, nil, fmt.Errorf("unable to encode address: %v", err) } // We'll also encode the address/amount in a machine readable raw PSBT // format. If the user supplied a base PSBT, we'll add the output to // that one, otherwise we'll create a new one. packet := i.BasePsbt if packet == nil { packet, err = psbt.New(nil, nil, 2, 0, nil) if err != nil { return nil, 0, nil, fmt.Errorf("unable to create "+ "PSBT: %v", err) } } packet.UnsignedTx.TxOut = append(packet.UnsignedTx.TxOut, out) packet.Outputs = append(packet.Outputs, psbt.POutput{}) return addr, out.Value, packet, nil } // Verify makes sure the PSBT that is given to the intent has an output that // sends to the channel funding multisig address with the correct amount. A // simple check that at least a single input has been specified is performed. func (i *PsbtIntent) Verify(packet *psbt.Packet) error { if packet == nil { return fmt.Errorf("PSBT is nil") } if i.State != PsbtOutputKnown { return fmt.Errorf("invalid state. got %v expected %v", i.State, PsbtOutputKnown) } // Try to locate the channel funding multisig output. _, expectedOutput, err := i.FundingOutput() if err != nil { return fmt.Errorf("funding output cannot be created: %v", err) } outputFound := false outputSum := int64(0) for _, out := range packet.UnsignedTx.TxOut { outputSum += out.Value if psbt.TxOutsEqual(out, expectedOutput) { outputFound = true } } if !outputFound { return fmt.Errorf("funding output not found in PSBT") } // At least one input needs to be specified and it must be large enough // to pay for all outputs. We don't want to dive into fee estimation // here so we just assume that if the input amount exceeds the output // amount, the chosen fee is sufficient. if len(packet.UnsignedTx.TxIn) == 0 { return fmt.Errorf("PSBT has no inputs") } sum, err := psbt.SumUtxoInputValues(packet) if err != nil { return fmt.Errorf("error determining input sum: %v", err) } if sum <= outputSum { return fmt.Errorf("input amount sum must be larger than " + "output amount sum") } // SumUtxoInputValues checks that packet.Inputs is non-empty. Here we // check that each Input has a WitnessUtxo field, to avoid possible // malleability. for _, in := range packet.Inputs { if in.WitnessUtxo == nil { return fmt.Errorf("not all inputs are segwit spends") } } i.PendingPsbt = packet i.State = PsbtVerified return nil } // Finalize makes sure the final PSBT that is given to the intent is fully valid // and signed but still contains the same UTXOs and outputs as the pending // transaction we previously verified. If everything checks out, the funding // manager is informed that the channel can now be opened and the funding // transaction be broadcast. func (i *PsbtIntent) Finalize(packet *psbt.Packet) error { if packet == nil { return fmt.Errorf("PSBT is nil") } if i.State != PsbtVerified { return fmt.Errorf("invalid state. got %v expected %v", i.State, PsbtVerified) } // Make sure the PSBT itself thinks it's finalized and ready to be // broadcast. err := psbt.MaybeFinalizeAll(packet) if err != nil { return fmt.Errorf("error finalizing PSBT: %v", err) } rawTx, err := psbt.Extract(packet) if err != nil { return fmt.Errorf("unable to extract funding TX: %v", err) } return i.FinalizeRawTX(rawTx) } // FinalizeRawTX makes sure the final raw transaction that is given to the // intent is fully valid and signed but still contains the same UTXOs and // outputs as the pending transaction we previously verified. If everything // checks out, the funding manager is informed that the channel can now be // opened and the funding transaction be broadcast. func (i *PsbtIntent) FinalizeRawTX(rawTx *wire.MsgTx) error { if rawTx == nil { return fmt.Errorf("raw transaction is nil") } if i.State != PsbtVerified { return fmt.Errorf("invalid state. got %v expected %v", i.State, PsbtVerified) } // Do a basic check that this is still the same TX that we verified in // the previous step. This is to protect the user from unwanted // modifications. We only check the outputs and previous outpoints of // the inputs of the wire transaction because the fields in the PSBT // part are allowed to change. if i.PendingPsbt == nil { return fmt.Errorf("PSBT was not verified first") } err := psbt.VerifyOutputsEqual( rawTx.TxOut, i.PendingPsbt.UnsignedTx.TxOut, ) if err != nil { return fmt.Errorf("outputs differ from verified PSBT: %v", err) } err = psbt.VerifyInputPrevOutpointsEqual( rawTx.TxIn, i.PendingPsbt.UnsignedTx.TxIn, ) if err != nil { return fmt.Errorf("inputs differ from verified PSBT: %v", err) } // We also check that we have a signed TX. This is only necessary if the // FinalizeRawTX is called directly with a wire format TX instead of // extracting the TX from a PSBT. err = verifyInputsSigned(rawTx.TxIn) if err != nil { return fmt.Errorf("inputs not signed: %v", err) } // As far as we can tell, this TX is ok to be used as a funding // transaction. i.State = PsbtFinalized i.FinalTX = rawTx // Signal the funding manager that it can now finally continue with its // funding flow as the PSBT is now ready to be converted into a real // transaction and be published. i.signalPsbtReady.Do(func() { close(i.PsbtReady) }) return nil } // CompileFundingTx finalizes the previously verified PSBT and returns the // extracted binary serialized transaction from it. It also prepares the channel // point for which this funding intent was initiated for. func (i *PsbtIntent) CompileFundingTx() (*wire.MsgTx, error) { if i.State != PsbtFinalized { return nil, fmt.Errorf("invalid state. got %v expected %v", i.State, PsbtFinalized) } // Identify our funding outpoint now that we know everything's ready. _, txOut, err := i.FundingOutput() if err != nil { return nil, fmt.Errorf("cannot get funding output: %v", err) } ok, idx := input.FindScriptOutputIndex(i.FinalTX, txOut.PkScript) if !ok { return nil, fmt.Errorf("funding output not found in PSBT") } i.chanPoint = &wire.OutPoint{ Hash: i.FinalTX.TxHash(), Index: idx, } i.State = PsbtFundingTxCompiled return i.FinalTX, nil } // RemoteCanceled informs the listener of the PSBT ready channel that the // funding has been canceled by the remote peer and that we can no longer // continue with it. func (i *PsbtIntent) RemoteCanceled() { log.Debugf("PSBT funding intent canceled by remote, state=%v", i.State) i.signalPsbtReady.Do(func() { i.PsbtReady <- ErrRemoteCanceled i.State = PsbtResponderCanceled }) i.ShimIntent.Cancel() } // Cancel allows the caller to cancel a funding Intent at any time. This will // return make sure the channel funding flow with the remote peer is failed and // any reservations are canceled. // // NOTE: Part of the chanfunding.Intent interface. func (i *PsbtIntent) Cancel() { log.Debugf("PSBT funding intent canceled, state=%v", i.State) i.signalPsbtReady.Do(func() { i.PsbtReady <- ErrUserCanceled i.State = PsbtInitiatorCanceled }) i.ShimIntent.Cancel() } // Inputs returns all inputs to the final funding transaction that we know // about. These are only known after the PSBT has been verified. func (i *PsbtIntent) Inputs() []wire.OutPoint { var inputs []wire.OutPoint switch i.State { // We return the inputs to the pending psbt. case PsbtVerified: for _, in := range i.PendingPsbt.UnsignedTx.TxIn { inputs = append(inputs, in.PreviousOutPoint) } // We return the inputs to the final funding tx. case PsbtFinalized, PsbtFundingTxCompiled: for _, in := range i.FinalTX.TxIn { inputs = append(inputs, in.PreviousOutPoint) } // In all other states we cannot know the inputs to the funding tx, and // return an empty list. default: } return inputs } // Outputs returns all outputs of the final funding transaction that we // know about. These are only known after the PSBT has been verified. func (i *PsbtIntent) Outputs() []*wire.TxOut { switch i.State { // We return the outputs of the pending psbt. case PsbtVerified: return i.PendingPsbt.UnsignedTx.TxOut // We return the outputs of the final funding tx. case PsbtFinalized, PsbtFundingTxCompiled: return i.FinalTX.TxOut // In all other states we cannot know the final outputs, and return an // empty list. default: return nil } } // PsbtAssembler is a type of chanfunding.Assembler wherein the funding // transaction is constructed outside of lnd by using partially signed bitcoin // transactions (PSBT). type PsbtAssembler struct { // fundingAmt is the total amount of coins in the funding output. fundingAmt btcutil.Amount // basePsbt is the user-supplied base PSBT the channel output should be // added to. basePsbt *psbt.Packet // netParams are the network parameters used to encode the P2WSH funding // address. netParams *chaincfg.Params // shouldPublish specifies if the assembler should publish the // transaction once the channel funding has completed. shouldPublish bool } // NewPsbtAssembler creates a new CannedAssembler from the material required // to construct a funding output and channel point. An optional base PSBT can // be supplied which will be used to add the channel output to instead of // creating a new one. func NewPsbtAssembler(fundingAmt btcutil.Amount, basePsbt *psbt.Packet, netParams *chaincfg.Params, shouldPublish bool) *PsbtAssembler { return &PsbtAssembler{ fundingAmt: fundingAmt, basePsbt: basePsbt, netParams: netParams, shouldPublish: shouldPublish, } } // ProvisionChannel creates a new ShimIntent given the passed funding Request. // The returned intent is immediately able to provide the channel point and // funding output as they've already been created outside lnd. // // NOTE: This method satisfies the chanfunding.Assembler interface. func (p *PsbtAssembler) ProvisionChannel(req *Request) (Intent, error) { // We'll exit out if this field is set as the funding transaction will // be assembled externally, so we don't influence coin selection. if req.SubtractFees { return nil, fmt.Errorf("SubtractFees not supported for PSBT") } intent := &PsbtIntent{ ShimIntent: ShimIntent{ localFundingAmt: p.fundingAmt, }, State: PsbtShimRegistered, BasePsbt: p.basePsbt, PsbtReady: make(chan error, 1), netParams: p.netParams, } // A simple sanity check to ensure the provisioned request matches the // re-made shim intent. if req.LocalAmt+req.RemoteAmt != p.fundingAmt { return nil, fmt.Errorf("intent doesn't match PSBT "+ "assembler: local_amt=%v, remote_amt=%v, funding_amt=%v", req.LocalAmt, req.RemoteAmt, p.fundingAmt) } return intent, nil } // ShouldPublishFundingTx is a method of the assembler that signals if the // funding transaction should be published after the channel negotiations are // completed with the remote peer. // // NOTE: This method is a part of the ConditionalPublishAssembler interface. func (p *PsbtAssembler) ShouldPublishFundingTx() bool { return p.shouldPublish } // A compile-time assertion to ensure PsbtAssembler meets the // ConditionalPublishAssembler interface. var _ ConditionalPublishAssembler = (*PsbtAssembler)(nil) // verifyInputsSigned verifies that the given list of inputs is non-empty and // that all the inputs either contain a script signature or a witness stack. func verifyInputsSigned(ins []*wire.TxIn) error { if len(ins) == 0 { return fmt.Errorf("no inputs in transaction") } for idx, in := range ins { if len(in.SignatureScript) == 0 && len(in.Witness) == 0 { return fmt.Errorf("input %d has no signature data "+ "attached", idx) } } return nil }