lnwallet: add new AuxSigner interface to mirror SigPool

In this commit, we add a new aux signer interface that's meant to mirror the SigPool. If present, this'll be used to (maybe) obtain signatures for second level HTLCs for certain classes of custom channels.

Author: Roasbeef

lnwallet/aux_signer.go

@@ -0,0 +1,146 @@
+package lnwallet
+
+import (
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+// BaseAuxJob is a struct that contains the common fields that are shared among
+// the aux sign/verify jobs.
+type BaseAuxJob struct {
+	// OutputIndex is the output index of the HTLC on the commitment
+	// transaction being signed.
+	OutputIndex int32
+
+	// KeyRing is the commitment key ring that contains the keys needed to
+	// generate the second level HTLC signatures.
+	KeyRing CommitmentKeyRing
+
+	// HTLC is the HTLC that is being signed or verified.
+	HTLC PaymentDescriptor
+
+	// Incoming is a boolean that indicates if the HTLC is incoming or
+	// outgoing.
+	Incoming bool
+
+	// CommitBlob is the commitment transaction blob that contains the aux
+	// information for this channel.
+	CommitBlob fn.Option[tlv.Blob]
+
+	// HtlcLeaf is the aux tap leaf that corresponds to the HTLC being
+	// signed/verified.
+	HtlcLeaf input.AuxTapLeaf
+}
+
+// AuxSigJob is a struct that contains all the information needed to sign an
+// HTLC for custom channels.
+type AuxSigJob struct {
+	// SignDesc is the sign desc for this HTLC.
+	SignDesc input.SignDescriptor
+
+	BaseAuxJob
+
+	// Resp is a channel that will be used to send the result of the sign
+	// job.
+	Resp chan AuxSigJobResp
+
+	// Cancel is a channel that should be closed if the caller wishes to
+	// abandon all pending sign jobs part of a single batch.
+	Cancel chan struct{}
+}
+
+// NewAuxSigJob creates a new AuxSigJob.
+func NewAuxSigJob(sigJob SignJob, keyRing CommitmentKeyRing, incoming bool,
+	htlc PaymentDescriptor, commitBlob fn.Option[tlv.Blob],
+	htlcLeaf input.AuxTapLeaf, cancelChan chan struct{}) AuxSigJob {
+
+	return AuxSigJob{
+		SignDesc: sigJob.SignDesc,
+		BaseAuxJob: BaseAuxJob{
+			OutputIndex: sigJob.OutputIndex,
+			KeyRing:     keyRing,
+			HTLC:        htlc,
+			Incoming:    incoming,
+			CommitBlob:  commitBlob,
+			HtlcLeaf:    htlcLeaf,
+		},
+		Resp:   make(chan AuxSigJobResp, 1),
+		Cancel: cancelChan,
+	}
+}
+
+// AuxSigJobResp is a struct that contains the result of a sign job.
+type AuxSigJobResp struct {
+	// SigBlob is the signature blob that was generated for the HTLC. This
+	// is an opaque TLV field that may contain the signature and other data.
+	SigBlob fn.Option[tlv.Blob]
+
+	// HtlcIndex is the index of the HTLC that was signed.
+	HtlcIndex uint64
+
+	// Err is the error that occurred when executing the specified
+	// signature job. In the case that no error occurred, this value will
+	// be nil.
+	Err error
+}
+
+// AuxVerifyJob is a struct that contains all the information needed to verify
+// an HTLC for custom channels.
+type AuxVerifyJob struct {
+	// SigBlob is the signature blob that was generated for the HTLC. This
+	// is an opaque TLV field that may contain the signature and other data.
+	SigBlob fn.Option[tlv.Blob]
+
+	BaseAuxJob
+
+	// Cancel is a channel that should be closed if the caller wishes to
+	// abandon the job.
+	Cancel chan struct{}
+
+	// ErrResp is a channel that will be used to send the result of the
+	// verify job.
+	ErrResp chan error
+}
+
+// NewAuxVerifyJob creates a new AuxVerifyJob.
+func NewAuxVerifyJob(sig fn.Option[tlv.Blob], keyRing CommitmentKeyRing,
+	incoming bool, htlc PaymentDescriptor, commitBlob fn.Option[tlv.Blob],
+	htlcLeaf input.AuxTapLeaf) AuxVerifyJob {
+
+	return AuxVerifyJob{
+		SigBlob: sig,
+		BaseAuxJob: BaseAuxJob{
+			KeyRing:    keyRing,
+			HTLC:       htlc,
+			Incoming:   incoming,
+			CommitBlob: commitBlob,
+			HtlcLeaf:   htlcLeaf,
+		},
+	}
+}
+
+// AuxSigner is an interface that is used to sign and verify HTLCs for custom
+// channels. It is similar to the existing SigPool, but uses opaque blobs to
+// shuffle around signature information and other metadata.
+type AuxSigner interface {
+	// SubmitSecondLevelSigBatch takes a batch of aux sign jobs and
+	// processes them asynchronously.
+	SubmitSecondLevelSigBatch(chanState *channeldb.OpenChannel,
+		commitTx *wire.MsgTx, sigJob []AuxSigJob) error
+
+	// PackSigs takes a series of aux signatures and packs them into a
+	// single blob that can be sent alongside the CommitSig messages.
+	PackSigs([]fn.Option[tlv.Blob]) (fn.Option[tlv.Blob], error)
+
+	// UnpackSigs takes a packed blob of signatures and returns the
+	// original signatures for each HTLC, keyed by HTLC index.
+	UnpackSigs(fn.Option[tlv.Blob]) ([]fn.Option[tlv.Blob], error)
+
+	// VerifySecondLevelSigs attempts to synchronously verify a batch of aux
+	// sig jobs.
+	VerifySecondLevelSigs(chanState *channeldb.OpenChannel,
+		commitTx *wire.MsgTx, verifyJob []AuxVerifyJob) error
+}

lnwallet/commitment.go

@@ -833,6 +833,36 @@ func updateAuxBlob(chanState *channeldb.OpenChannel,
 	)
 }
 
+// packSigs is a helper function that attempts to pack a series of aux
+// signatures and packs them into a single blob that can be sent alongside the
+// CommitSig messages.
+func packSigs(auxSigs []fn.Option[tlv.Blob],
+	signer fn.Option[AuxSigner]) (tlv.Blob, error) {
+
+	if signer.IsNone() {
+		return nil, nil
+	}
+
+	blobOption, err := signer.UnsafeFromSome().PackSigs(auxSigs)
+	if err != nil {
+		return nil, fmt.Errorf("error packing aux sigs: %w", err)
+	}
+
+	return blobOption.UnwrapOr(nil), nil
+}
+
+// unpackSigs is a helper function that takes a packed blob of signatures and
+// returns the original signatures for each HTLC, keyed by HTLC index.
+func unpackSigs(blob fn.Option[tlv.Blob],
+	signer fn.Option[AuxSigner]) ([]fn.Option[tlv.Blob], error) {
+
+	if signer.IsNone() {
+		return nil, nil
+	}
+
+	return signer.UnsafeFromSome().UnpackSigs(blob)
+}
+
 // createUnsignedCommitmentTx generates the unsigned commitment transaction for
 // a commitment view and returns it as part of the unsignedCommitmentTx. The
 // passed in balances should be balances *before* subtracting any commitment