watchtower/lookout/justice_descriptor: anchor justice txn

This commit modifies the JusticeDescriptor to support creation of
justice transactions spending from anchor commitments. Rather than the
unencumbered p2wkh scripts from before, the tower will now use the
to-remote-confirmed that includes the additional CSV delay of 1. This
also requires setting the sequence number appropriately on the to-remote
input.

watchtower/lookout/justice_descriptor.go

@@ -48,6 +48,7 @@ type breachedInput struct {
 	txOut    *wire.TxOut
 	outPoint wire.OutPoint
 	witness  [][]byte
+	sequence uint32
 }
 
 // commitToLocalInput extracts the information required to spend the commit
@@ -104,20 +105,35 @@ func (p *JusticeDescriptor) commitToRemoteInput() (*breachedInput, error) {
 		return nil, err
 	}
 
-	// Since the to-remote witness script should just be a regular p2wkh
+	var (
-	// output, we'll parse it to retrieve the public key.
+		toRemoteScriptHash []byte
-	toRemotePubKey, err := btcec.ParsePubKey(toRemoteScript, btcec.S256())
+		toRemoteSequence   uint32
-	if err != nil {
-		return nil, err
-	}
-
-	// Compute the witness script hash from the to-remote pubkey, which will
-	// be used to locate the input on the breach commitment transaction.
-	toRemoteScriptHash, err := input.CommitScriptUnencumbered(
-		toRemotePubKey,
 	)
-	if err != nil {
+	if p.JusticeKit.BlobType.IsAnchorChannel() {
-		return nil, err
+		toRemoteScriptHash, err = input.WitnessScriptHash(
+			toRemoteScript,
+		)
+		if err != nil {
+			return nil, err
+		}
+
+		toRemoteSequence = 1
+	} else {
+		// Since the to-remote witness script should just be a regular p2wkh
+		// output, we'll parse it to retrieve the public key.
+		toRemotePubKey, err := btcec.ParsePubKey(toRemoteScript, btcec.S256())
+		if err != nil {
+			return nil, err
+		}
+
+		// Compute the witness script hash from the to-remote pubkey, which will
+		// be used to locate the input on the breach commitment transaction.
+		toRemoteScriptHash, err = input.CommitScriptUnencumbered(
+			toRemotePubKey,
+		)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	// Locate the to-remote output on the breaching commitment transaction.
@@ -146,6 +162,7 @@ func (p *JusticeDescriptor) commitToRemoteInput() (*breachedInput, error) {
 		txOut:    toRemoteTxOut,
 		outPoint: toRemoteOutPoint,
 		witness:  buildWitness(witnessStack, toRemoteScript),
+		sequence: toRemoteSequence,
 	}, nil
 }
 
@@ -164,6 +181,7 @@ func (p *JusticeDescriptor) assembleJusticeTxn(txWeight int64,
 		totalAmt += btcutil.Amount(input.txOut.Value)
 		justiceTxn.AddTxIn(&wire.TxIn{
 			PreviousOutPoint: input.outPoint,
+			Sequence:         input.sequence,
 		})
 	}
 
@@ -279,8 +297,13 @@ func (p *JusticeDescriptor) CreateJusticeTxn() (*wire.MsgTx, error) {
 		if err != nil {
 			return nil, err
 		}
-		weightEstimate.AddWitnessInput(input.P2WKHWitnessSize)
 		sweepInputs = append(sweepInputs, toRemoteInput)
+
+		if p.JusticeKit.BlobType.IsAnchorChannel() {
+			weightEstimate.AddWitnessInput(input.ToRemoteConfirmedWitnessSize)
+		} else {
+			weightEstimate.AddWitnessInput(input.P2WKHWitnessSize)
+		}
 	}
 
 	// TODO(conner): sweep htlc outputs

watchtower/lookout/justice_descriptor_test.go

@@ -50,6 +50,8 @@ var (
 	)
 
 	altruistCommitType = blob.FlagCommitOutputs.Type()
+
+	altruistAnchorCommitType = blob.TypeAltruistAnchorCommit
 )
 
 // TestJusticeDescriptor asserts that a JusticeDescriptor is able to produce the
@@ -67,6 +69,10 @@ func TestJusticeDescriptor(t *testing.T) {
 			name:     "altruist and commit type",
 			blobType: altruistCommitType,
 		},
+		{
+			name:     "altruist anchor commit type",
+			blobType: altruistAnchorCommitType,
+		},
 	}
 
 	for _, test := range tests {
@@ -77,6 +83,8 @@ func TestJusticeDescriptor(t *testing.T) {
 }
 
 func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
+	isAnchorChannel := blobType.IsAnchorChannel()
+
 	const (
 		localAmount  = btcutil.Amount(100000)
 		remoteAmount = btcutil.Amount(200000)
@@ -111,9 +119,54 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 	toLocalScriptHash, err := input.WitnessScriptHash(toLocalScript)
 	require.Nil(t, err)
 
-	// Compute the to-remote witness script hash.
+	// Compute the to-remote redeem script, witness script hash, and
-	toRemoteScriptHash, err := input.CommitScriptUnencumbered(toRemotePK)
+	// sequence numbers.
-	require.Nil(t, err)
+	//
+	// NOTE: This is pretty subtle.
+	//
+	// The actual redeem script for a p2wkh output is just the pubkey, but
+	// the witness sighash calculation injects the classic p2kh script:
+	// OP_DUP OP_HASH160 <pubkey-hash160> OP_EQUALVERIFY OP_CHECKSIG. When
+	// signing for p2wkh we don't pass the raw pubkey as the witness script
+	// to the sign descriptor (since that's also not a valid script).
+	// Instead we give it the _pkscript_ of the form OP_0 <pubkey-hash160>
+	// from which pubkey-hash160 is extracted during sighash calculation.
+	//
+	// On the other hand, signing for the anchor p2wsh to-remote outputs
+	// requires the sign descriptor to contain the redeem script ver batim.
+	// This difference in behavior forces us to use a distinct
+	// toRemoteSigningScript to handle both cases.
+	var (
+		toRemoteSequence      uint32
+		toRemoteRedeemScript  []byte
+		toRemoteScriptHash    []byte
+		toRemoteSigningScript []byte
+	)
+	if isAnchorChannel {
+		toRemoteSequence = 1
+		toRemoteRedeemScript, err = input.CommitScriptToRemoteConfirmed(
+			toRemotePK,
+		)
+		require.Nil(t, err)
+
+		toRemoteScriptHash, err = input.WitnessScriptHash(
+			toRemoteRedeemScript,
+		)
+		require.Nil(t, err)
+
+		// As it should be.
+		toRemoteSigningScript = toRemoteRedeemScript
+
+	} else {
+		toRemoteRedeemScript = toRemotePK.SerializeCompressed()
+		toRemoteScriptHash, err = input.CommitScriptUnencumbered(
+			toRemotePK,
+		)
+		require.Nil(t, err)
+
+		// NOTE: This is the _pkscript_.
+		toRemoteSigningScript = toRemoteScriptHash
+	}
 
 	// Construct the breaching commitment txn, containing the to-local and
 	// to-remote outputs. We don't need any inputs for this test.
@@ -142,7 +195,11 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 	// create signatures using the original weight estimate.
 	weightEstimate.AddWitnessInput(input.ToLocalPenaltyWitnessSize - 1)
 
-	weightEstimate.AddWitnessInput(input.P2WKHWitnessSize)
+	if isAnchorChannel {
+		weightEstimate.AddWitnessInput(input.ToRemoteConfirmedWitnessSize)
+	} else {
+		weightEstimate.AddWitnessInput(input.P2WKHWitnessSize)
+	}
 	weightEstimate.AddP2WKHOutput()
 	if blobType.Has(blob.FlagReward) {
 		weightEstimate.AddP2WKHOutput()
@@ -167,6 +224,7 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 	// Begin to assemble the justice kit, starting with the sweep address,
 	// pubkeys, and csv delay.
 	justiceKit := &blob.JusticeKit{
+		BlobType:     blobType,
 		SweepAddress: makeAddrSlice(22),
 		CSVDelay:     csvDelay,
 	}
@@ -192,6 +250,7 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 					Hash:  breachTxID,
 					Index: 1,
 				},
+				Sequence: toRemoteSequence,
 			},
 		},
 	}
@@ -226,7 +285,7 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 			KeyLocator: toRemoteKeyLoc,
 			PubKey:     toRemotePK,
 		},
-		WitnessScript: toRemoteScriptHash,
+		WitnessScript: toRemoteSigningScript,
 		Output:        breachTxn.TxOut[1],
 		SigHashes:     hashCache,
 		InputIndex:    1,
@@ -245,18 +304,15 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 	// Compute the witness for the to-remote input. The first element is a
 	// DER-encoded signature under the to-remote pubkey. The sighash flag is
 	// also present, so we trim it.
-	toRemoteWitness, err := input.CommitSpendNoDelay(
+	toRemoteSigRaw, err := signer.SignOutputRaw(justiceTxn, toRemoteSignDesc)
-		signer, toRemoteSignDesc, justiceTxn, false,
-	)
 	require.Nil(t, err)
-	toRemoteSigRaw := toRemoteWitness[0][:len(toRemoteWitness[0])-1]
 
 	// Convert the DER to-local sig into a fixed-size signature.
 	toLocalSig, err := lnwire.NewSigFromSignature(toLocalSigRaw)
 	require.Nil(t, err)
 
 	// Convert the DER to-remote sig into a fixed-size signature.
-	toRemoteSig, err := lnwire.NewSigFromRawSignature(toRemoteSigRaw)
+	toRemoteSig, err := lnwire.NewSigFromSignature(toRemoteSigRaw)
 	require.Nil(t, err)
 
 	// Complete our justice kit by copying the signatures into the payload.
@@ -301,9 +357,9 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 
 	// Construct the test's to-remote witness.
 	justiceTxn.TxIn[1].Witness = make([][]byte, 2)
-	justiceTxn.TxIn[1].Witness[0] = append(toRemoteSigRaw,
+	justiceTxn.TxIn[1].Witness[0] = append(toRemoteSigRaw.Serialize(),
 		byte(txscript.SigHashAll))
-	justiceTxn.TxIn[1].Witness[1] = toRemotePK.SerializeCompressed()
+	justiceTxn.TxIn[1].Witness[1] = toRemoteRedeemScript
 
 	// Assert that the watchtower derives the same justice txn.
 	require.Equal(t, justiceTxn, wtJusticeTxn)