lnwallet: extend the SignDescriptor to include a PrivateTweak

This commit extends the SignDescriptor with a single attribute, the
‘PrivateTweak’. The duties of the Signer interface have also been
augmented to properly derive a private key using the specified tweak,
iff it’s non-nil.

As currently defined in order to generate the proper private key based
off of a PrivateTweak, the signer is to add the tweak value to the
private key for the specified public key. This generated value is to be
used for signing within the specified context.

This change paves the way for automatic revoked output sweeping with
signatures generated directly by the Signer interface, maintaining the
structure of the abstraction.

A test has been added at the interface level in order to excerise each
WalletController’s implementation of the key derivation as currently
defined.

lnwallet/btcwallet/signer.go

@@ -24,7 +24,7 @@ func (b *BtcWallet) FetchInputInfo(prevOut *wire.OutPoint) (*wire.TxOut, error)
 	)
 
 	// First check to see if the output is already within the utxo cache.
-	// If so we can return directly saving usk a disk access.
+	// If so we can return directly saving a disk access.
 	b.cacheMtx.RLock()
 	if output, ok := b.utxoCache[*prevOut]; ok {
 		b.cacheMtx.RUnlock()
@@ -72,7 +72,7 @@ func (b *BtcWallet) fetchOutputAddr(script []byte) (waddrmgr.ManagedAddress, err
 	return nil, fmt.Errorf("address not found")
 }
 
-// fetchPrivKey attempts to retrieve the raw private key coresponding to the
+// fetchPrivKey attempts to retrieve the raw private key corresponding to the
 // passed public key.
 // TODO(roasbeef): alternatively can extract all the data pushes within the
 // script, then attempt to match keys one by one
@@ -98,14 +98,24 @@ func (b *BtcWallet) fetchPrivKey(pub *btcec.PublicKey) (*btcec.PrivateKey, error
 func (b *BtcWallet) SignOutputRaw(tx *wire.MsgTx, signDesc *lnwallet.SignDescriptor) ([]byte, error) {
 	witnessScript := signDesc.WitnessScript
 
+	// First attempt to fetch the private key which corresponds to the
+	// specified public key.
 	privKey, err := b.fetchPrivKey(signDesc.PubKey)
 	if err != nil {
 		return nil, err
 	}
 
+	// If a tweak is specified, then we'll need to use this tweak to derive
+	// the final private key to be used for signing this output.
+	if signDesc.PrivateTweak != nil {
+		privKey = lnwallet.DeriveRevocationPrivKey(privKey,
+			signDesc.PrivateTweak)
+	}
+
 	amt := signDesc.Output.Value
-	sig, err := txscript.RawTxInWitnessSignature(tx, signDesc.SigHashes, 0,
-		amt, witnessScript, txscript.SigHashAll, privKey)
+	sig, err := txscript.RawTxInWitnessSignature(tx, signDesc.SigHashes,
+		signDesc.InputIndex, amt, witnessScript, txscript.SigHashAll,
+		privKey)
 	if err != nil {
 		return nil, err
 	}