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/wallet.go

@@ -271,9 +271,9 @@ type LightningWallet struct {
 	// update the commitment state.
 	Signer Signer
 
-	// chainIO is an instance of the BlockChainIO interface. chainIO is
+	// ChainIO is an instance of the BlockChainIO interface. ChainIO is
 	// used to lookup the existence of outputs within the UTXO set.
-	chainIO BlockChainIO
+	ChainIO BlockChainIO
 
 	// rootKey is the root HD key derived from a WalletController private
 	// key. This rootKey is used to derive all LN specific secrets.
@@ -341,7 +341,7 @@ func NewLightningWallet(cdb *channeldb.DB, notifier chainntnfs.ChainNotifier,
 		chainNotifier:    notifier,
 		Signer:           signer,
 		WalletController: wallet,
-		chainIO:          bio,
+		ChainIO:          bio,
 		ChannelDB:        cdb,
 		msgChan:          make(chan interface{}, msgBufferSize),
 		nextFundingID:    0,
@@ -961,7 +961,7 @@ func (l *LightningWallet) handleFundingCounterPartySigs(msg *addCounterPartySigs
 			// Fetch the alleged previous output along with the
 			// pkscript referenced by this input.
 			prevOut := txin.PreviousOutPoint
-			output, err := l.chainIO.GetUtxo(&prevOut.Hash, prevOut.Index)
+			output, err := l.ChainIO.GetUtxo(&prevOut.Hash, prevOut.Index)
 			if output == nil {
 				msg.err <- fmt.Errorf("input to funding tx does not exist: %v", err)
 				return
@@ -1200,7 +1200,7 @@ func (l *LightningWallet) handleChannelOpen(req *channelOpenMsg) {
 
 	// Finally, create and officially open the payment channel!
 	// TODO(roasbeef): CreationTime once tx is 'open'
-	channel, _ := NewLightningChannel(l.Signer, l.chainIO, l.chainNotifier,
+	channel, _ := NewLightningChannel(l.Signer, l.ChainIO, l.chainNotifier,
 		res.partialState)
 
 	res.chanOpen <- channel
@@ -1242,7 +1242,7 @@ out:
 
 	// Finally, create and officially open the payment channel!
 	// TODO(roasbeef): CreationTime once tx is 'open'
-	channel, _ := NewLightningChannel(l.Signer, l.chainIO, l.chainNotifier,
+	channel, _ := NewLightningChannel(l.Signer, l.ChainIO, l.chainNotifier,
 		res.partialState)
 	res.chanOpen <- channel
 }