lnwire: add new Sig type to handle conversion to/from btcec.Signature

In this commit, we add a new signature type. We’ll use this type to
avoid fully decoding a signature on the wire into a btcec.Signature.
This type is only really needed when we need to do signature
validation, as a result, always encoding it is a waste. Several helper
methods have been added to the new struct in order to ensure that we
can use it in the existing codebase without substantial issues.

lnwire/signature.go

@@ -6,12 +6,16 @@ import (
 	"github.com/roasbeef/btcd/btcec"
 )
 
-// SerializeSigToWire serializes a *Signature to [64]byte in the format
-// specified by the Lightning RFC.
-func SerializeSigToWire(b *[64]byte, e *btcec.Signature) error {
+// Sig is a fixed-sized ECDSA signature. Unlike Bitcoin, we use fixed sized
+// signatures on the wire, instead of DER encoded signatures. This type
+// provides several methods to convert to/from a regular Bitcoin DER encoded
+// signature (raw bytes and *btcec.Signature).
+type Sig [64]byte
 
-	// Serialize the signature with all the checks that entails.
-	sig := e.Serialize()
+// NewSigFromRawSignature returns a Sig from a Bitcoin raw signature encoded in
+// the cannonical DER encoding.
+func NewSigFromRawSignature(sig []byte) (Sig, error) {
+	var b Sig
 
 	// Extract lengths of R and S. The DER representation is laid out as
 	// 0x30 <length> 0x02 <length r> r 0x02 <length s> s
@@ -23,14 +27,14 @@ func SerializeSigToWire(b *[64]byte, e *btcec.Signature) error {
 	sLen := sig[5+rLen]
 
 	// Check to make sure R and S can both fit into their intended buffers.
-	// We check S first because these code blocks decrement sLen and
-	// rLen in the case of a 33-byte 0-padded integer returned from
-	// Serialize() and rLen is used in calculating array indices for
-	// S. We can track this with additional variables, but it's more
-	// efficient to just check S first.
+	// We check S first because these code blocks decrement sLen and rLen
+	// in the case of a 33-byte 0-padded integer returned from Serialize()
+	// and rLen is used in calculating array indices for S. We can track
+	// this with additional variables, but it's more efficient to just
+	// check S first.
 	if sLen > 32 {
 		if (sLen > 33) || (sig[6+rLen] != 0x00) {
-			return fmt.Errorf("S is over 32 bytes long " +
+			return b, fmt.Errorf("S is over 32 bytes long " +
 				"without padding")
 		}
 		sLen--
@@ -42,7 +46,7 @@ func SerializeSigToWire(b *[64]byte, e *btcec.Signature) error {
 	// Do the same for R as we did for S
 	if rLen > 32 {
 		if (rLen > 33) || (sig[4] != 0x00) {
-			return fmt.Errorf("R is over 32 bytes long " +
+			return b, fmt.Errorf("R is over 32 bytes long " +
 				"without padding")
 		}
 		rLen--
@@ -50,13 +54,33 @@ func SerializeSigToWire(b *[64]byte, e *btcec.Signature) error {
 	} else {
 		copy(b[32-rLen:], sig[4:4+rLen])
 	}
-	return nil
+
+	return b, nil
 }
 
-// DeserializeSigFromWire deserializes a *Signature from [64]byte in the format
-// specified by the Lightning RFC.
-func DeserializeSigFromWire(e **btcec.Signature, b [64]byte) error {
+// NewSigFromSignature creates a new signature as used on the wire, from an
+// existing btcec.Signature.
+func NewSigFromSignature(e *btcec.Signature) (Sig, error) {
+	// Serialize the signature with all the checks that entails.
+	return NewSigFromRawSignature(e.Serialize())
+}
 
+// ToSignature converts the fixed-sized signature to a btcec.Signature objects
+// which can be used for signature validation checks.
+func (b *Sig) ToSignature() (*btcec.Signature, error) {
+	// Parse the signature with strict checks.
+	sigBytes := b.ToSignatureBytes()
+	sig, err := btcec.ParseDERSignature(sigBytes, btcec.S256())
+	if err != nil {
+		return nil, err
+	}
+
+	return sig, nil
+}
+
+// ToSignatureBytes serializes the target fixed-sized signature into the raw
+// bytes of a DER encoding.
+func (b *Sig) ToSignatureBytes() []byte {
 	// Extract canonically-padded bigint representations from buffer
 	r := extractCanonicalPadding(b[0:32])
 	s := extractCanonicalPadding(b[32:64])
@@ -75,13 +99,7 @@ func DeserializeSigFromWire(e **btcec.Signature, b [64]byte) error {
 	copy(sigBytes[4:], r)         // Copy R
 	copy(sigBytes[rLen+6:], s)    // Copy S
 
-	// Parse the signature with strict checks.
-	sig, err := btcec.ParseDERSignature(sigBytes, btcec.S256())
-	if err != nil {
-		return err
-	}
-	*e = sig
-	return nil
+	return sigBytes
 }
 
 // extractCanonicalPadding is a utility function to extract the canonical