Modify the function of SignCanonical in btcec, do makeCompact after i…

…sCanonicalSig.

Since function recoverKeyFromSignature is called inside function makeCompact, the performance loss calculated by function recoverKeyFromSignature is relatively large.
So do makeCompact after isCanonicalSig can reduce the number of times it is executed, when isCanonicalSig is false then continue the next loop.
The average performance improvement after modification is nearly 40%－50%.

btcsuite/btcd/btcec/privkey.go

@@ -65,21 +65,23 @@ func (p *PrivateKey) Sign(hash []byte) (*Signature, error) {
 
 // SignCanonical goes through signatures and returns only a canonical
 // representations.  This matches the EOS blockchain expectations.
+// do isCanonical before compactSig, for performance improvement
 func (p *PrivateKey) SignCanonical(curve *KoblitzCurve, hash []byte) ([]byte, error) {
 	for i := 0; i < 25; i++ {
 		sig, err := signRFC6979(p, hash, i)
 		if err != nil {
 			return nil, err
 		}
 
-		compactSig, err := makeCompact(curve, sig, p, hash, true)
-		if err != nil {
+		if !isCanonicalSig(sig.R.Bytes(), sig.S.Bytes()) {
 			continue
 		}
 
-		if isCanonical(compactSig) {
-			return compactSig, nil
+		compactSig, err := makeCompact(curve, sig, p, hash, true)
+		if err != nil {
+			continue
 		}
+		return compactSig, nil
 	}
 	return nil, errors.New("couldn't find a canonical signature")
 }
@@ -110,3 +112,14 @@ func isCanonical(compactSig []byte) bool {
 	t4 := !(d[33] == 0 && ((d[34] & 0x80) == 0))
 	return t1 && t2 && t3 && t4
 }
+
+// add function for original signature, do isCanonical before compactSig
+func isCanonicalSig(r []byte, s []byte) bool {
+	rlen := len(r)
+	slen := len(s)
+	t1 := rlen < 32 || (r[0] & 0x80) == 0
+	t2 := !( (rlen < 32 || r[0] == 0) && ((rlen >= 31 && (r[rlen-31] & 0x80) == 0) || rlen < 31) )
+	t3 := slen < 32 || (s[0] & 0x80) == 0
+	t4 := !( (slen < 32 || s[0] == 0) && ((slen >= 31 && (s[slen-31] & 0x80) == 0) || slen < 31) )
+	return t1 && t2 && t3 && t4
+}