internal/radix51: replace ToBytes with AppendBytes

internal/radix51/fe.go

@@ -263,10 +263,10 @@ func (v *FieldElement) FromBytes(x []byte) *FieldElement {
 	return v
 }
 
-func (v *FieldElement) ToBytes(r []byte) {
-	if len(r) != 32 {
-		panic("invalid input size")
-	}
+// AppendBytes appends a 32 bytes little-endian encoding of v to b.
+func (v *FieldElement) AppendBytes(b []byte) []byte {
+	res, r := sliceForAppend(b, 32)
+
 	t := new(FieldElement).Reduce(v)
 
 	r[0] = byte(t[0] & 0xff)
@@ -309,6 +309,22 @@ func (v *FieldElement) ToBytes(r []byte) {
 	r[29] = byte((t[4] >> 28) & 0xff)
 	r[30] = byte((t[4] >> 36) & 0xff)
 	r[31] = byte((t[4] >> 44))
+
+	return res
+}
+
+// sliceForAppend extends the input slice by n bytes. head is the full extended
+// slice, while tail is the appended part. If the original slice has sufficient
+// capacity no allocation is performed.
+func sliceForAppend(in []byte, n int) (head, tail []byte) {
+	if total := len(in) + n; cap(in) >= total {
+		head = in[:total]
+	} else {
+		head = make([]byte, total)
+		copy(head, in)
+	}
+	tail = head[len(in):]
+	return
 }
 
 func (v *FieldElement) FromBig(num *big.Int) *FieldElement {
@@ -333,8 +349,7 @@ func (v *FieldElement) FromBig(num *big.Int) *FieldElement {
 }
 
 func (v *FieldElement) ToBig() *big.Int {
-	var buf [32]byte
-	v.ToBytes(buf[:]) // does a reduction
+	buf := v.AppendBytes(nil)
 
 	numWords := 256 / bits.UintSize
 	words := make([]big.Word, numWords)
@@ -360,8 +375,8 @@ func (v *FieldElement) ToBig() *big.Int {
 // Equal returns 1 if v and u are equal, and 0 otherwise.
 func (v *FieldElement) Equal(u *FieldElement) int {
 	var sa, sv [32]byte
-	u.ToBytes(sa[:])
-	v.ToBytes(sv[:])
+	u.AppendBytes(sa[:0])
+	v.AppendBytes(sv[:0])
 	return subtle.ConstantTimeCompare(sa[:], sv[:])
 }
 
@@ -388,7 +403,7 @@ func (v *FieldElement) CondNeg(u *FieldElement, cond int) *FieldElement {
 // IsNegative returns 1 if v is negative, and 0 otherwise.
 func (v *FieldElement) IsNegative() int {
 	var b [32]byte
-	v.ToBytes(b[:])
+	v.AppendBytes(b[:0])
 	return int(b[0] & 1)
 }
 

internal/radix51/fe_test.go

@@ -124,7 +124,7 @@ func TestFeFromBytesRoundTrip(t *testing.T) {
 		18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32}
 
 	fe.FromBytes(in[:])
-	fe.ToBytes(out[:])
+	fe.AppendBytes(out[:0])
 
 	if !bytes.Equal(in[:], out[:]) {
 		t.Error("Bytes<>FE doesn't roundtrip")
@@ -137,7 +137,7 @@ func TestFeFromBytesRoundTrip(t *testing.T) {
 	fe[3] = 0x5e8fca9e0881c
 	fe[4] = 0x5c490f087d796
 
-	fe.ToBytes(out[:])
+	fe.AppendBytes(out[:0])
 	r.FromBytes(out[:])
 
 	for i := 0; i < len(fe); i++ {