common/math, crypto/secp256k1: make ReadBits public in package math

common/math/big.go

@@ -102,14 +102,15 @@ func PaddedBigBytes(bigint *big.Int, n int) []byte {
 		return bigint.Bytes()
 	}
 	ret := make([]byte, n)
-	readBits(ret, bigint.Bits())
+	ReadBits(bigint, ret)
 	return ret
 }
 
-// reads little-endian words as big-endian bytes into buf.
-func readBits(buf []byte, words []big.Word) {
+// ReadBits encodes the absolute value of bigint as big-endian bytes. Callers must ensure
+// that buf has enough space. If buf is too short the result will be incomplete.
+func ReadBits(bigint *big.Int, buf []byte) {
 	i := len(buf)
-	for _, d := range words {
+	for _, d := range bigint.Bits() {
 		for j := 0; j < wordBytes && i > 0; j++ {
 			i--
 			buf[i] = byte(d)

common/math/big_test.go

@@ -18,6 +18,7 @@ package math
 
 import (
 	"bytes"
+	"encoding/hex"
 	"math/big"
 	"testing"
 )
@@ -138,6 +139,21 @@ func BenchmarkPaddedBigBytes(b *testing.B) {
 	}
 }
 
+func TestReadBits(t *testing.T) {
+	check := func(input string) {
+		want, _ := hex.DecodeString(input)
+		int, _ := new(big.Int).SetString(input, 16)
+		buf := make([]byte, len(want))
+		ReadBits(int, buf)
+		if !bytes.Equal(buf, want) {
+			t.Errorf("have: %x\nwant: %x", buf, want)
+		}
+	}
+	check("000000000000000000000000000000000000000000000000000000FEFCF3F8F0")
+	check("0000000000012345000000000000000000000000000000000000FEFCF3F8F0")
+	check("18F8F8F1000111000110011100222004330052300000000000000000FEFCF3F8F0")
+}
+
 func TestU256(t *testing.T) {
 	tests := []struct{ x, y *big.Int }{
 		{x: big.NewInt(0), y: big.NewInt(0)},

crypto/secp256k1/curve.go

@@ -36,6 +36,8 @@ import (
 	"math/big"
 	"sync"
 	"unsafe"
+
+	"github.com/ethereum/go-ethereum/common/math"
 )
 
 /*
@@ -230,8 +232,8 @@ func (BitCurve *BitCurve) ScalarMult(Bx, By *big.Int, scalar []byte) (*big.Int,
 
 	// Do the multiplication in C, updating point.
 	point := make([]byte, 64)
-	readBits(point[:32], Bx)
-	readBits(point[32:], By)
+	math.ReadBits(Bx, point[:32])
+	math.ReadBits(By, point[32:])
 	pointPtr := (*C.uchar)(unsafe.Pointer(&point[0]))
 	scalarPtr := (*C.uchar)(unsafe.Pointer(&scalar[0]))
 	res := C.secp256k1_pubkey_scalar_mul(context, pointPtr, scalarPtr)

crypto/secp256k1/curve_test.go

@@ -1,39 +0,0 @@
-// Copyright 2015 The go-ethereum Authors
-// This file is part of the go-ethereum library.
-//
-// The go-ethereum library is free software: you can redistribute it and/or modify
-// it under the terms of the GNU Lesser General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// The go-ethereum library is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-// GNU Lesser General Public License for more details.
-//
-// You should have received a copy of the GNU Lesser General Public License
-// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
-
-package secp256k1
-
-import (
-	"bytes"
-	"encoding/hex"
-	"math/big"
-	"testing"
-)
-
-func TestReadBits(t *testing.T) {
-	check := func(input string) {
-		want, _ := hex.DecodeString(input)
-		int, _ := new(big.Int).SetString(input, 16)
-		buf := make([]byte, len(want))
-		readBits(buf, int)
-		if !bytes.Equal(buf, want) {
-			t.Errorf("have: %x\nwant: %x", buf, want)
-		}
-	}
-	check("000000000000000000000000000000000000000000000000000000FEFCF3F8F0")
-	check("0000000000012345000000000000000000000000000000000000FEFCF3F8F0")
-	check("18F8F8F1000111000110011100222004330052300000000000000000FEFCF3F8F0")
-}

crypto/secp256k1/secp256.go

@@ -38,7 +38,6 @@ import "C"
 
 import (
 	"errors"
-	"math/big"
 	"unsafe"
 )
 
@@ -129,16 +128,3 @@ func checkSignature(sig []byte) error {
 	}
 	return nil
 }
-
-// reads num into buf as big-endian bytes.
-func readBits(buf []byte, num *big.Int) {
-	const wordLen = int(unsafe.Sizeof(big.Word(0)))
-	i := len(buf)
-	for _, d := range num.Bits() {
-		for j := 0; j < wordLen && i > 0; j++ {
-			i--
-			buf[i] = byte(d)
-			d >>= 8
-		}
-	}
-}

crypto/secp256k1/secp256_test.go

@@ -24,6 +24,7 @@ import (
 	"encoding/hex"
 	"testing"
 
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto/randentropy"
 )
 
@@ -35,9 +36,7 @@ func generateKeyPair() (pubkey, privkey []byte) {
 		panic(err)
 	}
 	pubkey = elliptic.Marshal(S256(), key.X, key.Y)
-	privkey = make([]byte, 32)
-	readBits(privkey, key.D)
-	return pubkey, privkey
+	return pubkey, math.PaddedBigBytes(key.D, 32)
 }
 
 func randSig() []byte {