crypto, crypto/ecies, crypto/secp256k1: libsecp256k1 scalar mult

thanks to Felix Lange (fjl) for help with design & impl

crypto/secp256k1/curve_test.go

@@ -0,0 +1,39 @@
+// 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")
+}