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precompute.go
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precompute.go
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// Copyright 2015 Conformal Systems LLC. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package btcec
import (
"bytes"
"compress/zlib"
"encoding/base64"
"encoding/binary"
"io/ioutil"
)
//go:generate go run -tags gensecp256k1 genprecomps.go
// loadS256BytePoints decompresses and deserializes the pre-computed byte points
// used to accelerate scalar base multiplication for the secp256k1 curve. This
// approach is used since it allows the compile to use significantly less ram
// and be performed much faster than it is with hard-coding the final in-memory
// data structure. At the same time, it is quite fast to generate the in-memory
// data structure at init time with this approach versus computing the table.
func loadS256BytePoints() error {
// There will be no byte points to load when generating them.
bp := secp256k1BytePoints
if len(secp256k1BytePoints) == 0 {
return nil
}
// Decompress the pre-computed table used to accelerate scalar base
// multiplication.
decoded := make([]byte, base64.StdEncoding.DecodedLen(len(bp)))
if _, err := base64.StdEncoding.Decode(decoded, bp); err != nil {
return err
}
r, err := zlib.NewReader(bytes.NewReader(decoded))
if err != nil {
return err
}
serialized, err := ioutil.ReadAll(r)
if err != nil {
return err
}
// Deserialize the precomputed byte points and set the curve to them.
offset := 0
var bytePoints [32][256][3]fieldVal
for byteNum := 0; byteNum < 32; byteNum++ {
// All points in this window.
for i := 0; i < 256; i++ {
px, py, pz := new(fieldVal), new(fieldVal), new(fieldVal)
for i := 0; i < 10; i++ {
px.n[i] = binary.LittleEndian.Uint32(serialized[offset:])
offset += 4
}
for i := 0; i < 10; i++ {
py.n[i] = binary.LittleEndian.Uint32(serialized[offset:])
offset += 4
}
for i := 0; i < 10; i++ {
pz.n[i] = binary.LittleEndian.Uint32(serialized[offset:])
offset += 4
}
bytePoints[byteNum][i][0] = *px
bytePoints[byteNum][i][1] = *py
bytePoints[byteNum][i][2] = *pz
}
}
secp256k1.bytePoints = &bytePoints
return nil
}