92 lines
2.9 KiB
Go
92 lines
2.9 KiB
Go
// Copyright 2015 The btcsuite developers
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// Copyright (c) 2015-2021 The Decred developers
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// Use of this source code is governed by an ISC
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// license that can be found in the LICENSE file.
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package secp256k1
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import (
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"compress/zlib"
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"encoding/base64"
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"encoding/binary"
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"io/ioutil"
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"strings"
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"sync"
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)
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//go:generate go run -tags gensecp256k1 genprecomps.go
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// bytePointTable describes a table used to house pre-computed values for
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// accelerating scalar base multiplication.
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type bytePointTable [32][256][2]FieldVal
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// s256BytePoints houses pre-computed values used to accelerate scalar base
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// multiplication such that they are only loaded on first use.
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var s256BytePoints = func() func() *bytePointTable {
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// mustLoadBytePoints decompresses and deserializes the pre-computed byte
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// points used to accelerate scalar base multiplication for the secp256k1
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// curve.
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//
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// This approach is used since it allows the compile to use significantly
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// less ram and be performed much faster than it is with hard-coding the
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// final in-memory data structure. At the same time, it is quite fast to
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// generate the in-memory data structure on first use with this approach
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// versus computing the table.
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//
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// It will panic on any errors because the data is hard coded and thus any
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// errors means something is wrong in the source code.
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var data *bytePointTable
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mustLoadBytePoints := func() {
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// There will be no byte points to load when generating them.
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bp := compressedBytePoints
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if len(bp) == 0 {
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return
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}
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// Decompress the pre-computed table used to accelerate scalar base
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// multiplication.
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decoder := base64.NewDecoder(base64.StdEncoding, strings.NewReader(bp))
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r, err := zlib.NewReader(decoder)
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if err != nil {
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panic(err)
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}
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serialized, err := ioutil.ReadAll(r)
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if err != nil {
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panic(err)
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}
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// Deserialize the precomputed byte points and set the memory table to
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// them.
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offset := 0
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var bytePoints bytePointTable
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for byteNum := 0; byteNum < len(bytePoints); byteNum++ {
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// All points in this window.
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for i := 0; i < len(bytePoints[byteNum]); i++ {
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px := &bytePoints[byteNum][i][0]
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py := &bytePoints[byteNum][i][1]
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for i := 0; i < len(px.n); i++ {
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px.n[i] = binary.LittleEndian.Uint32(serialized[offset:])
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offset += 4
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}
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for i := 0; i < len(py.n); i++ {
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py.n[i] = binary.LittleEndian.Uint32(serialized[offset:])
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offset += 4
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}
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}
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}
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data = &bytePoints
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}
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// Return a closure that initializes the data on first access. This is done
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// because the table takes a non-trivial amount of memory and initializing
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// it unconditionally would cause anything that imports the package, either
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// directly, or indirectly via transitive deps, to use that memory even if
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// the caller never accesses any parts of the package that actually needs
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// access to it.
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var loadBytePointsOnce sync.Once
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return func() *bytePointTable {
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loadBytePointsOnce.Do(mustLoadBytePoints)
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return data
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}
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}()
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