src/crypto/ecdsa/boring.go

// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ecdsa

import (
	"crypto/internal/boring"
	"math/big"
	"sync/atomic"
	"unsafe"
)

// Cached conversions from Go PublicKey/PrivateKey to BoringCrypto.
//
// A new 'boring atomic.Value' field in both PublicKey and PrivateKey
// serves as a cache for the most recent conversion. The cache is an
// atomic.Value because code might reasonably set up a key and then
// (thinking it immutable) use it from multiple goroutines simultaneously.
// The first operation initializes the cache; if there are multiple simultaneous
// first operations, they will do redundant work but not step on each other.
//
// We could just assume that once used in a Sign or Verify operation,
// a particular key is never again modified, but that has not been a
// stated assumption before. Just in case there is any existing code that
// does modify the key between operations, we save the original values
// alongside the cached BoringCrypto key and check that the real key
// still matches before using the cached key. The theory is that the real
// operations are significantly more expensive than the comparison.

type boringPub struct {
	key  *boring.PublicKeyECDSA
	orig PublicKey
}

func boringPublicKey(pub *PublicKey) (*boring.PublicKeyECDSA, error) {
	b := (*boringPub)(atomic.LoadPointer(&pub.boring))
	if b != nil && publicKeyEqual(&b.orig, pub) {
		return b.key, nil
	}

	b = new(boringPub)
	b.orig = copyPublicKey(pub)
	key, err := boring.NewPublicKeyECDSA(b.orig.Curve.Params().Name, b.orig.X, b.orig.Y)
	if err != nil {
		return nil, err
	}
	b.key = key
	atomic.StorePointer(&pub.boring, unsafe.Pointer(b))
	return key, nil
}

type boringPriv struct {
	key  *boring.PrivateKeyECDSA
	orig PrivateKey
}

func boringPrivateKey(priv *PrivateKey) (*boring.PrivateKeyECDSA, error) {
	b := (*boringPriv)(atomic.LoadPointer(&priv.boring))
	if b != nil && privateKeyEqual(&b.orig, priv) {
		return b.key, nil
	}

	b = new(boringPriv)
	b.orig = copyPrivateKey(priv)
	key, err := boring.NewPrivateKeyECDSA(b.orig.Curve.Params().Name, b.orig.X, b.orig.Y, b.orig.D)
	if err != nil {
		return nil, err
	}
	b.key = key
	atomic.StorePointer(&priv.boring, unsafe.Pointer(b))
	return key, nil
}

func publicKeyEqual(k1, k2 *PublicKey) bool {
	return k1.X != nil &&
		k1.Curve.Params() == k2.Curve.Params() &&
		k1.X.Cmp(k2.X) == 0 &&
		k1.Y.Cmp(k2.Y) == 0
}

func privateKeyEqual(k1, k2 *PrivateKey) bool {
	return publicKeyEqual(&k1.PublicKey, &k2.PublicKey) &&
		k1.D.Cmp(k2.D) == 0
}

func copyPublicKey(k *PublicKey) PublicKey {
	return PublicKey{
		Curve: k.Curve,
		X:     new(big.Int).Set(k.X),
		Y:     new(big.Int).Set(k.Y),
	}
}

func copyPrivateKey(k *PrivateKey) PrivateKey {
	return PrivateKey{
		PublicKey: copyPublicKey(&k.PublicKey),
		D:         new(big.Int).Set(k.D),
	}
}