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// Copyright 2022 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 ecdh
import (
"crypto/internal/boring"
"crypto/internal/nistec"
"crypto/internal/randutil"
"encoding/binary"
"errors"
"io"
"math/bits"
)
type nistCurve[Point nistPoint[Point]] struct {
name string
newPoint func() Point
scalarOrder []byte
}
// nistPoint is a generic constraint for the nistec Point types.
type nistPoint[T any] interface {
Bytes() []byte
BytesX() ([]byte, error)
SetBytes([]byte) (T, error)
ScalarMult(T, []byte) (T, error)
ScalarBaseMult([]byte) (T, error)
}
func (c *nistCurve[Point]) String() string {
return c.name
}
var errInvalidPrivateKey = errors.New("crypto/ecdh: invalid private key")
func (c *nistCurve[Point]) GenerateKey(rand io.Reader) (*PrivateKey, error) {
if boring.Enabled && rand == boring.RandReader {
key, bytes, err := boring.GenerateKeyECDH(c.name)
if err != nil {
return nil, err
}
return newBoringPrivateKey(c, key, bytes)
}
key := make([]byte, len(c.scalarOrder))
randutil.MaybeReadByte(rand)
for {
if _, err := io.ReadFull(rand, key); err != nil {
return nil, err
}
// Mask off any excess bits if the size of the underlying field is not a
// whole number of bytes, which is only the case for P-521. We use a
// pointer to the scalarOrder field because comparing generic and
// instantiated types is not supported.
if &c.scalarOrder[0] == &p521Order[0] {
key[0] &= 0b0000_0001
}
// In tests, rand will return all zeros and NewPrivateKey will reject
// the zero key as it generates the identity as a public key. This also
// makes this function consistent with crypto/elliptic.GenerateKey.
key[1] ^= 0x42
k, err := c.NewPrivateKey(key)
if err == errInvalidPrivateKey {
continue
}
return k, err
}
}
func (c *nistCurve[Point]) NewPrivateKey(key []byte) (*PrivateKey, error) {
if len(key) != len(c.scalarOrder) {
return nil, errors.New("crypto/ecdh: invalid private key size")
}
if isZero(key) || !isLess(key, c.scalarOrder) {
return nil, errInvalidPrivateKey
}
if boring.Enabled {
bk, err := boring.NewPrivateKeyECDH(c.name, key)
if err != nil {
return nil, err
}
return newBoringPrivateKey(c, bk, key)
}
k := &PrivateKey{
curve: c,
privateKey: append([]byte{}, key...),
}
return k, nil
}
func newBoringPrivateKey(c Curve, bk *boring.PrivateKeyECDH, privateKey []byte) (*PrivateKey, error) {
k := &PrivateKey{
curve: c,
boring: bk,
privateKey: append([]byte(nil), privateKey...),
}
return k, nil
}
func (c *nistCurve[Point]) privateKeyToPublicKey(key *PrivateKey) *PublicKey {
boring.Unreachable()
if key.curve != c {
panic("crypto/ecdh: internal error: converting the wrong key type")
}
p, err := c.newPoint().ScalarBaseMult(key.privateKey)
if err != nil {
// This is unreachable because the only error condition of
// ScalarBaseMult is if the input is not the right size.
panic("crypto/ecdh: internal error: nistec ScalarBaseMult failed for a fixed-size input")
}
publicKey := p.Bytes()
if len(publicKey) == 1 {
// The encoding of the identity is a single 0x00 byte. This is
// unreachable because the only scalar that generates the identity is
// zero, which is rejected by NewPrivateKey.
panic("crypto/ecdh: internal error: nistec ScalarBaseMult returned the identity")
}
return &PublicKey{
curve: key.curve,
publicKey: publicKey,
}
}
// isZero returns whether a is all zeroes in constant time.
func isZero(a []byte) bool {
var acc byte
for _, b := range a {
acc |= b
}
return acc == 0
}
// isLess returns whether a < b, where a and b are big-endian buffers of the
// same length and shorter than 72 bytes.
func isLess(a, b []byte) bool {
if len(a) != len(b) {
panic("crypto/ecdh: internal error: mismatched isLess inputs")
}
// Copy the values into a fixed-size preallocated little-endian buffer.
// 72 bytes is enough for every scalar in this package, and having a fixed
// size lets us avoid heap allocations.
if len(a) > 72 {
panic("crypto/ecdh: internal error: isLess input too large")
}
bufA, bufB := make([]byte, 72), make([]byte, 72)
for i := range a {
bufA[i], bufB[i] = a[len(a)-i-1], b[len(b)-i-1]
}
// Perform a subtraction with borrow.
var borrow uint64
for i := 0; i < len(bufA); i += 8 {
limbA, limbB := binary.LittleEndian.Uint64(bufA[i:]), binary.LittleEndian.Uint64(bufB[i:])
_, borrow = bits.Sub64(limbA, limbB, borrow)
}
// If there is a borrow at the end of the operation, then a < b.
return borrow == 1
}
func (c *nistCurve[Point]) NewPublicKey(key []byte) (*PublicKey, error) {
// Reject the point at infinity and compressed encodings.
if len(key) == 0 || key[0] != 4 {
return nil, errors.New("crypto/ecdh: invalid public key")
}
k := &PublicKey{
curve: c,
publicKey: append([]byte{}, key...),
}
if boring.Enabled {
bk, err := boring.NewPublicKeyECDH(c.name, k.publicKey)
if err != nil {
return nil, err
}
k.boring = bk
} else {
// SetBytes also checks that the point is on the curve.
if _, err := c.newPoint().SetBytes(key); err != nil {
return nil, err
}
}
return k, nil
}
func (c *nistCurve[Point]) ecdh(local *PrivateKey, remote *PublicKey) ([]byte, error) {
// Note that this function can't return an error, as NewPublicKey rejects
// invalid points and the point at infinity, and NewPrivateKey rejects
// invalid scalars and the zero value. BytesX returns an error for the point
// at infinity, but in a prime order group such as the NIST curves that can
// only be the result of a scalar multiplication if one of the inputs is the
// zero scalar or the point at infinity.
if boring.Enabled {
return boring.ECDH(local.boring, remote.boring)
}
boring.Unreachable()
p, err := c.newPoint().SetBytes(remote.publicKey)
if err != nil {
return nil, err
}
if _, err := p.ScalarMult(p, local.privateKey); err != nil {
return nil, err
}
return p.BytesX()
}
// P256 returns a Curve which implements NIST P-256 (FIPS 186-3, section D.2.3),
// also known as secp256r1 or prime256v1.
//
// Multiple invocations of this function will return the same value, which can
// be used for equality checks and switch statements.
func P256() Curve { return p256 }
var p256 = &nistCurve[*nistec.P256Point]{
name: "P-256",
newPoint: nistec.NewP256Point,
scalarOrder: p256Order,
}
var p256Order = []byte{
0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xbc, 0xe6, 0xfa, 0xad, 0xa7, 0x17, 0x9e, 0x84,
0xf3, 0xb9, 0xca, 0xc2, 0xfc, 0x63, 0x25, 0x51}
// P384 returns a Curve which implements NIST P-384 (FIPS 186-3, section D.2.4),
// also known as secp384r1.
//
// Multiple invocations of this function will return the same value, which can
// be used for equality checks and switch statements.
func P384() Curve { return p384 }
var p384 = &nistCurve[*nistec.P384Point]{
name: "P-384",
newPoint: nistec.NewP384Point,
scalarOrder: p384Order,
}
var p384Order = []byte{
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xc7, 0x63, 0x4d, 0x81, 0xf4, 0x37, 0x2d, 0xdf,
0x58, 0x1a, 0x0d, 0xb2, 0x48, 0xb0, 0xa7, 0x7a,
0xec, 0xec, 0x19, 0x6a, 0xcc, 0xc5, 0x29, 0x73}
// P521 returns a Curve which implements NIST P-521 (FIPS 186-3, section D.2.5),
// also known as secp521r1.
//
// Multiple invocations of this function will return the same value, which can
// be used for equality checks and switch statements.
func P521() Curve { return p521 }
var p521 = &nistCurve[*nistec.P521Point]{
name: "P-521",
newPoint: nistec.NewP521Point,
scalarOrder: p521Order,
}
var p521Order = []byte{0x01, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xfa,
0x51, 0x86, 0x87, 0x83, 0xbf, 0x2f, 0x96, 0x6b,
0x7f, 0xcc, 0x01, 0x48, 0xf7, 0x09, 0xa5, 0xd0,
0x3b, 0xb5, 0xc9, 0xb8, 0x89, 0x9c, 0x47, 0xae,
0xbb, 0x6f, 0xb7, 0x1e, 0x91, 0x38, 0x64, 0x09}