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// Copyright 2019 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.
//go:build !go1.20
package curve25519
import (
"crypto/subtle"
"errors"
"strconv"
"golang.org/x/crypto/curve25519/internal/field"
)
func scalarMult(dst, scalar, point *[32]byte) {
var e [32]byte
copy(e[:], scalar[:])
e[0] &= 248
e[31] &= 127
e[31] |= 64
var x1, x2, z2, x3, z3, tmp0, tmp1 field.Element
x1.SetBytes(point[:])
x2.One()
x3.Set(&x1)
z3.One()
swap := 0
for pos := 254; pos >= 0; pos-- {
b := e[pos/8] >> uint(pos&7)
b &= 1
swap ^= int(b)
x2.Swap(&x3, swap)
z2.Swap(&z3, swap)
swap = int(b)
tmp0.Subtract(&x3, &z3)
tmp1.Subtract(&x2, &z2)
x2.Add(&x2, &z2)
z2.Add(&x3, &z3)
z3.Multiply(&tmp0, &x2)
z2.Multiply(&z2, &tmp1)
tmp0.Square(&tmp1)
tmp1.Square(&x2)
x3.Add(&z3, &z2)
z2.Subtract(&z3, &z2)
x2.Multiply(&tmp1, &tmp0)
tmp1.Subtract(&tmp1, &tmp0)
z2.Square(&z2)
z3.Mult32(&tmp1, 121666)
x3.Square(&x3)
tmp0.Add(&tmp0, &z3)
z3.Multiply(&x1, &z2)
z2.Multiply(&tmp1, &tmp0)
}
x2.Swap(&x3, swap)
z2.Swap(&z3, swap)
z2.Invert(&z2)
x2.Multiply(&x2, &z2)
copy(dst[:], x2.Bytes())
}
func scalarBaseMult(dst, scalar *[32]byte) {
checkBasepoint()
scalarMult(dst, scalar, &basePoint)
}
func x25519(dst *[32]byte, scalar, point []byte) ([]byte, error) {
var in [32]byte
if l := len(scalar); l != 32 {
return nil, errors.New("bad scalar length: " + strconv.Itoa(l) + ", expected 32")
}
if l := len(point); l != 32 {
return nil, errors.New("bad point length: " + strconv.Itoa(l) + ", expected 32")
}
copy(in[:], scalar)
if &point[0] == &Basepoint[0] {
scalarBaseMult(dst, &in)
} else {
var base, zero [32]byte
copy(base[:], point)
scalarMult(dst, &in, &base)
if subtle.ConstantTimeCompare(dst[:], zero[:]) == 1 {
return nil, errors.New("bad input point: low order point")
}
}
return dst[:], nil
}
func checkBasepoint() {
if subtle.ConstantTimeCompare(Basepoint, []byte{
0x09, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
}) != 1 {
panic("curve25519: global Basepoint value was modified")
}
}
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