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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/edwards25519/field"
"crypto/internal/randutil"
"errors"
"io"
)
var (
x25519PublicKeySize = 32
x25519PrivateKeySize = 32
x25519SharedSecretSize = 32
)
// X25519 returns a Curve which implements the X25519 function over Curve25519
// (RFC 7748, Section 5).
//
// Multiple invocations of this function will return the same value, so it can
// be used for equality checks and switch statements.
func X25519() Curve { return x25519 }
var x25519 = &x25519Curve{}
type x25519Curve struct{}
func (c *x25519Curve) String() string {
return "X25519"
}
func (c *x25519Curve) GenerateKey(rand io.Reader) (*PrivateKey, error) {
key := make([]byte, x25519PrivateKeySize)
randutil.MaybeReadByte(rand)
if _, err := io.ReadFull(rand, key); err != nil {
return nil, err
}
return c.NewPrivateKey(key)
}
func (c *x25519Curve) NewPrivateKey(key []byte) (*PrivateKey, error) {
if len(key) != x25519PrivateKeySize {
return nil, errors.New("crypto/ecdh: invalid private key size")
}
return &PrivateKey{
curve: c,
privateKey: append([]byte{}, key...),
}, nil
}
func (c *x25519Curve) privateKeyToPublicKey(key *PrivateKey) *PublicKey {
if key.curve != c {
panic("crypto/ecdh: internal error: converting the wrong key type")
}
k := &PublicKey{
curve: key.curve,
publicKey: make([]byte, x25519PublicKeySize),
}
x25519Basepoint := [32]byte{9}
x25519ScalarMult(k.publicKey, key.privateKey, x25519Basepoint[:])
return k
}
func (c *x25519Curve) NewPublicKey(key []byte) (*PublicKey, error) {
if len(key) != x25519PublicKeySize {
return nil, errors.New("crypto/ecdh: invalid public key")
}
return &PublicKey{
curve: c,
publicKey: append([]byte{}, key...),
}, nil
}
func (c *x25519Curve) ecdh(local *PrivateKey, remote *PublicKey) ([]byte, error) {
out := make([]byte, x25519SharedSecretSize)
x25519ScalarMult(out, local.privateKey, remote.publicKey)
if isZero(out) {
return nil, errors.New("crypto/ecdh: bad X25519 remote ECDH input: low order point")
}
return out, nil
}
func x25519ScalarMult(dst, scalar, point []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())
}
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