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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.
// Package curve25519 provides an implementation of the X25519 function, which
// performs scalar multiplication on the elliptic curve known as Curve25519.
// See RFC 7748.
//
// This package is a wrapper for the X25519 implementation
// in the crypto/ecdh package.
package curve25519
import "crypto/ecdh"
// ScalarMult sets dst to the product scalar * point.
//
// Deprecated: when provided a low-order point, ScalarMult will set dst to all
// zeroes, irrespective of the scalar. Instead, use the X25519 function, which
// will return an error.
func ScalarMult(dst, scalar, point *[32]byte) {
if _, err := x25519(dst, scalar[:], point[:]); err != nil {
// The only error condition for x25519 when the inputs are 32 bytes long
// is if the output would have been the all-zero value.
for i := range dst {
dst[i] = 0
}
}
}
// ScalarBaseMult sets dst to the product scalar * base where base is the
// standard generator.
//
// It is recommended to use the X25519 function with Basepoint instead, as
// copying into fixed size arrays can lead to unexpected bugs.
func ScalarBaseMult(dst, scalar *[32]byte) {
curve := ecdh.X25519()
priv, err := curve.NewPrivateKey(scalar[:])
if err != nil {
panic("curve25519: internal error: scalarBaseMult was not 32 bytes")
}
copy(dst[:], priv.PublicKey().Bytes())
}
const (
// ScalarSize is the size of the scalar input to X25519.
ScalarSize = 32
// PointSize is the size of the point input to X25519.
PointSize = 32
)
// Basepoint is the canonical Curve25519 generator.
var Basepoint []byte
var basePoint = [32]byte{9}
func init() { Basepoint = basePoint[:] }
// X25519 returns the result of the scalar multiplication (scalar * point),
// according to RFC 7748, Section 5. scalar, point and the return value are
// slices of 32 bytes.
//
// scalar can be generated at random, for example with crypto/rand. point should
// be either Basepoint or the output of another X25519 call.
//
// If point is Basepoint (but not if it's a different slice with the same
// contents) a precomputed implementation might be used for performance.
func X25519(scalar, point []byte) ([]byte, error) {
// Outline the body of function, to let the allocation be inlined in the
// caller, and possibly avoid escaping to the heap.
var dst [32]byte
return x25519(&dst, scalar, point)
}
func x25519(dst *[32]byte, scalar, point []byte) ([]byte, error) {
curve := ecdh.X25519()
pub, err := curve.NewPublicKey(point)
if err != nil {
return nil, err
}
priv, err := curve.NewPrivateKey(scalar)
if err != nil {
return nil, err
}
out, err := priv.ECDH(pub)
if err != nil {
return nil, err
}
copy(dst[:], out)
return dst[:], nil
}
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