95 lines
3.1 KiB
Go
95 lines
3.1 KiB
Go
// Copyright 2019 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package curve25519 provides an implementation of the X25519 function, which
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// performs scalar multiplication on the elliptic curve known as Curve25519.
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// See RFC 7748.
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package curve25519 // import "golang.org/x/crypto/curve25519"
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import (
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"crypto/subtle"
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"fmt"
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)
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// ScalarMult sets dst to the product scalar * point.
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//
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// Deprecated: when provided a low-order point, ScalarMult will set dst to all
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// zeroes, irrespective of the scalar. Instead, use the X25519 function, which
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// will return an error.
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func ScalarMult(dst, scalar, point *[32]byte) {
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scalarMult(dst, scalar, point)
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}
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// ScalarBaseMult sets dst to the product scalar * base where base is the
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// standard generator.
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//
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// It is recommended to use the X25519 function with Basepoint instead, as
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// copying into fixed size arrays can lead to unexpected bugs.
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func ScalarBaseMult(dst, scalar *[32]byte) {
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ScalarMult(dst, scalar, &basePoint)
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}
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const (
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// ScalarSize is the size of the scalar input to X25519.
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ScalarSize = 32
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// PointSize is the size of the point input to X25519.
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PointSize = 32
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)
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// Basepoint is the canonical Curve25519 generator.
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var Basepoint []byte
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var basePoint = [32]byte{9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
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func init() { Basepoint = basePoint[:] }
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func checkBasepoint() {
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if subtle.ConstantTimeCompare(Basepoint, []byte{
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0x09, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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}) != 1 {
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panic("curve25519: global Basepoint value was modified")
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}
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}
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// X25519 returns the result of the scalar multiplication (scalar * point),
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// according to RFC 7748, Section 5. scalar, point and the return value are
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// slices of 32 bytes.
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//
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// scalar can be generated at random, for example with crypto/rand. point should
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// be either Basepoint or the output of another X25519 call.
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//
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// If point is Basepoint (but not if it's a different slice with the same
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// contents) a precomputed implementation might be used for performance.
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func X25519(scalar, point []byte) ([]byte, error) {
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// Outline the body of function, to let the allocation be inlined in the
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// caller, and possibly avoid escaping to the heap.
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var dst [32]byte
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return x25519(&dst, scalar, point)
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}
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func x25519(dst *[32]byte, scalar, point []byte) ([]byte, error) {
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var in [32]byte
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if l := len(scalar); l != 32 {
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return nil, fmt.Errorf("bad scalar length: %d, expected %d", l, 32)
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}
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if l := len(point); l != 32 {
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return nil, fmt.Errorf("bad point length: %d, expected %d", l, 32)
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}
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copy(in[:], scalar)
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if &point[0] == &Basepoint[0] {
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checkBasepoint()
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ScalarBaseMult(dst, &in)
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} else {
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var base, zero [32]byte
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copy(base[:], point)
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ScalarMult(dst, &in, &base)
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if subtle.ConstantTimeCompare(dst[:], zero[:]) == 1 {
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return nil, fmt.Errorf("bad input point: low order point")
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}
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}
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return dst[:], nil
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}
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