// 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.
package curve25519 // import "golang.org/x/crypto/curve25519" import ( ) // 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 (, , *[32]byte) { var [32]byte copy([:], [:]) [0] &= 248 [31] &= 127 [31] |= 64 var , , , , , , field.Element .SetBytes([:]) .One() .Set(&) .One() := 0 for := 254; >= 0; -- { := [/8] >> uint(&7) &= 1 ^= int() .Swap(&, ) .Swap(&, ) = int() .Subtract(&, &) .Subtract(&, &) .Add(&, &) .Add(&, &) .Multiply(&, &) .Multiply(&, &) .Square(&) .Square(&) .Add(&, &) .Subtract(&, &) .Multiply(&, &) .Subtract(&, &) .Square(&) .Mult32(&, 121666) .Square(&) .Add(&, &) .Multiply(&, &) .Multiply(&, &) } .Swap(&, ) .Swap(&, ) .Invert(&) .Multiply(&, &) copy([:], .Bytes()) } // 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 (, *[32]byte) { ScalarMult(, , &basePoint) } 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, 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} func init() { Basepoint = basePoint[:] } 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") } } // 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 (, []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 [32]byte return x25519(&, , ) } func x25519( *[32]byte, , []byte) ([]byte, error) { var [32]byte if := len(); != 32 { return nil, fmt.Errorf("bad scalar length: %d, expected %d", , 32) } if := len(); != 32 { return nil, fmt.Errorf("bad point length: %d, expected %d", , 32) } copy([:], ) if &[0] == &Basepoint[0] { checkBasepoint() ScalarBaseMult(, &) } else { var , [32]byte copy([:], ) ScalarMult(, &, &) if subtle.ConstantTimeCompare([:], [:]) == 1 { return nil, fmt.Errorf("bad input point: low order point") } } return [:], nil }