path: root/src/crypto/elliptic/p256_generic.go

// Copyright 2013 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 !amd64 && !arm64

package elliptic

// This file contains a constant-time, 32-bit implementation of P256.

import "math/big"

type p256Curve struct {
	*CurveParams
}

func (curve p256Curve) Params() *CurveParams {
	return curve.CurveParams
}

// p256GetScalar endian-swaps the big-endian scalar value from in and writes it
// to out. If the scalar is equal or greater than the order of the group, it's
// reduced modulo that order.
func p256GetScalar(out *[32]byte, in []byte) {
	n := new(big.Int).SetBytes(in)
	var scalarBytes []byte

	if n.Cmp(p256Params.N) >= 0 || len(in) > len(out) {
		n.Mod(n, p256Params.N)
		scalarBytes = n.Bytes()
	} else {
		scalarBytes = in
	}

	for i, v := range scalarBytes {
		out[len(scalarBytes)-(1+i)] = v
	}
}

func (p256Curve) ScalarBaseMult(scalar []byte) (x, y *big.Int) {
	var scalarReversed [32]byte
	p256GetScalar(&scalarReversed, scalar)

	var x1, y1, z1 [p256Limbs]uint32
	p256ScalarBaseMult(&x1, &y1, &z1, &scalarReversed)
	return p256ToAffine(&x1, &y1, &z1)
}

func (p256Curve) ScalarMult(bigX, bigY *big.Int, scalar []byte) (x, y *big.Int) {
	var scalarReversed [32]byte
	p256GetScalar(&scalarReversed, scalar)

	var px, py, x1, y1, z1 [p256Limbs]uint32
	p256FromBig(&px, bigX)
	p256FromBig(&py, bigY)
	p256ScalarMult(&x1, &y1, &z1, &px, &py, &scalarReversed)
	return p256ToAffine(&x1, &y1, &z1)
}

// p256Precomputed contains precomputed values to aid the calculation of scalar
// multiples of the base point, G. It's actually two, equal length, tables
// concatenated.
//
// The first table contains (x,y) field element pairs for 16 multiples of the
// base point, G.
//
//	Index  |  Index (binary) | Value
//	    0  |           0000  | 0G (all zeros, omitted)
//	    1  |           0001  | G
//	    2  |           0010  | 2**64G
//	    3  |           0011  | 2**64G + G
//	    4  |           0100  | 2**128G
//	    5  |           0101  | 2**128G + G
//	    6  |           0110  | 2**128G + 2**64G
//	    7  |           0111  | 2**128G + 2**64G + G
//	    8  |           1000  | 2**192G
//	    9  |           1001  | 2**192G + G
//	   10  |           1010  | 2**192G + 2**64G
//	   11  |           1011  | 2**192G + 2**64G + G
//	   12  |           1100  | 2**192G + 2**128G
//	   13  |           1101  | 2**192G + 2**128G + G
//	   14  |           1110  | 2**192G + 2**128G + 2**64G
//	   15  |           1111  | 2**192G + 2**128G + 2**64G + G
//
// The second table follows the same style, but the terms are 2**32G,
// 2**96G, 2**160G, 2**224G.
//
// This is ~2KB of data.
var p256Precomputed = [p256Limbs * 2 * 15 * 2]uint32{
	0x11522878, 0xe730d41, 0xdb60179, 0x4afe2ff, 0x12883add, 0xcaddd88, 0x119e7edc, 0xd4a6eab, 0x3120bee,
	0x1d2aac15, 0xf25357c, 0x19e45cdd, 0x5c721d0, 0x1992c5a5, 0xa237487, 0x154ba21, 0x14b10bb, 0xae3fe3,
	0xd41a576, 0x922fc51, 0x234994f, 0x60b60d3, 0x164586ae, 0xce95f18, 0x1fe49073, 0x3fa36cc, 0x5ebcd2c,
	0xb402f2f, 0x15c70bf, 0x1561925c, 0x5a26704, 0xda91e90, 0xcdc1c7f, 0x1ea12446, 0xe1ade1e, 0xec91f22,
	0x26f7778, 0x566847e, 0xa0bec9e, 0x234f453, 0x1a31f21a, 0xd85e75c, 0x56c7109, 0xa267a00, 0xb57c050,
	0x98fb57, 0xaa837cc, 0x60c0792, 0xcfa5e19, 0x61bab9e, 0x589e39b, 0xa324c5, 0x7d6dee7, 0x2976e4b,
	0x1fc4124a, 0xa8c244b, 0x1ce86762, 0xcd61c7e, 0x1831c8e0, 0x75774e1, 0x1d96a5a9, 0x843a649, 0xc3ab0fa,
	0x6e2e7d5, 0x7673a2a, 0x178b65e8, 0x4003e9b, 0x1a1f11c2, 0x7816ea, 0xf643e11, 0x58c43df, 0xf423fc2,
	0x19633ffa, 0x891f2b2, 0x123c231c, 0x46add8c, 0x54700dd, 0x59e2b17, 0x172db40f, 0x83e277d, 0xb0dd609,
	0xfd1da12, 0x35c6e52, 0x19ede20c, 0xd19e0c0, 0x97d0f40, 0xb015b19, 0x449e3f5, 0xe10c9e, 0x33ab581,
	0x56a67ab, 0x577734d, 0x1dddc062, 0xc57b10d, 0x149b39d, 0x26a9e7b, 0xc35df9f, 0x48764cd, 0x76dbcca,
	0xca4b366, 0xe9303ab, 0x1a7480e7, 0x57e9e81, 0x1e13eb50, 0xf466cf3, 0x6f16b20, 0x4ba3173, 0xc168c33,
	0x15cb5439, 0x6a38e11, 0x73658bd, 0xb29564f, 0x3f6dc5b, 0x53b97e, 0x1322c4c0, 0x65dd7ff, 0x3a1e4f6,
	0x14e614aa, 0x9246317, 0x1bc83aca, 0xad97eed, 0xd38ce4a, 0xf82b006, 0x341f077, 0xa6add89, 0x4894acd,
	0x9f162d5, 0xf8410ef, 0x1b266a56, 0xd7f223, 0x3e0cb92, 0xe39b672, 0x6a2901a, 0x69a8556, 0x7e7c0,
	0x9b7d8d3, 0x309a80, 0x1ad05f7f, 0xc2fb5dd, 0xcbfd41d, 0x9ceb638, 0x1051825c, 0xda0cf5b, 0x812e881,
	0x6f35669, 0x6a56f2c, 0x1df8d184, 0x345820, 0x1477d477, 0x1645db1, 0xbe80c51, 0xc22be3e, 0xe35e65a,
	0x1aeb7aa0, 0xc375315, 0xf67bc99, 0x7fdd7b9, 0x191fc1be, 0x61235d, 0x2c184e9, 0x1c5a839, 0x47a1e26,
	0xb7cb456, 0x93e225d, 0x14f3c6ed, 0xccc1ac9, 0x17fe37f3, 0x4988989, 0x1a90c502, 0x2f32042, 0xa17769b,
	0xafd8c7c, 0x8191c6e, 0x1dcdb237, 0x16200c0, 0x107b32a1, 0x66c08db, 0x10d06a02, 0x3fc93, 0x5620023,
	0x16722b27, 0x68b5c59, 0x270fcfc, 0xfad0ecc, 0xe5de1c2, 0xeab466b, 0x2fc513c, 0x407f75c, 0xbaab133,
	0x9705fe9, 0xb88b8e7, 0x734c993, 0x1e1ff8f, 0x19156970, 0xabd0f00, 0x10469ea7, 0x3293ac0, 0xcdc98aa,
	0x1d843fd, 0xe14bfe8, 0x15be825f, 0x8b5212, 0xeb3fb67, 0x81cbd29, 0xbc62f16, 0x2b6fcc7, 0xf5a4e29,
	0x13560b66, 0xc0b6ac2, 0x51ae690, 0xd41e271, 0xf3e9bd4, 0x1d70aab, 0x1029f72, 0x73e1c35, 0xee70fbc,
	0xad81baf, 0x9ecc49a, 0x86c741e, 0xfe6be30, 0x176752e7, 0x23d416, 0x1f83de85, 0x27de188, 0x66f70b8,
	0x181cd51f, 0x96b6e4c, 0x188f2335, 0xa5df759, 0x17a77eb6, 0xfeb0e73, 0x154ae914, 0x2f3ec51, 0x3826b59,
	0xb91f17d, 0x1c72949, 0x1362bf0a, 0xe23fddf, 0xa5614b0, 0xf7d8f, 0x79061, 0x823d9d2, 0x8213f39,
	0x1128ae0b, 0xd095d05, 0xb85c0c2, 0x1ecb2ef, 0x24ddc84, 0xe35e901, 0x18411a4a, 0xf5ddc3d, 0x3786689,
	0x52260e8, 0x5ae3564, 0x542b10d, 0x8d93a45, 0x19952aa4, 0x996cc41, 0x1051a729, 0x4be3499, 0x52b23aa,
	0x109f307e, 0x6f5b6bb, 0x1f84e1e7, 0x77a0cfa, 0x10c4df3f, 0x25a02ea, 0xb048035, 0xe31de66, 0xc6ecaa3,
	0x28ea335, 0x2886024, 0x1372f020, 0xf55d35, 0x15e4684c, 0xf2a9e17, 0x1a4a7529, 0xcb7beb1, 0xb2a78a1,
	0x1ab21f1f, 0x6361ccf, 0x6c9179d, 0xb135627, 0x1267b974, 0x4408bad, 0x1cbff658, 0xe3d6511, 0xc7d76f,
	0x1cc7a69, 0xe7ee31b, 0x54fab4f, 0x2b914f, 0x1ad27a30, 0xcd3579e, 0xc50124c, 0x50daa90, 0xb13f72,
	0xb06aa75, 0x70f5cc6, 0x1649e5aa, 0x84a5312, 0x329043c, 0x41c4011, 0x13d32411, 0xb04a838, 0xd760d2d,
	0x1713b532, 0xbaa0c03, 0x84022ab, 0x6bcf5c1, 0x2f45379, 0x18ae070, 0x18c9e11e, 0x20bca9a, 0x66f496b,
	0x3eef294, 0x67500d2, 0xd7f613c, 0x2dbbeb, 0xb741038, 0xe04133f, 0x1582968d, 0xbe985f7, 0x1acbc1a,
	0x1a6a939f, 0x33e50f6, 0xd665ed4, 0xb4b7bd6, 0x1e5a3799, 0x6b33847, 0x17fa56ff, 0x65ef930, 0x21dc4a,
	0x2b37659, 0x450fe17, 0xb357b65, 0xdf5efac, 0x15397bef, 0x9d35a7f, 0x112ac15f, 0x624e62e, 0xa90ae2f,
	0x107eecd2, 0x1f69bbe, 0x77d6bce, 0x5741394, 0x13c684fc, 0x950c910, 0x725522b, 0xdc78583, 0x40eeabb,
	0x1fde328a, 0xbd61d96, 0xd28c387, 0x9e77d89, 0x12550c40, 0x759cb7d, 0x367ef34, 0xae2a960, 0x91b8bdc,
	0x93462a9, 0xf469ef, 0xb2e9aef, 0xd2ca771, 0x54e1f42, 0x7aaa49, 0x6316abb, 0x2413c8e, 0x5425bf9,
	0x1bed3e3a, 0xf272274, 0x1f5e7326, 0x6416517, 0xea27072, 0x9cedea7, 0x6e7633, 0x7c91952, 0xd806dce,
	0x8e2a7e1, 0xe421e1a, 0x418c9e1, 0x1dbc890, 0x1b395c36, 0xa1dc175, 0x1dc4ef73, 0x8956f34, 0xe4b5cf2,
	0x1b0d3a18, 0x3194a36, 0x6c2641f, 0xe44124c, 0xa2f4eaa, 0xa8c25ba, 0xf927ed7, 0x627b614, 0x7371cca,
	0xba16694, 0x417bc03, 0x7c0a7e3, 0x9c35c19, 0x1168a205, 0x8b6b00d, 0x10e3edc9, 0x9c19bf2, 0x5882229,
	0x1b2b4162, 0xa5cef1a, 0x1543622b, 0x9bd433e, 0x364e04d, 0x7480792, 0x5c9b5b3, 0xe85ff25, 0x408ef57,
	0x1814cfa4, 0x121b41b, 0xd248a0f, 0x3b05222, 0x39bb16a, 0xc75966d, 0xa038113, 0xa4a1769, 0x11fbc6c,
	0x917e50e, 0xeec3da8, 0x169d6eac, 0x10c1699, 0xa416153, 0xf724912, 0x15cd60b7, 0x4acbad9, 0x5efc5fa,
	0xf150ed7, 0x122b51, 0x1104b40a, 0xcb7f442, 0xfbb28ff, 0x6ac53ca, 0x196142cc, 0x7bf0fa9, 0x957651,
	0x4e0f215, 0xed439f8, 0x3f46bd5, 0x5ace82f, 0x110916b6, 0x6db078, 0xffd7d57, 0xf2ecaac, 0xca86dec,
	0x15d6b2da, 0x965ecc9, 0x1c92b4c2, 0x1f3811, 0x1cb080f5, 0x2d8b804, 0x19d1c12d, 0xf20bd46, 0x1951fa7,
	0xa3656c3, 0x523a425, 0xfcd0692, 0xd44ddc8, 0x131f0f5b, 0xaf80e4a, 0xcd9fc74, 0x99bb618, 0x2db944c,
	0xa673090, 0x1c210e1, 0x178c8d23, 0x1474383, 0x10b8743d, 0x985a55b, 0x2e74779, 0x576138, 0x9587927,
	0x133130fa, 0xbe05516, 0x9f4d619, 0xbb62570, 0x99ec591, 0xd9468fe, 0x1d07782d, 0xfc72e0b, 0x701b298,
	0x1863863b, 0x85954b8, 0x121a0c36, 0x9e7fedf, 0xf64b429, 0x9b9d71e, 0x14e2f5d8, 0xf858d3a, 0x942eea8,
	0xda5b765, 0x6edafff, 0xa9d18cc, 0xc65e4ba, 0x1c747e86, 0xe4ea915, 0x1981d7a1, 0x8395659, 0x52ed4e2,
	0x87d43b7, 0x37ab11b, 0x19d292ce, 0xf8d4692, 0x18c3053f, 0x8863e13, 0x4c146c0, 0x6bdf55a, 0x4e4457d,
	0x16152289, 0xac78ec2, 0x1a59c5a2, 0x2028b97, 0x71c2d01, 0x295851f, 0x404747b, 0x878558d, 0x7d29aa4,
	0x13d8341f, 0x8daefd7, 0x139c972d, 0x6b7ea75, 0xd4a9dde, 0xff163d8, 0x81d55d7, 0xa5bef68, 0xb7b30d8,
	0xbe73d6f, 0xaa88141, 0xd976c81, 0x7e7a9cc, 0x18beb771, 0xd773cbd, 0x13f51951, 0x9d0c177, 0x1c49a78,
}

// Group operations:
//
// Elements of the elliptic curve group are represented in Jacobian
// coordinates: (x, y, z). An affine point (x', y') is x'=x/z**2, y'=y/z**3 in
// Jacobian form.

// p256PointDouble sets {xOut,yOut,zOut} = 2*{x,y,z}.
//
// See https://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#doubling-dbl-2009-l
func p256PointDouble(xOut, yOut, zOut, x, y, z *[p256Limbs]uint32) {
	var delta, gamma, alpha, beta, tmp, tmp2 [p256Limbs]uint32

	p256Square(&delta, z)
	p256Square(&gamma, y)
	p256Mul(&beta, x, &gamma)

	p256Sum(&tmp, x, &delta)
	p256Diff(&tmp2, x, &delta)
	p256Mul(&alpha, &tmp, &tmp2)
	p256Scalar3(&alpha)

	p256Sum(&tmp, y, z)
	p256Square(&tmp, &tmp)
	p256Diff(&tmp, &tmp, &gamma)
	p256Diff(zOut, &tmp, &delta)

	p256Scalar4(&beta)
	p256Square(xOut, &alpha)
	p256Diff(xOut, xOut, &beta)
	p256Diff(xOut, xOut, &beta)

	p256Diff(&tmp, &beta, xOut)
	p256Mul(&tmp, &alpha, &tmp)
	p256Square(&tmp2, &gamma)
	p256Scalar8(&tmp2)
	p256Diff(yOut, &tmp, &tmp2)
}

// p256PointAddMixed sets {xOut,yOut,zOut} = {x1,y1,z1} + {x2,y2,1}.
// (i.e. the second point is affine.)
//
// See https://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#addition-add-2007-bl
//
// Note that this function does not handle P+P, infinity+P nor P+infinity
// correctly.
func p256PointAddMixed(xOut, yOut, zOut, x1, y1, z1, x2, y2 *[p256Limbs]uint32) {
	var z1z1, z1z1z1, s2, u2, h, i, j, r, rr, v, tmp [p256Limbs]uint32

	p256Square(&z1z1, z1)
	p256Sum(&tmp, z1, z1)

	p256Mul(&u2, x2, &z1z1)
	p256Mul(&z1z1z1, z1, &z1z1)
	p256Mul(&s2, y2, &z1z1z1)
	p256Diff(&h, &u2, x1)
	p256Sum(&i, &h, &h)
	p256Square(&i, &i)
	p256Mul(&j, &h, &i)
	p256Diff(&r, &s2, y1)
	p256Sum(&r, &r, &r)
	p256Mul(&v, x1, &i)

	p256Mul(zOut, &tmp, &h)
	p256Square(&rr, &r)
	p256Diff(xOut, &rr, &j)
	p256Diff(xOut, xOut, &v)
	p256Diff(xOut, xOut, &v)

	p256Diff(&tmp, &v, xOut)
	p256Mul(yOut, &tmp, &r)
	p256Mul(&tmp, y1, &j)
	p256Diff(yOut, yOut, &tmp)
	p256Diff(yOut, yOut, &tmp)
}

// p256PointAdd sets {xOut,yOut,zOut} = {x1,y1,z1} + {x2,y2,z2}.
//
// See https://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#addition-add-2007-bl
//
// Note that this function does not handle P+P, infinity+P nor P+infinity
// correctly.
func p256PointAdd(xOut, yOut, zOut, x1, y1, z1, x2, y2, z2 *[p256Limbs]uint32) {
	var z1z1, z1z1z1, z2z2, z2z2z2, s1, s2, u1, u2, h, i, j, r, rr, v, tmp [p256Limbs]uint32

	p256Square(&z1z1, z1)
	p256Square(&z2z2, z2)
	p256Mul(&u1, x1, &z2z2)

	p256Sum(&tmp, z1, z2)
	p256Square(&tmp, &tmp)
	p256Diff(&tmp, &tmp, &z1z1)
	p256Diff(&tmp, &tmp, &z2z2)

	p256Mul(&z2z2z2, z2, &z2z2)
	p256Mul(&s1, y1, &z2z2z2)

	p256Mul(&u2, x2, &z1z1)
	p256Mul(&z1z1z1, z1, &z1z1)
	p256Mul(&s2, y2, &z1z1z1)
	p256Diff(&h, &u2, &u1)
	p256Sum(&i, &h, &h)
	p256Square(&i, &i)
	p256Mul(&j, &h, &i)
	p256Diff(&r, &s2, &s1)
	p256Sum(&r, &r, &r)
	p256Mul(&v, &u1, &i)

	p256Mul(zOut, &tmp, &h)
	p256Square(&rr, &r)
	p256Diff(xOut, &rr, &j)
	p256Diff(xOut, xOut, &v)
	p256Diff(xOut, xOut, &v)

	p256Diff(&tmp, &v, xOut)
	p256Mul(yOut, &tmp, &r)
	p256Mul(&tmp, &s1, &j)
	p256Diff(yOut, yOut, &tmp)
	p256Diff(yOut, yOut, &tmp)
}

// p256SelectAffinePoint sets {out_x,out_y} to the index'th entry of table.
//
// On entry: index < 16, table[0] must be zero.
func p256SelectAffinePoint(xOut, yOut *[p256Limbs]uint32, table []uint32, index uint32) {
	for i := range xOut {
		xOut[i] = 0
	}
	for i := range yOut {
		yOut[i] = 0
	}

	for i := uint32(1); i < 16; i++ {
		mask := i ^ index
		mask |= mask >> 2
		mask |= mask >> 1
		mask &= 1
		mask--
		for j := range xOut {
			xOut[j] |= table[0] & mask
			table = table[1:]
		}
		for j := range yOut {
			yOut[j] |= table[0] & mask
			table = table[1:]
		}
	}
}

// p256SelectJacobianPoint sets {out_x,out_y,out_z} to the index'th entry of
// table.
//
// On entry: index < 16, table[0] must be zero.
func p256SelectJacobianPoint(xOut, yOut, zOut *[p256Limbs]uint32, table *[16][3][p256Limbs]uint32, index uint32) {
	for i := range xOut {
		xOut[i] = 0
	}
	for i := range yOut {
		yOut[i] = 0
	}
	for i := range zOut {
		zOut[i] = 0
	}

	// The implicit value at index 0 is all zero. We don't need to perform that
	// iteration of the loop because we already set out_* to zero.
	for i := uint32(1); i < 16; i++ {
		mask := i ^ index
		mask |= mask >> 2
		mask |= mask >> 1
		mask &= 1
		mask--
		for j := range xOut {
			xOut[j] |= table[i][0][j] & mask
		}
		for j := range yOut {
			yOut[j] |= table[i][1][j] & mask
		}
		for j := range zOut {
			zOut[j] |= table[i][2][j] & mask
		}
	}
}

// p256GetBit returns the bit'th bit of scalar.
func p256GetBit(scalar *[32]uint8, bit uint) uint32 {
	return uint32(((scalar[bit>>3]) >> (bit & 7)) & 1)
}

// p256ScalarBaseMult sets {xOut,yOut,zOut} = scalar*G where scalar is a
// little-endian number. Note that the value of scalar must be less than the
// order of the group.
func p256ScalarBaseMult(xOut, yOut, zOut *[p256Limbs]uint32, scalar *[32]uint8) {
	nIsInfinityMask := ^uint32(0)
	var pIsNoninfiniteMask, mask, tableOffset uint32
	var px, py, tx, ty, tz [p256Limbs]uint32

	for i := range xOut {
		xOut[i] = 0
	}
	for i := range yOut {
		yOut[i] = 0
	}
	for i := range zOut {
		zOut[i] = 0
	}

	// The loop adds bits at positions 0, 64, 128 and 192, followed by
	// positions 32,96,160 and 224 and does this 32 times.
	for i := uint(0); i < 32; i++ {
		if i != 0 {
			p256PointDouble(xOut, yOut, zOut, xOut, yOut, zOut)
		}
		tableOffset = 0
		for j := uint(0); j <= 32; j += 32 {
			bit0 := p256GetBit(scalar, 31-i+j)
			bit1 := p256GetBit(scalar, 95-i+j)
			bit2 := p256GetBit(scalar, 159-i+j)
			bit3 := p256GetBit(scalar, 223-i+j)
			index := bit0 | (bit1 << 1) | (bit2 << 2) | (bit3 << 3)

			p256SelectAffinePoint(&px, &py, p256Precomputed[tableOffset:], index)
			tableOffset += 30 * p256Limbs

			// Since scalar is less than the order of the group, we know that
			// {xOut,yOut,zOut} != {px,py,1}, unless both are zero, which we handle
			// below.
			p256PointAddMixed(&tx, &ty, &tz, xOut, yOut, zOut, &px, &py)
			// The result of pointAddMixed is incorrect if {xOut,yOut,zOut} is zero
			// (a.k.a.  the point at infinity). We handle that situation by
			// copying the point from the table.
			p256CopyConditional(xOut, &px, nIsInfinityMask)
			p256CopyConditional(yOut, &py, nIsInfinityMask)
			p256CopyConditional(zOut, &p256One, nIsInfinityMask)

			// Equally, the result is also wrong if the point from the table is
			// zero, which happens when the index is zero. We handle that by
			// only copying from {tx,ty,tz} to {xOut,yOut,zOut} if index != 0.
			pIsNoninfiniteMask = nonZeroToAllOnes(index)
			mask = pIsNoninfiniteMask & ^nIsInfinityMask
			p256CopyConditional(xOut, &tx, mask)
			p256CopyConditional(yOut, &ty, mask)
			p256CopyConditional(zOut, &tz, mask)
			// If p was not zero, then n is now non-zero.
			nIsInfinityMask &^= pIsNoninfiniteMask
		}
	}
}

// p256PointToAffine converts a Jacobian point to an affine point. If the input
// is the point at infinity then it returns (0, 0) in constant time.
func p256PointToAffine(xOut, yOut, x, y, z *[p256Limbs]uint32) {
	var zInv, zInvSq [p256Limbs]uint32

	p256Invert(&zInv, z)
	p256Square(&zInvSq, &zInv)
	p256Mul(xOut, x, &zInvSq)
	p256Mul(&zInv, &zInv, &zInvSq)
	p256Mul(yOut, y, &zInv)
}

// p256ToAffine returns a pair of *big.Int containing the affine representation
// of {x,y,z}.
func p256ToAffine(x, y, z *[p256Limbs]uint32) (xOut, yOut *big.Int) {
	var xx, yy [p256Limbs]uint32
	p256PointToAffine(&xx, &yy, x, y, z)
	return p256ToBig(&xx), p256ToBig(&yy)
}

// p256ScalarMult sets {xOut,yOut,zOut} = scalar*{x,y}.
func p256ScalarMult(xOut, yOut, zOut, x, y *[p256Limbs]uint32, scalar *[32]uint8) {
	var px, py, pz, tx, ty, tz [p256Limbs]uint32
	var precomp [16][3][p256Limbs]uint32
	var nIsInfinityMask, index, pIsNoninfiniteMask, mask uint32

	// We precompute 0,1,2,... times {x,y}.
	precomp[1][0] = *x
	precomp[1][1] = *y
	precomp[1][2] = p256One

	for i := 2; i < 16; i += 2 {
		p256PointDouble(&precomp[i][0], &precomp[i][1], &precomp[i][2], &precomp[i/2][0], &precomp[i/2][1], &precomp[i/2][2])
		p256PointAddMixed(&precomp[i+1][0], &precomp[i+1][1], &precomp[i+1][2], &precomp[i][0], &precomp[i][1], &precomp[i][2], x, y)
	}

	for i := range xOut {
		xOut[i] = 0
	}
	for i := range yOut {
		yOut[i] = 0
	}
	for i := range zOut {
		zOut[i] = 0
	}
	nIsInfinityMask = ^uint32(0)

	// We add in a window of four bits each iteration and do this 64 times.
	for i := 0; i < 64; i++ {
		if i != 0 {
			p256PointDouble(xOut, yOut, zOut, xOut, yOut, zOut)
			p256PointDouble(xOut, yOut, zOut, xOut, yOut, zOut)
			p256PointDouble(xOut, yOut, zOut, xOut, yOut, zOut)
			p256PointDouble(xOut, yOut, zOut, xOut, yOut, zOut)
		}

		index = uint32(scalar[31-i/2])
		if (i & 1) == 1 {
			index &= 15
		} else {
			index >>= 4
		}

		// See the comments in scalarBaseMult about handling infinities.
		p256SelectJacobianPoint(&px, &py, &pz, &precomp, index)
		p256PointAdd(&tx, &ty, &tz, xOut, yOut, zOut, &px, &py, &pz)
		p256CopyConditional(xOut, &px, nIsInfinityMask)
		p256CopyConditional(yOut, &py, nIsInfinityMask)
		p256CopyConditional(zOut, &pz, nIsInfinityMask)

		pIsNoninfiniteMask = nonZeroToAllOnes(index)
		mask = pIsNoninfiniteMask & ^nIsInfinityMask
		p256CopyConditional(xOut, &tx, mask)
		p256CopyConditional(yOut, &ty, mask)
		p256CopyConditional(zOut, &tz, mask)
		nIsInfinityMask &^= pIsNoninfiniteMask
	}
}