// Copyright 2020 The BoringSSL Authors

//

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

//     https://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software

// distributed under the License is distributed on an "AS IS" BASIS,

// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

// See the License for the specific language governing permissions and

// limitations under the License.

// An implementation of the NIST P-256 elliptic curve point multiplication.

// 256-bit Montgomery form for 64 and 32-bit. Field operations are generated by

// Fiat, which lives in //third_party/fiat.

#include <openssl/base.h>

#include <openssl/bn.h>

#include <openssl/ec.h>

#include <openssl/err.h>

#include <openssl/mem.h>

#include <assert.h>

#include <string.h>

#include <iterator>

#include "../../internal.h"

#include "../delocate.h"

#include "./internal.h"

#include "../../../third_party/fiat/p256_field.c.inc"

#include "../../../third_party/fiat/p256_point.br.c.inc"

using namespace bssl;

// utility functions, handwritten

#if defined(OPENSSL_64_BIT)

#define FIAT_P256_NLIMBS 4

typedef uint64_t fiat_p256_limb_t;

typedef uint64_t fiat_p256_felem[FIAT_P256_NLIMBS];

static const fiat_p256_felem fiat_p256_one = {0x1, 0xffffffff00000000,

                                              0xffffffffffffffff, 0xfffffffe};

#else  // 64BIT; else 32BIT

#define FIAT_P256_NLIMBS 8

typedef uint32_t fiat_p256_limb_t;

typedef uint32_t fiat_p256_felem[FIAT_P256_NLIMBS];

static const fiat_p256_felem fiat_p256_one = {

    0x1, 0x0, 0x0, 0xffffffff, 0xffffffff, 0xffffffff, 0xfffffffe, 0x0};

#endif  // 64BIT

static void fiat_p256_copy(fiat_p256_limb_t out[FIAT_P256_NLIMBS],

                           const fiat_p256_limb_t in1[FIAT_P256_NLIMBS]) {

  for (size_t i = 0; i < FIAT_P256_NLIMBS; i++) {

    out[i] = in1[i];

  }

}

static void fiat_p256_cmovznz(fiat_p256_limb_t out[FIAT_P256_NLIMBS],

                              fiat_p256_limb_t t,

                              const fiat_p256_limb_t z[FIAT_P256_NLIMBS],

                              const fiat_p256_limb_t nz[FIAT_P256_NLIMBS]) {

  fiat_p256_selectznz(out, !!t, z, nz);

}

static void fiat_p256_from_words(fiat_p256_felem out,

                                 const BN_ULONG in[32 / sizeof(BN_ULONG)]) {

  // Typically, |BN_ULONG| and |fiat_p256_limb_t| will be the same type, but on

  // 64-bit platforms without |uint128_t|, they are different. However, on

  // little-endian systems, |uint64_t[4]| and |uint32_t[8]| have the same

  // layout.

  OPENSSL_memcpy(out, in, 32);

}

static void fiat_p256_from_generic(fiat_p256_felem out, const EC_FELEM *in) {

  fiat_p256_from_words(out, in->words);

}

static void fiat_p256_to_generic(EC_FELEM *out, const fiat_p256_felem in) {

  // See |fiat_p256_from_words|.

  OPENSSL_memcpy(out->words, in, 32);

}

// fiat_p256_inv_square calculates |out| = |in|^{-2}

//

// Based on Fermat's Little Theorem:

//   a^p = a (mod p)

//   a^{p-1} = 1 (mod p)

//   a^{p-3} = a^{-2} (mod p)

static void fiat_p256_inv_square(fiat_p256_felem out,

                                 const fiat_p256_felem in) {

  // This implements the addition chain described in

  // https://briansmith.org/ecc-inversion-addition-chains-01#p256_field_inversion

  fiat_p256_felem x2, x3, x6, x12, x15, x30, x32;

  fiat_p256_square(x2, in);   // 2^2 - 2^1

  fiat_p256_mul(x2, x2, in);  // 2^2 - 2^0

  fiat_p256_square(x3, x2);   // 2^3 - 2^1

  fiat_p256_mul(x3, x3, in);  // 2^3 - 2^0

  fiat_p256_square(x6, x3);

  for (int i = 1; i < 3; i++) {

    fiat_p256_square(x6, x6);

  }                           // 2^6 - 2^3

  fiat_p256_mul(x6, x6, x3);  // 2^6 - 2^0

  fiat_p256_square(x12, x6);

  for (int i = 1; i < 6; i++) {

    fiat_p256_square(x12, x12);

  }                             // 2^12 - 2^6

  fiat_p256_mul(x12, x12, x6);  // 2^12 - 2^0

  fiat_p256_square(x15, x12);

  for (int i = 1; i < 3; i++) {

    fiat_p256_square(x15, x15);

  }                             // 2^15 - 2^3

  fiat_p256_mul(x15, x15, x3);  // 2^15 - 2^0

  fiat_p256_square(x30, x15);

  for (int i = 1; i < 15; i++) {

    fiat_p256_square(x30, x30);

  }                              // 2^30 - 2^15

  fiat_p256_mul(x30, x30, x15);  // 2^30 - 2^0

  fiat_p256_square(x32, x30);

  fiat_p256_square(x32, x32);   // 2^32 - 2^2

  fiat_p256_mul(x32, x32, x2);  // 2^32 - 2^0

  fiat_p256_felem ret;

  fiat_p256_square(ret, x32);

  for (int i = 1; i < 31 + 1; i++) {

    fiat_p256_square(ret, ret);

  }                             // 2^64 - 2^32

  fiat_p256_mul(ret, ret, in);  // 2^64 - 2^32 + 2^0

  for (int i = 0; i < 96 + 32; i++) {

    fiat_p256_square(ret, ret);

  }                              // 2^192 - 2^160 + 2^128

  fiat_p256_mul(ret, ret, x32);  // 2^192 - 2^160 + 2^128 + 2^32 - 2^0

  for (int i = 0; i < 32; i++) {

    fiat_p256_square(ret, ret);

  }                              // 2^224 - 2^192 + 2^160 + 2^64 - 2^32

  fiat_p256_mul(ret, ret, x32);  // 2^224 - 2^192 + 2^160 + 2^64 - 2^0

  for (int i = 0; i < 30; i++) {

    fiat_p256_square(ret, ret);

  }                              // 2^254 - 2^222 + 2^190 + 2^94 - 2^30

  fiat_p256_mul(ret, ret, x30);  // 2^254 - 2^222 + 2^190 + 2^94 - 2^0

  fiat_p256_square(ret, ret);

  fiat_p256_square(out, ret);  // 2^256 - 2^224 + 2^192 + 2^96 - 2^2

}

// Group operations

// ----------------

//

// Building on top of the field operations we have the operations on the

// elliptic curve group itself. Points on the curve are represented in Jacobian

// coordinates.

static void fiat_p256_point_double(fiat_p256_felem x_out, fiat_p256_felem y_out,

                                   fiat_p256_felem z_out,

                                   const fiat_p256_felem x_in,

                                   const fiat_p256_felem y_in,

                                   const fiat_p256_felem z_in) {

  uint8_t out[3*32], in[3*32];

  static_assert(sizeof(fiat_p256_felem) == 32);

  OPENSSL_memcpy(&in[0], x_in, 32);

  OPENSSL_memcpy(&in[32], y_in, 32);

  OPENSSL_memcpy(&in[64], z_in, 32);

  p256_point_double((br_word_t)out, (br_word_t)in);

  OPENSSL_memcpy(x_out, &out[0], 32);

  OPENSSL_memcpy(y_out, &out[32], 32);

  OPENSSL_memcpy(z_out, &out[64], 32);

}

static void fiat_p256_point_add(fiat_p256_felem x3, fiat_p256_felem y3,

                                fiat_p256_felem z3, const fiat_p256_felem x1,

                                const fiat_p256_felem y1,

                                const fiat_p256_felem z1,

                                const fiat_p256_felem x2,

                                const fiat_p256_felem y2,

                                const fiat_p256_felem z2) {

  uint8_t out[3 * 32], in1[3 * 32], in2[3 * 32];

  static_assert(sizeof(fiat_p256_felem) == 32);

  OPENSSL_memcpy(&in1[0], x1, 32);

  OPENSSL_memcpy(&in1[32], y1, 32);

  OPENSSL_memcpy(&in1[64], z1, 32);

  OPENSSL_memcpy(&in2[0], x2, 32);

  OPENSSL_memcpy(&in2[32], y2, 32);

  OPENSSL_memcpy(&in2[64], z2, 32);

  p256_point_add_vartime_if_doubling((br_word_t)out, (br_word_t)in1,

                                     (br_word_t)in2);

  OPENSSL_memcpy(x3, &out[0], 32);

  OPENSSL_memcpy(y3, &out[32], 32);

  OPENSSL_memcpy(z3, &out[64], 32);

}

#include "./p256_table.h"

// fiat_p256_select_point_affine selects the |idx-1|th point from a

// precomputation table and copies it to out. If |idx| is zero, the output is

// the point at infinity.

static void fiat_p256_select_point_affine(

    const fiat_p256_limb_t idx, size_t size,

    const fiat_p256_felem pre_comp[/*size*/][2], fiat_p256_felem out[3]) {

  OPENSSL_memset(out, 0, sizeof(fiat_p256_felem) * 3);

  for (size_t i = 0; i < size; i++) {

    fiat_p256_limb_t mismatch = i ^ (idx - 1);

    fiat_p256_cmovznz(out[0], mismatch, pre_comp[i][0], out[0]);

    fiat_p256_cmovznz(out[1], mismatch, pre_comp[i][1], out[1]);

  }

  fiat_p256_cmovznz(out[2], idx, out[2], fiat_p256_one);

}

// fiat_p256_select_point selects the |idx|th point from a precomputation table

// and copies it to out.

static void fiat_p256_select_point(const fiat_p256_limb_t idx, size_t size,

                                   const fiat_p256_felem pre_comp[/*size*/][3],

                                   fiat_p256_felem out[3]) {

  OPENSSL_memset(out, 0, sizeof(fiat_p256_felem) * 3);

  for (size_t i = 0; i < size; i++) {

    fiat_p256_limb_t mismatch = i ^ idx;

    fiat_p256_cmovznz(out[0], mismatch, pre_comp[i][0], out[0]);

    fiat_p256_cmovznz(out[1], mismatch, pre_comp[i][1], out[1]);

    fiat_p256_cmovznz(out[2], mismatch, pre_comp[i][2], out[2]);

  }

}

// fiat_p256_get_bit returns the |i|th bit in |in|.

static crypto_word_t fiat_p256_get_bit(const EC_SCALAR *in, int i) {

  if (i < 0 || i >= 256) {

    return 0;

  }

#if defined(OPENSSL_64_BIT)

  static_assert(sizeof(BN_ULONG) == 8, "BN_ULONG was not 64-bit");

  return (in->words[i >> 6] >> (i & 63)) & 1;

#else

  static_assert(sizeof(BN_ULONG) == 4, "BN_ULONG was not 32-bit");

  return (in->words[i >> 5] >> (i & 31)) & 1;

#endif

}

// OPENSSL EC_METHOD FUNCTIONS

// Takes the Jacobian coordinates (X, Y, Z) of a point and returns (X', Y') =

// (X/Z^2, Y/Z^3).

static int ec_GFp_nistp256_point_get_affine_coordinates(

    const EC_GROUP *group, const EC_JACOBIAN *point, EC_FELEM *x_out,

    EC_FELEM *y_out) {

  if (constant_time_declassify_int(

          ec_GFp_simple_is_at_infinity(group, point))) {

    OPENSSL_PUT_ERROR(EC, EC_R_POINT_AT_INFINITY);

    return 0;

  }

  fiat_p256_felem z1, z2;

  fiat_p256_from_generic(z1, &point->Z);

  fiat_p256_inv_square(z2, z1);

  if (x_out != nullptr) {

    fiat_p256_felem x;

    fiat_p256_from_generic(x, &point->X);

    fiat_p256_mul(x, x, z2);

    fiat_p256_to_generic(x_out, x);

  }

  if (y_out != nullptr) {

    fiat_p256_felem y;

    fiat_p256_from_generic(y, &point->Y);

    fiat_p256_square(z2, z2);  // z^-4

    fiat_p256_mul(y, y, z1);   // y * z

    fiat_p256_mul(y, y, z2);   // y * z^-3

    fiat_p256_to_generic(y_out, y);

  }

  return 1;

}

static void ec_GFp_nistp256_add(const EC_GROUP *group, EC_JACOBIAN *r,

                                const EC_JACOBIAN *a, const EC_JACOBIAN *b) {

  fiat_p256_felem x1, y1, z1, x2, y2, z2;

  fiat_p256_from_generic(x1, &a->X);

  fiat_p256_from_generic(y1, &a->Y);

  fiat_p256_from_generic(z1, &a->Z);

  fiat_p256_from_generic(x2, &b->X);

  fiat_p256_from_generic(y2, &b->Y);

  fiat_p256_from_generic(z2, &b->Z);

  fiat_p256_point_add(x1, y1, z1, x1, y1, z1, x2, y2, z2);

  fiat_p256_to_generic(&r->X, x1);

  fiat_p256_to_generic(&r->Y, y1);

  fiat_p256_to_generic(&r->Z, z1);

}

static void ec_GFp_nistp256_dbl(const EC_GROUP *group, EC_JACOBIAN *r,

                                const EC_JACOBIAN *a) {

  fiat_p256_felem x, y, z;

  fiat_p256_from_generic(x, &a->X);

  fiat_p256_from_generic(y, &a->Y);

  fiat_p256_from_generic(z, &a->Z);

  fiat_p256_point_double(x, y, z, x, y, z);

  fiat_p256_to_generic(&r->X, x);

  fiat_p256_to_generic(&r->Y, y);

  fiat_p256_to_generic(&r->Z, z);

}

static void ec_GFp_nistp256_point_mul(const EC_GROUP *group, EC_JACOBIAN *r,

                                      const EC_JACOBIAN *p,

                                      const EC_SCALAR *scalar) {

  fiat_p256_felem p_pre_comp[17][3];

  OPENSSL_memset(&p_pre_comp, 0, sizeof(p_pre_comp));

  // Precompute multiples.

  fiat_p256_from_generic(p_pre_comp[1][0], &p->X);

  fiat_p256_from_generic(p_pre_comp[1][1], &p->Y);

  fiat_p256_from_generic(p_pre_comp[1][2], &p->Z);

  for (size_t j = 2; j <= 16; ++j) {

    if (j & 1) {

      fiat_p256_point_add(p_pre_comp[j][0], p_pre_comp[j][1], p_pre_comp[j][2],

                          p_pre_comp[1][0], p_pre_comp[1][1], p_pre_comp[1][2],

                          p_pre_comp[j - 1][0], p_pre_comp[j - 1][1],

                          p_pre_comp[j - 1][2]);

    } else {

      fiat_p256_point_double(p_pre_comp[j][0], p_pre_comp[j][1],

                             p_pre_comp[j][2], p_pre_comp[j / 2][0],

                             p_pre_comp[j / 2][1], p_pre_comp[j / 2][2]);

    }

  }

  // Set nq to the point at infinity.

  fiat_p256_felem nq[3] = {{0}, {0}, {0}}, ftmp, tmp[3];

  // Loop over |scalar| msb-to-lsb, incorporating |p_pre_comp| every 5th round.

  int skip = 1;  // Save two point operations in the first round.

  for (size_t i = 255; i < 256; i--) {

    // double

    if (!skip) {

      fiat_p256_point_double(nq[0], nq[1], nq[2], nq[0], nq[1], nq[2]);

    }

    // do other additions every 5 doublings

    if (i % 5 == 0) {

      crypto_word_t bits = fiat_p256_get_bit(scalar, i + 4) << 5;

      bits |= fiat_p256_get_bit(scalar, i + 3) << 4;

      bits |= fiat_p256_get_bit(scalar, i + 2) << 3;

      bits |= fiat_p256_get_bit(scalar, i + 1) << 2;

      bits |= fiat_p256_get_bit(scalar, i) << 1;

      bits |= fiat_p256_get_bit(scalar, i - 1);

      crypto_word_t sign, digit;

      ec_GFp_nistp_recode_scalar_bits(&sign, &digit, bits);

      // select the point to add or subtract, in constant time.

      fiat_p256_select_point((fiat_p256_limb_t)digit, 17,

                             (const fiat_p256_felem(*)[3])p_pre_comp, tmp);

      fiat_p256_opp(ftmp, tmp[1]);  // (X, -Y, Z) is the negative point.

      fiat_p256_cmovznz(tmp[1], (fiat_p256_limb_t)sign, tmp[1], ftmp);

      if (!skip) {

        fiat_p256_point_add(nq[0], nq[1], nq[2], nq[0], nq[1], nq[2], tmp[0],

                            tmp[1], tmp[2]);

      } else {

        fiat_p256_copy(nq[0], tmp[0]);

        fiat_p256_copy(nq[1], tmp[1]);

        fiat_p256_copy(nq[2], tmp[2]);

        skip = 0;

      }

    }

  }

  fiat_p256_to_generic(&r->X, nq[0]);

  fiat_p256_to_generic(&r->Y, nq[1]);

  fiat_p256_to_generic(&r->Z, nq[2]);

}

static void ec_GFp_nistp256_point_mul_base(const EC_GROUP *group,

                                           EC_JACOBIAN *r,

                                           const EC_SCALAR *scalar) {

  // Set nq to the point at infinity.

  fiat_p256_felem nq[3] = {{0}, {0}, {0}}, tmp[3];

  int skip = 1;  // Save two point operations in the first round.

  for (size_t i = 31; i < 32; i--) {

    if (!skip) {

      fiat_p256_point_double(nq[0], nq[1], nq[2], nq[0], nq[1], nq[2]);

    }

    // First, look 32 bits upwards.

    crypto_word_t bits = fiat_p256_get_bit(scalar, i + 224) << 3;

    bits |= fiat_p256_get_bit(scalar, i + 160) << 2;

    bits |= fiat_p256_get_bit(scalar, i + 96) << 1;

    bits |= fiat_p256_get_bit(scalar, i + 32);

    // Select the point to add, in constant time.

    fiat_p256_select_point_affine((fiat_p256_limb_t)bits, 15,

                                  fiat_p256_g_pre_comp[1], tmp);

    if (!skip) {

      fiat_p256_point_add(nq[0], nq[1], nq[2], nq[0], nq[1], nq[2], tmp[0],

                          tmp[1], tmp[2]);

    } else {

      fiat_p256_copy(nq[0], tmp[0]);

      fiat_p256_copy(nq[1], tmp[1]);

      fiat_p256_copy(nq[2], tmp[2]);

      skip = 0;

    }

    // Second, look at the current position.

    bits = fiat_p256_get_bit(scalar, i + 192) << 3;

    bits |= fiat_p256_get_bit(scalar, i + 128) << 2;

    bits |= fiat_p256_get_bit(scalar, i + 64) << 1;

    bits |= fiat_p256_get_bit(scalar, i);

    // Select the point to add, in constant time.

    fiat_p256_select_point_affine((fiat_p256_limb_t)bits, 15,

                                  fiat_p256_g_pre_comp[0], tmp);

    fiat_p256_point_add(nq[0], nq[1], nq[2], nq[0], nq[1], nq[2], tmp[0],

                        tmp[1], tmp[2]);

  }

  fiat_p256_to_generic(&r->X, nq[0]);

  fiat_p256_to_generic(&r->Y, nq[1]);

  fiat_p256_to_generic(&r->Z, nq[2]);

}

static void ec_GFp_nistp256_point_mul_public(const EC_GROUP *group,

                                             EC_JACOBIAN *r,

                                             const EC_SCALAR *g_scalar,

                                             const EC_JACOBIAN *p,

                                             const EC_SCALAR *p_scalar) {

#define P256_WSIZE_PUBLIC 4

  // Precompute multiples of |p|. p_pre_comp[i] is (2*i+1) * |p|.

  fiat_p256_felem p_pre_comp[1 << (P256_WSIZE_PUBLIC - 1)][3];

  fiat_p256_from_generic(p_pre_comp[0][0], &p->X);

  fiat_p256_from_generic(p_pre_comp[0][1], &p->Y);

  fiat_p256_from_generic(p_pre_comp[0][2], &p->Z);

  fiat_p256_felem p2[3];

  fiat_p256_point_double(p2[0], p2[1], p2[2], p_pre_comp[0][0],

                         p_pre_comp[0][1], p_pre_comp[0][2]);

  for (size_t i = 1; i < std::size(p_pre_comp); i++) {

    fiat_p256_point_add(p_pre_comp[i][0], p_pre_comp[i][1], p_pre_comp[i][2],

                        p_pre_comp[i - 1][0], p_pre_comp[i - 1][1],

                        p_pre_comp[i - 1][2], p2[0], p2[1], p2[2]);

  }

  // Set up the coefficients for |p_scalar|.

  int8_t p_wNAF[257];

  ec_compute_wNAF(group, p_wNAF, p_scalar, 256, P256_WSIZE_PUBLIC);

  // Set |ret| to the point at infinity.

  int skip = 1;  // Save some point operations.

  fiat_p256_felem ret[3] = {{0}, {0}, {0}};

  for (int i = 256; i >= 0; i--) {

    if (!skip) {

      fiat_p256_point_double(ret[0], ret[1], ret[2], ret[0], ret[1], ret[2]);

    }

    // For the |g_scalar|, we use the precomputed table without the

    // constant-time lookup.

    if (i <= 31) {

      // First, look 32 bits upwards.

      crypto_word_t bits = fiat_p256_get_bit(g_scalar, i + 224) << 3;

      bits |= fiat_p256_get_bit(g_scalar, i + 160) << 2;

      bits |= fiat_p256_get_bit(g_scalar, i + 96) << 1;

      bits |= fiat_p256_get_bit(g_scalar, i + 32);

      if (bits != 0) {

        size_t index = (size_t)(bits - 1);

        fiat_p256_point_add(ret[0], ret[1], ret[2], ret[0], ret[1], ret[2],

                            fiat_p256_g_pre_comp[1][index][0],

                            fiat_p256_g_pre_comp[1][index][1], fiat_p256_one);

        skip = 0;

      }

      // Second, look at the current position.

      bits = fiat_p256_get_bit(g_scalar, i + 192) << 3;

      bits |= fiat_p256_get_bit(g_scalar, i + 128) << 2;

      bits |= fiat_p256_get_bit(g_scalar, i + 64) << 1;

      bits |= fiat_p256_get_bit(g_scalar, i);

      if (bits != 0) {

        size_t index = (size_t)(bits - 1);

        fiat_p256_point_add(ret[0], ret[1], ret[2], ret[0], ret[1], ret[2],

                            fiat_p256_g_pre_comp[0][index][0],

                            fiat_p256_g_pre_comp[0][index][1], fiat_p256_one);

        skip = 0;

      }

    }

    int digit = p_wNAF[i];

    if (digit != 0) {

      assert(digit & 1);

      size_t idx = (size_t)(digit < 0 ? (-digit) >> 1 : digit >> 1);

      fiat_p256_felem *y = &p_pre_comp[idx][1], tmp;

      if (digit < 0) {

        fiat_p256_opp(tmp, p_pre_comp[idx][1]);

        y = &tmp;

      }

      if (!skip) {

        fiat_p256_point_add(ret[0], ret[1], ret[2], ret[0], ret[1], ret[2],

                            p_pre_comp[idx][0], *y, p_pre_comp[idx][2]);

      } else {

        fiat_p256_copy(ret[0], p_pre_comp[idx][0]);

        fiat_p256_copy(ret[1], *y);

        fiat_p256_copy(ret[2], p_pre_comp[idx][2]);

        skip = 0;

      }

    }

  }

  fiat_p256_to_generic(&r->X, ret[0]);

  fiat_p256_to_generic(&r->Y, ret[1]);

  fiat_p256_to_generic(&r->Z, ret[2]);

}

static int ec_GFp_nistp256_cmp_x_coordinate(const EC_GROUP *group,

                                            const EC_JACOBIAN *p,

                                            const EC_SCALAR *r) {

  if (ec_GFp_simple_is_at_infinity(group, p)) {

    return 0;

  }

  // We wish to compare X/Z^2 with r. This is equivalent to comparing X with

  // r*Z^2. Note that X and Z are represented in Montgomery form, while r is

  // not.

  fiat_p256_felem Z2_mont;

  fiat_p256_from_generic(Z2_mont, &p->Z);

  fiat_p256_mul(Z2_mont, Z2_mont, Z2_mont);

  fiat_p256_felem r_Z2;

  fiat_p256_from_words(r_Z2, r->words);  // r < order < p, so this is valid.

  fiat_p256_mul(r_Z2, r_Z2, Z2_mont);

  fiat_p256_felem X;

  fiat_p256_from_generic(X, &p->X);

  fiat_p256_from_montgomery(X, X);

  if (OPENSSL_memcmp(&r_Z2, &X, sizeof(r_Z2)) == 0) {

    return 1;

  }

  // During signing the x coefficient is reduced modulo the group order.

  // Therefore there is a small possibility, less than 1/2^128, that group_order

  // < p.x < P. in that case we need not only to compare against |r| but also to

  // compare against r+group_order.

  assert(group->field.N.width == group->order.N.width);

  EC_FELEM tmp;

  BN_ULONG carry =

      bn_add_words(tmp.words, r->words, group->order.N.d, group->field.N.width);

  if (carry == 0 &&

      bn_less_than_words(tmp.words, group->field.N.d, group->field.N.width)) {

    fiat_p256_from_generic(r_Z2, &tmp);

    fiat_p256_mul(r_Z2, r_Z2, Z2_mont);

    if (OPENSSL_memcmp(&r_Z2, &X, sizeof(r_Z2)) == 0) {

      return 1;

    }

  }

  return 0;

}

BSSL_NAMESPACE_BEGIN

DEFINE_METHOD_FUNCTION(EC_METHOD, EC_GFp_nistp256_method) {

  out->point_get_affine_coordinates =

      ec_GFp_nistp256_point_get_affine_coordinates;

  out->add = ec_GFp_nistp256_add;

  out->dbl = ec_GFp_nistp256_dbl;

  out->mul = ec_GFp_nistp256_point_mul;

  out->mul_base = ec_GFp_nistp256_point_mul_base;

  out->mul_public = ec_GFp_nistp256_point_mul_public;

  out->felem_mul = ec_GFp_mont_felem_mul;

  out->felem_sqr = ec_GFp_mont_felem_sqr;

  out->felem_to_bytes = ec_GFp_mont_felem_to_bytes;

  out->felem_from_bytes = ec_GFp_mont_felem_from_bytes;

  out->felem_reduce = ec_GFp_mont_felem_reduce;

  // TODO(davidben): This should use the specialized field arithmetic

  // implementation, rather than the generic one.

  out->felem_exp = ec_GFp_mont_felem_exp;

  out->scalar_inv0_montgomery = ec_simple_scalar_inv0_montgomery;

  out->scalar_to_montgomery_inv_vartime =

      ec_simple_scalar_to_montgomery_inv_vartime;

  out->cmp_x_coordinate = ec_GFp_nistp256_cmp_x_coordinate;

}

BSSL_NAMESPACE_END