dbl.c

/* Copyright 2008-2019 Douglas Wikstrom
 *
 * This file is part of Verificatum Elliptic Curve library (VEC).
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy,
 * modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include <gmp.h>

#include "vec.h"

#define t1 scratch->t1
#define t2 scratch->t2
#define s scratch->t3

#define modulus curve->modulus
#define a curve->a

void
vec_dbl(vec_scratch_mpz_t scratch,
        mpz_t rx, mpz_t ry,
        vec_curve *curve,
        mpz_t x, mpz_t y)
{

  /* If this is the unit point or its own inverse, then return the
     unit point. */
  if (mpz_cmp_si(x, -1) == 0 || mpz_cmp_si(y, 0) == 0)
    {
      mpz_set_si(rx, -1);
      mpz_set_si(ry, -1);
      return;
    }

  /* s = (3x^2 + a) / 2y */
  mpz_mul(t1, x, x);
  mpz_mod(t1, t1, modulus);
  mpz_mul_ui(t1, t1, 3);
  mpz_add(t1, t1, a);
  mpz_mod(t1, t1, modulus);

  mpz_mul_ui(t2, y, 2);
  mpz_invert(t2, t2, modulus);

  mpz_mul(s, t1, t2);
  mpz_mod(s, s, modulus);

  /* rx = s^2 - 2x */
  mpz_mul(t1, s, s);
  mpz_mul_ui(t2, x, 2);
  mpz_sub(t1, t1, t2);

  /* ry = s(x - rx) - y */
  mpz_sub(t2, x, t1);
  mpz_mul(t2, s, t2);
  mpz_sub(t2, t2, y);

  /* We assign the destination parameters in the end to allow them to
     be identical to the inputs. */
  mpz_mod(rx, t1, modulus);
  mpz_mod(ry, t2, modulus);

  return;
}