/* ecc-non-sec-add-jjj.c

   Copyright (C) 2013, 2022 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or

   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free

       Software Foundation; either version 3 of the License, or (at your

       option) any later version.

   or

     * the GNU General Public License as published by the Free

       Software Foundation; either version 2 of the License, or (at your

       option) any later version.

   or both in parallel, as here.

   GNU Nettle is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   General Public License for more details.

   You should have received copies of the GNU General Public License and

   the GNU Lesser General Public License along with this program.  If

   not, see http://www.gnu.org/licenses/.

*/

/* Development of Nettle's ECC support was funded by the .SE Internet Fund. */

#if HAVE_CONFIG_H

# include "config.h"

#endif

#include "ecc.h"

#include "ecc-internal.h"

/* Similar to ecc_add_jjj, but checks if x coordinates are equal (H =

   0) below, and if so, performs doubling if also y coordinates are

   equal, or returns 0 (failure) indicating that the result is the

   infinity point. */

int

ecc_nonsec_add_jjj (const struct ecc_curve *ecc,

        mp_limb_t *r, const mp_limb_t *p, const mp_limb_t *q,

        mp_limb_t *scratch)

{

#define x1 p

#define y1 (p + ecc->p.size)

#define z1 (p + 2*ecc->p.size)

#define x2 q

#define y2 (q + ecc->p.size)

#define z2 (q + 2*ecc->p.size)

#define x3 r

#define y3 (r + ecc->p.size)

#define z3 (r + 2*ecc->p.size)

  /* Formulas, from djb,

     http://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html#addition-add-2007-bl:

     Computation    Operation Live variables

      Z1Z1 = Z1^2   sqr   Z1Z1

      Z2Z2 = Z2^2   sqr   Z1Z1, Z2Z2

      U1 = X1*Z2Z2    mul   Z1Z1, Z2Z2, U1

      U2 = X2*Z1Z1    mul   Z1Z1, Z2Z2, U1, U2

      H = U2-U1         Z1Z1, Z2Z2, U1, H

      Z3 = ((Z1+Z2)^2-Z1Z1-Z2Z2)*H sqr, mul Z1Z1, Z2Z2, U1, H

      S1 = Y1*Z2*Z2Z2   mul, mul  Z1Z1, U1, H, S1

      S2 = Y2*Z1*Z1Z1   mul, mul  U1, H, S1, S2

      W = 2*(S2-S1) (djb: r)    U1, H, S1, W

      I = (2*H)^2   sqr   U1, H, S1, W, I

      J = H*I     mul   U1, S1, W, J, V

      V = U1*I      mul   S1, W, J, V

      X3 = W^2-J-2*V    sqr   S1, W, J, V

      Y3 = W*(V-X3)-2*S1*J  mul, mul

  */

#define h scratch

#define z1z1 (scratch + ecc->p.size)

#define z2z2 z1z1

#define z1z2 (scratch + 2*ecc->p.size)

#define w (scratch + ecc->p.size)

#define i (scratch + 2*ecc->p.size)

#define j h

#define v i

#define tp  (scratch + 3*ecc->p.size)

  ecc_mod_sqr (&ecc->p, z2z2, z2, tp);    /* z2z2 */

  /* Store u1 at x3 */

  ecc_mod_mul (&ecc->p, x3, x1, z2z2, tp);  /* z2z2 */

  ecc_mod_add (&ecc->p, z1z2, z1, z2);    /* z2z2, z1z2 */

  ecc_mod_sqr (&ecc->p, z1z2, z1z2, tp);

  ecc_mod_sub (&ecc->p, z1z2, z1z2, z2z2);  /* z2z2, z1z2 */

  /* Do s1 early, store at y3 */

  ecc_mod_mul (&ecc->p, z2z2, z2z2, z2, tp);  /* z2z2, z1z2 */

  ecc_mod_mul (&ecc->p, y3, z2z2, y1, tp);  /* z1z2 */

  ecc_mod_sqr (&ecc->p, z1z1, z1, tp);    /* z1z1, z1z2 */

  ecc_mod_sub (&ecc->p, z1z2, z1z2, z1z1);

  ecc_mod_mul (&ecc->p, h, x2, z1z1, tp);  /* z1z1, z1z2, h */

  ecc_mod_sub (&ecc->p, h, h, x3);

  /* z1^3 */

  ecc_mod_mul (&ecc->p, z1z1, z1z1, z1, tp);

  /* z3 <-- h z1 z2 delayed until now, since that may clobber z1. */

  ecc_mod_mul (&ecc->p, z3, z1z2, h, tp);  /* z1z1, h */

  /* w = 2 (s2 - s1) */

  ecc_mod_mul (&ecc->p, w, z1z1, y2, tp);  /* h, w */

  ecc_mod_sub (&ecc->p, w, w, y3);

  /* Note that use of ecc_mod_zero_p depends 0 <= h,w < 2p. */

  if (ecc_mod_zero_p (&ecc->p, h))

    {

      /* X1 == X2 */

      if (ecc_mod_zero_p (&ecc->p, w)) {

  /* Y1 == Y2. Do point duplication. Note that q input is

     unclobbered, and that scratch need is smaller. Implies some

     unnecessary recomputation, but performance it not so

     important for this very unlikely corner case. */

  ecc_dup_jj (ecc, r, q, scratch);

  return 1;

      }

      /* We must have Y1 == -Y2, and then the result is the infinity

   point, */

      mpn_zero (r, 3*ecc->p.size);

      return 0;

    }

  ecc_mod_add (&ecc->p, w, w, w);

  /* i = (2h)^2 */

  ecc_mod_add (&ecc->p, i, h, h);    /* h, w, i */

  ecc_mod_sqr (&ecc->p, i, i, tp);

  /* j and h can overlap */

  ecc_mod_mul (&ecc->p, j, h, i, tp);    /* j, w, i */

  /* v and i can overlap */

  ecc_mod_mul (&ecc->p, v, x3, i, tp);    /* j, w, v */

  /* x3 <-- w^2 - j - 2v */

  ecc_mod_sqr (&ecc->p, x3, w, tp);

  ecc_mod_sub (&ecc->p, x3, x3, j);

  ecc_mod_submul_1 (&ecc->p, x3, v, 2);

  /* y3 <-- w (v - x3) - 2 s1 j */

  ecc_mod_mul (&ecc->p, j, j, y3, tp);

  ecc_mod_sub (&ecc->p, v, v, x3);

  ecc_mod_mul (&ecc->p, y3, v, w, tp);

  ecc_mod_submul_1 (&ecc->p, y3, j, 2);

  return 1;

}