/* ecc-gostdsa-verify.c

   Copyright (C) 2015 Dmitry Eremin-Solenikov
   Copyright (C) 2013, 2014 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/.
*/

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>
#include <stdlib.h>

#include "gostdsa.h"
#include "ecc-internal.h"

/* Low-level GOST DSA verify */

static int
ecdsa_in_range (const struct ecc_curve *ecc, const mp_limb_t *xp)
{
  return !mpn_zero_p (xp, ecc->p.size)
    && mpn_cmp (xp, ecc->q.m, ecc->p.size) < 0;
}

mp_size_t
ecc_gostdsa_verify_itch (const struct ecc_curve *ecc)
{
  /* Largest storage need is for the ecc_mul_a call. */
  return 5*ecc->p.size + ECC_MUL_A_ITCH (ecc->p.size);
}

/* FIXME: Use faster primitives, not requiring side-channel silence. */
int
ecc_gostdsa_verify (const struct ecc_curve *ecc,
		  const mp_limb_t *pp, /* Public key */
		  size_t length, const uint8_t *digest,
		  const mp_limb_t *rp, const mp_limb_t *sp,
		  mp_limb_t *scratch)
{
  /* Procedure, according to GOST R 34.10. q denotes the group
     order.

     1. Check 0 < r, s < q.

     2. v <-- h^{-1}  (mod q)

     3. z1  <-- s * v (mod q)

     4. z2  <-- -r * v (mod q)

     5. R = u1 G + u2 Y

     6. Signature is valid if R_x = r (mod q).
  */

#define hp (scratch)
#define vp (scratch + ecc->p.size)
#define z1 (scratch + 3*ecc->p.size)
#define z2 (scratch + 4*ecc->p.size)

#define P1 (scratch + 4*ecc->p.size)
#define P2 (scratch)


  if (! (ecdsa_in_range (ecc, rp)
	 && ecdsa_in_range (ecc, sp)))
    return 0;

  gost_hash (&ecc->q, hp, length, digest);

  if (mpn_zero_p (hp, ecc->p.size))
    mpn_add_1 (hp, hp, ecc->p.size, 1);

  /* Compute v */
  ecc->q.invert (&ecc->q, vp, hp, vp + ecc->p.size);

  /* z1 = s / h, P1 = z1 * G */
  ecc_mod_mul_canonical (&ecc->q, z1, sp, vp, z1);

  /* z2 = - r / h, P2 = z2 * Y */
  mpn_sub_n (hp, ecc->q.m, rp, ecc->p.size);
  ecc_mod_mul_canonical (&ecc->q, z2, hp, vp, z2);

   /* Total storage: 5*ecc->p.size + ECC_MUL_A_ITCH */
  ecc_mul_a (ecc, P2, z2, pp, z2 + ecc->p.size);

  /* Total storage: 7*ecc->p.size + ECC_MUL_G_ITCH */
  ecc_mul_g (ecc, P1, z1, P1 + 3*ecc->p.size);

  /* Total storage: 6*ecc->p.size + ECC_ADD_JJJ_ITCH */
  if (!ecc_nonsec_add_jjj (ecc, P1, P1, P2, P1 + 3*ecc->p.size))
    return 0;

  /* x coordinate only, modulo q */
  ecc_j_to_a (ecc, 2, P2, P1, P1 + 3*ecc->p.size);

  return (mpn_cmp (rp, P2, ecc->p.size) == 0);
#undef P2
#undef P1
#undef z2
#undef z1
#undef hp
#undef vp
}
