/*

 * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>

 *

 * 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 "inner.h"

#define I15_LEN     ((BR_MAX_EC_SIZE + 29) / 15)

#define POINT_LEN   (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))

/* see bearssl_ec.h */

uint32_t

br_ecdsa_i15_vrfy_raw(const br_ec_impl *impl,

  const void *hash, size_t hash_len,

  const br_ec_public_key *pk,

  const void *sig, size_t sig_len)

{

  /*

   * IMPORTANT: this code is fit only for curves with a prime

   * order. This is needed so that modular reduction of the X

   * coordinate of a point can be done with a simple subtraction.

   */

  const br_ec_curve_def *cd;

  uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], t1[I15_LEN], t2[I15_LEN];

  unsigned char tx[(BR_MAX_EC_SIZE + 7) >> 3];

  unsigned char ty[(BR_MAX_EC_SIZE + 7) >> 3];

  unsigned char eU[POINT_LEN];

  size_t nlen, rlen, ulen;

  uint16_t n0i;

  uint32_t res;

  /*

   * If the curve is not supported, then report an error.

   */

  if (((impl->supported_curves >> pk->curve) & 1) == 0) {

    return 0;

  }

  /*

   * Get the curve parameters (generator and order).

   */

  switch (pk->curve) {

  case BR_EC_secp256r1:

    cd = &br_secp256r1;

    break;

  case BR_EC_secp384r1:

    cd = &br_secp384r1;

    break;

  case BR_EC_secp521r1:

    cd = &br_secp521r1;

    break;

  default:

    return 0;

  }

  /*

   * Signature length must be even.

   */

  if (sig_len & 1) {

    return 0;

  }

  rlen = sig_len >> 1;

  /*

   * Public key point must have the proper size for this curve.

   */

  if (pk->qlen != cd->generator_len) {

    return 0;

  }

  /*

   * Get modulus; then decode the r and s values. They must be

   * lower than the modulus, and s must not be null.

   */

  nlen = cd->order_len;

  br_i15_decode(n, cd->order, nlen);

  n0i = br_i15_ninv15(n[1]);

  if (!br_i15_decode_mod(r, sig, rlen, n)) {

    return 0;

  }

  if (!br_i15_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) {

    return 0;

  }

  if (br_i15_iszero(s)) {

    return 0;

  }

  /*

   * Invert s. We do that with a modular exponentiation; we use

   * the fact that for all the curves we support, the least

   * significant byte is not 0 or 1, so we can subtract 2 without

   * any carry to process.

   * We also want 1/s in Montgomery representation, which can be

   * done by converting _from_ Montgomery representation before

   * the inversion (because (1/s)*R = 1/(s/R)).

   */

  br_i15_from_monty(s, n, n0i);

  memcpy(tx, cd->order, nlen);

  tx[nlen - 1] -= 2;

  br_i15_modpow(s, tx, nlen, n, n0i, t1, t2);

  /*

   * Truncate the hash to the modulus length (in bits) and reduce

   * it modulo the curve order. The modular reduction can be done

   * with a subtraction since the truncation already reduced the

   * value to the modulus bit length.

   */

  br_ecdsa_i15_bits2int(t1, hash, hash_len, n[0]);

  br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);

  /*

   * Multiply the (truncated, reduced) hash value with 1/s, result in

   * t2, encoded in ty.

   */

  br_i15_montymul(t2, t1, s, n, n0i);

  br_i15_encode(ty, nlen, t2);

  /*

   * Multiply r with 1/s, result in t1, encoded in tx.

   */

  br_i15_montymul(t1, r, s, n, n0i);

  br_i15_encode(tx, nlen, t1);

  /*

   * Compute the point x*Q + y*G.

   */

  ulen = cd->generator_len;

  memcpy(eU, pk->q, ulen);

  res = impl->muladd(eU, NULL, ulen,

    tx, nlen, ty, nlen, cd->curve);

  /*

   * Get the X coordinate, reduce modulo the curve order, and

   * compare with the 'r' value.

   *

   * The modular reduction can be done with subtractions because

   * we work with curves of prime order, so the curve order is

   * close to the field order (Hasse's theorem).

   */

  br_i15_zero(t1, n[0]);

  br_i15_decode(t1, &eU[1], ulen >> 1);

  t1[0] = n[0];

  br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1);

  res &= ~br_i15_sub(t1, r, 1);

  res &= br_i15_iszero(t1);

  return res;

}