/*

 * 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))

#define ORDER_LEN   ((BR_MAX_EC_SIZE + 7) >> 3)

/* see bearssl_ec.h */

size_t

br_ecdsa_i15_sign_raw(const br_ec_impl *impl,

  const br_hash_class *hf, const void *hash_value,

  const br_ec_private_key *sk, void *sig)

{

  /*

   * 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.

   * We also rely on the last byte of the curve order to be distinct

   * from 0 and 1.

   */

  const br_ec_curve_def *cd;

  uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], x[I15_LEN];

  uint16_t m[I15_LEN], k[I15_LEN], t1[I15_LEN], t2[I15_LEN];

  unsigned char tt[ORDER_LEN << 1];

  unsigned char eU[POINT_LEN];

  size_t hash_len, nlen, ulen;

  uint16_t n0i;

  uint32_t ctl;

  br_hmac_drbg_context drbg;

  /*

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

   */

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

    return 0;

  }

  /*

   * Get the curve parameters (generator and order).

   */

  switch (sk->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;

  }

  /*

   * Get modulus.

   */

  nlen = cd->order_len;

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

  n0i = br_i15_ninv15(n[1]);

  /*

   * Get private key as an i15 integer. This also checks that the

   * private key is well-defined (not zero, and less than the

   * curve order).

   */

  if (!br_i15_decode_mod(x, sk->x, sk->xlen, n)) {

    return 0;

  }

  if (br_i15_iszero(x)) {

    return 0;

  }

  /*

   * Get hash length.

   */

  hash_len = (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;

  /*

   * Truncate and reduce the hash value modulo the curve order.

   */

  br_ecdsa_i15_bits2int(m, hash_value, hash_len, n[0]);

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

  /*

   * RFC 6979 generation of the "k" value.

   *

   * The process uses HMAC_DRBG (with the hash function used to

   * process the message that is to be signed). The seed is the

   * concatenation of the encodings of the private key and

   * the hash value (after truncation and modular reduction).

   */

  br_i15_encode(tt, nlen, x);

  br_i15_encode(tt + nlen, nlen, m);

  br_hmac_drbg_init(&drbg, hf, tt, nlen << 1);

  for (;;) {

    br_hmac_drbg_generate(&drbg, tt, nlen);

    br_ecdsa_i15_bits2int(k, tt, nlen, n[0]);

    if (br_i15_iszero(k)) {

      continue;

    }

    if (br_i15_sub(k, n, 0)) {

      break;

    }

  }

  /*

   * Compute k*G and extract the X coordinate, then reduce it

   * modulo the curve order. Since we support only curves with

   * prime order, that reduction is only a matter of computing

   * a subtraction.

   */

  br_i15_encode(tt, nlen, k);

  ulen = impl->mulgen(eU, tt, nlen, sk->curve);

  br_i15_zero(r, n[0]);

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

  r[0] = n[0];

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

  /*

   * Compute 1/k in double-Montgomery representation. We do so by

   * first converting _from_ Montgomery representation (twice),

   * then using a modular exponentiation.

   */

  br_i15_from_monty(k, n, n0i);

  br_i15_from_monty(k, n, n0i);

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

  tt[nlen - 1] -= 2;

  br_i15_modpow(k, tt, nlen, n, n0i, t1, t2);

  /*

   * Compute s = (m+xr)/k (mod n).

   * The k[] array contains R^2/k (double-Montgomery representation);

   * we thus can use direct Montgomery multiplications and conversions

   * from Montgomery, avoiding any call to br_i15_to_monty() (which

   * is slower).

   */

  br_i15_from_monty(m, n, n0i);

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

  ctl = br_i15_add(t1, m, 1);

  ctl |= br_i15_sub(t1, n, 0) ^ 1;

  br_i15_sub(t1, n, ctl);

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

  /*

   * Encode r and s in the signature.

   */

  br_i15_encode(sig, nlen, r);

  br_i15_encode((unsigned char *)sig + nlen, nlen, s);

  return nlen << 1;

}