/* Copyright 2016-2017 Brian Smith.

 *

 * Permission to use, copy, modify, and/or distribute this software for any

 * purpose with or without fee is hereby granted, provided that the above

 * copyright notice and this permission notice appear in all copies.

 *

 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES

 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF

 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY

 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES

 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION

 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN

 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#include "limbs.h"

#include "../internal.h"

#include "../fipsmodule/bn/internal.h"

#include "limbs.inl"

/* XXX: We assume that the conversion from |Carry| to |Limb| is constant-time,

 * but we haven't verified that assumption. TODO: Fix it so we don't need to

 * make that assumption. */

/* Returns 0xfff..f if |a| is zero, and zero otherwise. */

Limb LIMB_is_zero(const Limb a) {

  return constant_time_is_zero_w(a);

}

/* Returns 0xfff..f if |a| is all zero limbs, and zero otherwise. |num_limbs|

 * may be zero. */

Limb LIMBS_are_zero(const Limb a[], size_t num_limbs) {

  Limb all = 0;

  for (size_t i = 0; i < num_limbs; ++i) {

    all |= a[i];

  }

  return LIMB_is_zero(all);

}

/* Returns 0xffff..f if |a == b|, and zero otherwise. |num_limbs| may be zero. */

Limb LIMBS_equal(const Limb a[], const Limb b[], size_t num_limbs) {

  Limb eq = CONSTTIME_TRUE_W;

  for (size_t i = 0; i < num_limbs; ++i) {

    eq = constant_time_select_w(eq, constant_time_eq_w(a[i], b[i]), eq);

  }

  return eq;

}

/* Returns 0xffff...f if |a| is less than |b|, and zero otherwise. */

Limb LIMBS_less_than(const Limb a[], const Limb b[], size_t num_limbs) {

  debug_assert_nonsecret(num_limbs >= 1);

  /* There are lots of ways to implement this. It is implemented this way to

   * be consistent with |LIMBS_limbs_reduce_once| and other code that makes such

   * comparisons as part of doing conditional reductions. */

  Limb dummy;

  Carry borrow = limb_sub(&dummy, a[0], b[0]);

  for (size_t i = 1; i < num_limbs; ++i) {

    borrow = limb_sbb(&dummy, a[i], b[i], borrow);

  }

  return constant_time_is_nonzero_w(borrow);

}

/* if (r >= m) { r -= m; } */

void LIMBS_reduce_once(Limb r[], const Limb m[], size_t num_limbs) {

  debug_assert_nonsecret(num_limbs >= 1);

  /* This could be done more efficiently if we had |num_limbs| of extra space

   * available, by storing |r - m| and then doing a conditional copy of either

   * |r| or |r - m|. But, in order to operate in constant space, with an eye

   * towards this function being used in RSA in the future, we do things a

   * slightly less efficient way. */

  Limb lt = LIMBS_less_than(r, m, num_limbs);

  Carry borrow =

      limb_sub(&r[0], r[0], constant_time_select_w(lt, 0, m[0]));

  for (size_t i = 1; i < num_limbs; ++i) {

    /* XXX: This is probably particularly inefficient because the operations in

     * constant_time_select affect the carry flag, so there will likely be

     * loads and stores of |borrow|. */

    borrow =

        limb_sbb(&r[i], r[i], constant_time_select_w(lt, 0, m[i]), borrow);

  }

  dev_assert_secret(borrow == 0);

}

void LIMBS_add_mod(Limb r[], const Limb a[], const Limb b[], const Limb m[],

                   size_t num_limbs) {

  Limb overflow1 =

      constant_time_is_nonzero_w(limbs_add(r, a, b, num_limbs));

  Limb overflow2 = ~LIMBS_less_than(r, m, num_limbs);

  Limb overflow = overflow1 | overflow2;

  Carry borrow = limb_sub(&r[0], r[0], m[0] & overflow);

  for (size_t i = 1; i < num_limbs; ++i) {

    borrow = limb_sbb(&r[i], r[i], m[i] & overflow, borrow);

  }

}

void LIMBS_sub_mod(Limb r[], const Limb a[], const Limb b[], const Limb m[],

                   size_t num_limbs) {

  Limb underflow =

      constant_time_is_nonzero_w(limbs_sub(r, a, b, num_limbs));

  Carry carry = limb_add(&r[0], r[0], m[0] & underflow);

  for (size_t i = 1; i < num_limbs; ++i) {

    carry = limb_adc(&r[i], r[i], m[i] & underflow, carry);

  }

}

void LIMBS_shl_mod(Limb r[], const Limb a[], const Limb m[], size_t num_limbs) {

  Limb overflow1 =

      constant_time_is_nonzero_w(a[num_limbs - 1] & LIMB_HIGH_BIT);

  Limb carry = 0;

  for (size_t i = 0; i < num_limbs; ++i) {

    Limb limb = a[i];

    Limb new_carry = limb >> (LIMB_BITS - 1);

    r[i] = (limb << 1) | carry;

    carry = new_carry;

  }

  Limb overflow2 = ~LIMBS_less_than(r, m, num_limbs);

  Limb overflow = overflow1 | overflow2;

  Carry borrow = limb_sub(&r[0], r[0], m[0] & overflow);

  for (size_t i = 1; i < num_limbs; ++i) {

    borrow = limb_sbb(&r[i], r[i], m[i] & overflow, borrow);

  }

}

int LIMBS_select_512_32(Limb r[], const Limb table[], size_t num_limbs,

                        crypto_word_t index) {

  if (num_limbs % (512 / LIMB_BITS) != 0) {

    return 0;

  }

  limbs_select(r, table, num_limbs, 32, index);

  return 1;

}

static const Limb FIVE_BITS_MASK = 0x1f;

crypto_word_t LIMBS_window5_split_window(Limb lower_limb, Limb higher_limb, size_t index_within_word) {

  Limb high_bits = (higher_limb << (LIMB_BITS - index_within_word))

    & FIVE_BITS_MASK;

  // There are no bits outside the window above |index_within_word| (if there

  // were then this wouldn't be a split window), so we don't need to mask

  // |low_bits|.

  Limb low_bits = lower_limb >> index_within_word;

  return low_bits | high_bits;

}

crypto_word_t LIMBS_window5_unsplit_window(Limb limb, size_t index_within_word) {

  return (limb >> index_within_word) & FIVE_BITS_MASK;

}

Limb LIMB_shr(Limb a, size_t shift) {

  return a >> shift;

}

Limb limbs_mul_add_limb(Limb r[], const Limb a[], Limb b, size_t num_limbs) {

  Limb carried = 0;

  for (size_t i = 0; i < num_limbs; ++i) {

    Limb lo;

    Limb hi;

    bn_umult_lohi(&lo, &hi, a[i], b);

    Limb tmp;

    Carry c = limb_add(&tmp, lo, carried);

    c = limb_adc(&carried, hi, 0, c);

    dev_assert_secret(c == 0);

    c = limb_add(&r[i], r[i], tmp);

    c = limb_adc(&carried, carried, 0, c);

    // (A * B) + C + D never carries.

    dev_assert_secret(c == 0);

  }

  return carried;

}