/*

 * Copyright (c) 2016 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"

/* see inner.h */

void

br_i31_modpow(uint32_t *x,

  const unsigned char *e, size_t elen,

  const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)

{

  size_t mlen;

  uint32_t k;

  /*

   * 'mlen' is the length of m[] expressed in bytes (including

   * the "bit length" first field).

   */

  mlen = ((m[0] + 63) >> 5) * sizeof m[0];

  /*

   * Throughout the algorithm:

   * -- t1[] is in Montgomery representation; it contains x, x^2,

   * x^4, x^8...

   * -- The result is accumulated, in normal representation, in

   * the x[] array.

   * -- t2[] is used as destination buffer for each multiplication.

   *

   * Note that there is no need to call br_i32_from_monty().

   */

  memcpy(t1, x, mlen);

  br_i31_to_monty(t1, m);

  br_i31_zero(x, m[0]);

  x[1] = 1;

  for (k = 0; k < ((uint32_t)elen << 3); k ++) {

    uint32_t ctl;

    ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;

    br_i31_montymul(t2, x, t1, m, m0i);

    CCOPY(ctl, x, t2, mlen);

    br_i31_montymul(t2, t1, t1, m, m0i);

    memcpy(t1, t2, mlen);

  }

}