mp_mod_2d.c

   1 #include "tommath_private.h"
   2 #ifdef MP_MOD_2D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* calc a value mod 2**b */
   7 mp_err mp_mod_2d(const mp_int *a, int b, mp_int *c)
   8 {
   9    int x;
  10    mp_err err;
  11
  12    if (b < 0) {
  13       return MP_VAL;
  14    }
  15
  16    if (b == 0) {
  17       mp_zero(c);
  18       return MP_OKAY;
  19    }
  20
  21    /* if the modulus is larger than the value than return */
  22    if (b >= (a->used * MP_DIGIT_BIT)) {
  23       return mp_copy(a, c);
  24    }
  25
  26    if ((err = mp_copy(a, c)) != MP_OKAY) {
  27       return err;
  28    }
  29
  30    /* zero digits above the last digit of the modulus */
  31    x = (b / MP_DIGIT_BIT) + (((b % MP_DIGIT_BIT) == 0) ? 0 : 1);
  32    s_mp_zero_digs(c->dp + x, c->used - x);
  33
  34    /* clear the digit that is not completely outside/inside the modulus */
  35    c->dp[b / MP_DIGIT_BIT] &=
  36       ((mp_digit)1 << (mp_digit)(b % MP_DIGIT_BIT)) - (mp_digit)1;
  37    mp_clamp(c);
  38    return MP_OKAY;
  39 }
  40 #endif