lib/hcrypto/libtommath/bn_mp_reduce_2k_setup.c

   1 #include "tommath_private.h"
   2 #ifdef BN_MP_REDUCE_2K_SETUP_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* determines the setup value */
   7 mp_err mp_reduce_2k_setup(const mp_int *a, mp_digit *d)
   8 {
   9    mp_err err;
  10    mp_int tmp;
  11    int    p;
  12
  13    if ((err = mp_init(&tmp)) != MP_OKAY) {
  14       return err;
  15    }
  16
  17    p = mp_count_bits(a);
  18    if ((err = mp_2expt(&tmp, p)) != MP_OKAY) {
  19       mp_clear(&tmp);
  20       return err;
  21    }
  22
  23    if ((err = s_mp_sub(&tmp, a, &tmp)) != MP_OKAY) {
  24       mp_clear(&tmp);
  25       return err;
  26    }
  27
  28    *d = tmp.dp[0];
  29    mp_clear(&tmp);
  30    return MP_OKAY;
  31 }
  32 #endif