mp_invmod.c

   1 #include "tommath_private.h"
   2 #ifdef MP_INVMOD_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* hac 14.61, pp608 */
   7 mp_err mp_invmod(const mp_int *a, const mp_int *b, mp_int *c)
   8 {
   9    /* for all n in N and n > 0, n = 0 mod 1 */
  10    if (!mp_isneg(a) && mp_cmp_d(b, 1uL) == MP_EQ) {
  11       mp_zero(c);
  12       return MP_OKAY;
  13    }
  14
  15    /* b cannot be negative and has to be >1 */
  16    if (mp_isneg(b) || (mp_cmp_d(b, 1uL) != MP_GT)) {
  17       return MP_VAL;
  18    }
  19
  20    /* if the modulus is odd we can use a faster routine instead */
  21    if (MP_HAS(S_MP_INVMOD_ODD) && mp_isodd(b)) {
  22       return s_mp_invmod_odd(a, b, c);
  23    }
  24
  25    return MP_HAS(S_MP_INVMOD)
  26           ? s_mp_invmod(a, b, c)
  27           : MP_VAL;
  28 }
  29 #endif