mp_dr_is_modulus.c

   1 #include "tommath_private.h"
   2 #ifdef MP_DR_IS_MODULUS_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* determines if a number is a valid DR modulus */
   7 bool mp_dr_is_modulus(const mp_int *a)
   8 {
   9    int ix;
  10
  11    /* must be at least two digits */
  12    if (a->used < 2) {
  13       return false;
  14    }
  15
  16    /* must be of the form b**k - a [a <= b] so all
  17     * but the first digit must be equal to -1 (mod b).
  18     */
  19    for (ix = 1; ix < a->used; ix++) {
  20       if (a->dp[ix] != MP_MASK) {
  21          return false;
  22       }
  23    }
  24    return true;
  25 }
  26
  27 #endif