kc3-lang/libtommath/mp_exptmod.c

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• Hash : c7686f24
  Author :
  Date : 2022-10-02T12:58:53
  

  slightly edit, update and run typos.sh Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

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• mp_exptmod.c

• #include "tommath_private.h"
  #ifdef MP_EXPTMOD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* this is a shell function that calls either the normal or Montgomery
   * exptmod functions.  Originally the call to the montgomery code was
   * embedded in the normal function but that wasted a lot of stack space
   * for nothing (since 99% of the time the Montgomery code would be called)
   */
  mp_err mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y)
  {
     int dr;
  
     /* modulus P must be positive */
     if (mp_isneg(P)) {
        return MP_VAL;
     }
  
     /* if exponent X is negative we have to recurse */
     if (mp_isneg(X)) {
        mp_int tmpG, tmpX;
        mp_err err;
  
        if (!MP_HAS(MP_INVMOD)) {
           return MP_VAL;
        }
  
        if ((err = mp_init_multi(&tmpG, &tmpX, NULL)) != MP_OKAY) {
           return err;
        }
  
        /* first compute 1/G mod P */
        if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        /* now get |X| */
        if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        /* and now compute (1/G)**|X| instead of G**X [X < 0] */
        err = mp_exptmod(&tmpG, &tmpX, P, Y);
  LBL_ERR:
        mp_clear_multi(&tmpG, &tmpX, NULL);
        return err;
     }
  
     /* modified diminished radix reduction */
     if (MP_HAS(MP_REDUCE_IS_2K_L) && MP_HAS(MP_REDUCE_2K_L) && MP_HAS(S_MP_EXPTMOD) &&
         mp_reduce_is_2k_l(P)) {
        return s_mp_exptmod(G, X, P, Y, 1);
     }
  
     /* is it a DR modulus? default to no */
     dr = (MP_HAS(MP_DR_IS_MODULUS) && mp_dr_is_modulus(P)) ? 1 : 0;
  
     /* if not, is it a unrestricted DR modulus? */
     if (MP_HAS(MP_REDUCE_IS_2K) && (dr == 0)) {
        dr = (mp_reduce_is_2k(P)) ? 2 : 0;
     }
  
     /* if the modulus is odd or dr != 0 use the montgomery method */
     if (MP_HAS(S_MP_EXPTMOD_FAST) && (mp_isodd(P) || (dr != 0))) {
        return s_mp_exptmod_fast(G, X, P, Y, dr);
     }
  
     /* otherwise use the generic Barrett reduction technique */
     if (MP_HAS(S_MP_EXPTMOD)) {
        return s_mp_exptmod(G, X, P, Y, 0);
     }
  
     /* no exptmod for evens */
     return MP_VAL;
  }
  
  #endif
  

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