libtommath/bn_mp_prime_frobenius_underwood.c

#include "tommath_private.h"
#ifdef BN_MP_PRIME_FROBENIUS_UNDERWOOD_C

/* LibTomMath, multiple-precision integer library -- Tom St Denis
 *
 * LibTomMath is a library that provides multiple-precision
 * integer arithmetic as well as number theoretic functionality.
 *
 * The library was designed directly after the MPI library by
 * Michael Fromberger but has been written from scratch with
 * additional optimizations in place.
 *
 * SPDX-License-Identifier: Unlicense
 */

/*
 *  See file bn_mp_prime_is_prime.c or the documentation in doc/bn.tex for the details
 */
#ifndef LTM_USE_FIPS_ONLY

#ifdef MP_8BIT
/*
 * floor of positive solution of
 * (2^16)-1 = (a+4)*(2*a+5)
 * TODO: Both values are smaller than N^(1/4), would have to use a bigint
 *       for a instead but any a biger than about 120 are already so rare that
 *       it is possible to ignore them and still get enough pseudoprimes.
 *       But it is still a restriction of the set of available pseudoprimes
 *       which makes this implementation less secure if used stand-alone.
 */
#define LTM_FROBENIUS_UNDERWOOD_A 177
#else
#define LTM_FROBENIUS_UNDERWOOD_A 32764
#endif
int mp_prime_frobenius_underwood(const mp_int *N, int *result)
{
   mp_int T1z, T2z, Np1z, sz, tz;

   int a, ap2, length, i, j, isset;
   int e;

   *result = MP_NO;

   if ((e = mp_init_multi(&T1z, &T2z, &Np1z, &sz, &tz, NULL)) != MP_OKAY) {
      return e;
   }

   for (a = 0; a < LTM_FROBENIUS_UNDERWOOD_A; a++) {
      /* TODO: That's ugly! No, really, it is! */
      if ((a==2) || (a==4) || (a==7) || (a==8) || (a==10) ||
          (a==14) || (a==18) || (a==23) || (a==26) || (a==28)) {
         continue;
      }
      /* (32764^2 - 4) < 2^31, no bigint for >MP_8BIT needed) */
      if ((e = mp_set_long(&T1z, (unsigned long)a)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }

      if ((e = mp_sqr(&T1z, &T1z)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }

      if ((e = mp_sub_d(&T1z, 4uL, &T1z)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }

      if ((e = mp_kronecker(&T1z, N, &j)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }

      if (j == -1) {
         break;
      }

      if (j == 0) {
         /* composite */
         goto LBL_FU_ERR;
      }
   }
   /* Tell it a composite and set return value accordingly */
   if (a >= LTM_FROBENIUS_UNDERWOOD_A) {
      e = MP_ITER;
      goto LBL_FU_ERR;
   }
   /* Composite if N and (a+4)*(2*a+5) are not coprime */
   if ((e = mp_set_long(&T1z, (unsigned long)((a+4)*((2*a)+5)))) != MP_OKAY) {
      goto LBL_FU_ERR;
   }

   if ((e = mp_gcd(N, &T1z, &T1z)) != MP_OKAY) {
      goto LBL_FU_ERR;
   }

   if (!((T1z.used == 1) && (T1z.dp[0] == 1u))) {
      goto LBL_FU_ERR;
   }

   ap2 = a + 2;
   if ((e = mp_add_d(N, 1uL, &Np1z)) != MP_OKAY) {
      goto LBL_FU_ERR;
   }

   mp_set(&sz, 1uL);
   mp_set(&tz, 2uL);
   length = mp_count_bits(&Np1z);

   for (i = length - 2; i >= 0; i--) {
      /*
       * temp = (sz*(a*sz+2*tz))%N;
       * tz   = ((tz-sz)*(tz+sz))%N;
       * sz   = temp;
       */
      if ((e = mp_mul_2(&tz, &T2z)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }

      /* a = 0 at about 50% of the cases (non-square and odd input) */
      if (a != 0) {
         if ((e = mp_mul_d(&sz, (mp_digit)a, &T1z)) != MP_OKAY) {
            goto LBL_FU_ERR;
         }
         if ((e = mp_add(&T1z, &T2z, &T2z)) != MP_OKAY) {
            goto LBL_FU_ERR;
         }
      }

      if ((e = mp_mul(&T2z, &sz, &T1z)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((e = mp_sub(&tz, &sz, &T2z)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((e = mp_add(&sz, &tz, &sz)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((e = mp_mul(&sz, &T2z, &tz)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((e = mp_mod(&tz, N, &tz)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((e = mp_mod(&T1z, N, &sz)) != MP_OKAY) {
         goto LBL_FU_ERR;
      }
      if ((isset = mp_get_bit(&Np1z, i)) == MP_VAL) {
         e = isset;
         goto LBL_FU_ERR;
      }
      if (isset == MP_YES) {
         /*
          *  temp = (a+2) * sz + tz
          *  tz   = 2 * tz - sz
          *  sz   = temp
          */
         if (a == 0) {
            if ((e = mp_mul_2(&sz, &T1z)) != MP_OKAY) {
               goto LBL_FU_ERR;
            }
         } else {
            if ((e = mp_mul_d(&sz, (mp_digit)ap2, &T1z)) != MP_OKAY) {
               goto LBL_FU_ERR;
            }
         }
         if ((e = mp_add(&T1z, &tz, &T1z)) != MP_OKAY) {
            goto LBL_FU_ERR;
         }
         if ((e = mp_mul_2(&tz, &T2z)) != MP_OKAY) {
            goto LBL_FU_ERR;
         }
         if ((e = mp_sub(&T2z, &sz, &tz)) != MP_OKAY) {
            goto LBL_FU_ERR;
         }
         mp_exch(&sz, &T1z);
      }
   }

   if ((e = mp_set_long(&T1z, (unsigned long)((2 * a) + 5))) != MP_OKAY) {
      goto LBL_FU_ERR;
   }
   if ((e = mp_mod(&T1z, N, &T1z)) != MP_OKAY) {
      goto LBL_FU_ERR;
   }
   if ((mp_iszero(&sz) != MP_NO) && (mp_cmp(&tz, &T1z) == MP_EQ)) {
      *result = MP_YES;
      goto LBL_FU_ERR;
   }

LBL_FU_ERR:
   mp_clear_multi(&tz, &sz, &Np1z, &T2z, &T1z, NULL);
   return e;
}

#endif
#endif

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