pqc/external/flint-2.4.3/ulong_extras/discrete_log_bsgs.c

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2014-05-18 22:03:37 +00:00
/*=============================================================================
This file is part of FLINT.
FLINT is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
FLINT is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with FLINT; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
=============================================================================*/
/******************************************************************************
Copyright (C) 2013 Mike Hansen
******************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "flint.h"
#include "ulong_extras.h"
mp_limb_t n_discrete_log_bsgs(mp_limb_t b, mp_limb_t a, mp_limb_t n)
{
int i, j;
double ninv;
mp_limb_t m, a_m, c;
mp_limb_t * table;
ninv = n_precompute_inverse(n);
m = ceil(sqrt((double) n));
table = (mp_limb_t*)flint_malloc(m * sizeof(mp_limb_t));
table[0] = 1;
for (j = 1; j < m; j++)
{
table[j] = n_mulmod_precomp(table[j-1], a, n, ninv);
}
a_m = n_invmod(a, n);
a_m = n_powmod_precomp(a_m, m, n, ninv);
c = b;
for (i = 0; i < m; i++)
{
for (j = 0; j < m; j++)
{
if (c == table[j])
{
flint_free(table);
return i * m + j;
}
}
c = n_mulmod_precomp(c, a_m, n, ninv);
}
flint_free(table);
flint_printf("Exception (n_discrete_log_bsgs). discrete log not found.\n");
abort();
}