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/* mpih-const-time.c - Constant-time MPI helper functions
* Copyright (C) 2020 g10 Code GmbH
*
* This file is part of Libgcrypt.
*
* Libgcrypt is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* Libgcrypt 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 Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi-internal.h"
#include "g10lib.h"
#define A_LIMB_1 ((mpi_limb_t)1)
/* These variables are used to generate masks from conditional operation
* flag parameters. Use of volatile prevents compiler optimizations from
* converting AND-masking to conditional branches. */
static volatile mpi_limb_t vzero = 0;
static volatile mpi_limb_t vone = 1;
/*
* W = U when OP_ENABLED=1
* otherwise, W keeps old value
*/
void
_gcry_mpih_set_cond (mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize,
unsigned long op_enable)
{
mpi_size_t i;
mpi_limb_t mask1 = vzero - op_enable;
mpi_limb_t mask2 = op_enable - vone;
for (i = 0; i < usize; i++)
{
wp[i] = (wp[i] & mask2) | (up[i] & mask1);
}
}
/*
* W = U + V when OP_ENABLED=1
* otherwise, W = U
*/
mpi_limb_t
_gcry_mpih_add_n_cond (mpi_ptr_t wp, mpi_ptr_t up, mpi_ptr_t vp,
mpi_size_t usize, unsigned long op_enable)
{
mpi_size_t i;
mpi_limb_t cy;
mpi_limb_t mask1 = vzero - op_enable;
mpi_limb_t mask2 = op_enable - vone;
cy = 0;
for (i = 0; i < usize; i++)
{
mpi_limb_t u = up[i];
mpi_limb_t x = u + vp[i];
mpi_limb_t cy1 = x < u;
mpi_limb_t cy2;
x = x + cy;
cy2 = x < cy;
cy = cy1 | cy2;
wp[i] = (u & mask2) | (x & mask1);
}
return cy & mask1;
}
/*
* W = U - V when OP_ENABLED=1
* otherwise, W = U
*/
mpi_limb_t
_gcry_mpih_sub_n_cond (mpi_ptr_t wp, mpi_ptr_t up, mpi_ptr_t vp,
mpi_size_t usize, unsigned long op_enable)
{
mpi_size_t i;
mpi_limb_t cy;
mpi_limb_t mask1 = vzero - op_enable;
mpi_limb_t mask2 = op_enable - vone;
cy = 0;
for (i = 0; i < usize; i++)
{
mpi_limb_t u = up[i];
mpi_limb_t x = u - vp[i];
mpi_limb_t cy1 = x > u;
mpi_limb_t cy2;
cy2 = x < cy;
x = x - cy;
cy = cy1 | cy2;
wp[i] = (u & mask2) | (x & mask1);
}
return cy & mask1;
}
/*
* Swap value of U and V when OP_ENABLED=1
* otherwise, no change
*/
void
_gcry_mpih_swap_cond (mpi_ptr_t up, mpi_ptr_t vp, mpi_size_t usize,
unsigned long op_enable)
{
mpi_size_t i;
mpi_limb_t mask1 = vzero - op_enable;
mpi_limb_t mask2 = op_enable - vone;
for (i = 0; i < usize; i++)
{
mpi_limb_t u = up[i];
mpi_limb_t v = vp[i];
up[i] = (u & mask2) | (v & mask1);
vp[i] = (u & mask1) | (v & mask2);
}
}
/*
* W = -U when OP_ENABLED=1
* otherwise, W = U
*/
void
_gcry_mpih_abs_cond (mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize,
unsigned long op_enable)
{
mpi_size_t i;
mpi_limb_t mask1 = vzero - op_enable;
mpi_limb_t mask2 = op_enable - vone;
mpi_limb_t cy = op_enable;
for (i = 0; i < usize; i++)
{
mpi_limb_t u = up[i];
mpi_limb_t x = ~u + cy;
cy = (x < ~u);
wp[i] = (u & mask2) | (x & mask1);
}
}
/*
* Allocating memory for W,
* compute W = V % U, then return W
*/
mpi_ptr_t
_gcry_mpih_mod (mpi_ptr_t vp, mpi_size_t vsize,
mpi_ptr_t up, mpi_size_t usize)
{
int secure;
mpi_ptr_t rp;
mpi_size_t i;
secure = _gcry_is_secure (vp);
rp = mpi_alloc_limb_space (usize, secure);
MPN_ZERO (rp, usize);
for (i = 0; i < vsize * BITS_PER_MPI_LIMB; i++)
{
unsigned int j = vsize * BITS_PER_MPI_LIMB - 1 - i;
unsigned int limbno = j / BITS_PER_MPI_LIMB;
unsigned int bitno = j % BITS_PER_MPI_LIMB;
mpi_limb_t limb = vp[limbno];
unsigned int the_bit = ((limb & (A_LIMB_1 << bitno)) ? 1 : 0);
mpi_limb_t underflow;
mpi_limb_t overflow;
overflow = _gcry_mpih_lshift (rp, rp, usize, 1);
rp[0] |= the_bit;
underflow = _gcry_mpih_sub_n (rp, rp, up, usize);
mpih_add_n_cond (rp, rp, up, usize, overflow ^ underflow);
}
return rp;
}
int
_gcry_mpih_cmp_ui (mpi_ptr_t up, mpi_size_t usize, unsigned long v)
{
int is_all_zero = 1;
mpi_size_t i;
for (i = 1; i < usize; i++)
is_all_zero &= (up[i] == 0);
if (is_all_zero)
{
if (up[0] < v)
return -1;
else if (up[0] > v)
return 1;
else
return 0;
}
return 1;
}
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