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/* $OpenBSD: bn_shift.c,v 1.21 2023/02/13 04:25:37 jsing Exp $ */
/*
* Copyright (c) 2022, 2023 Joel Sing <jsing@openbsd.org>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
#include <openssl/bn.h>
#include <openssl/err.h>
#include "bn_local.h"
static inline int
bn_lshift(BIGNUM *r, const BIGNUM *a, int n)
{
size_t count, shift_bits, shift_words;
size_t lshift, rshift;
ssize_t rstride;
BN_ULONG *dst, *src;
if (n < 0) {
BNerror(BN_R_INVALID_LENGTH);
return 0;
}
shift_bits = n;
/*
* Left bit shift, potentially across word boundaries.
*
* When shift is not an exact multiple of BN_BITS2, the bottom bits of
* the previous word need to be right shifted and combined with the left
* shifted bits using bitwise OR. If shift is an exact multiple of
* BN_BITS2, the source for the left and right shifts are the same
* and the shifts become zero bits (which is effectively a memmove).
*/
shift_words = shift_bits / BN_BITS2;
lshift = shift_bits % BN_BITS2;
rshift = (BN_BITS2 - lshift) % BN_BITS2;
rstride = 0 - (lshift + rshift) / BN_BITS2;
if (a->top < 1) {
BN_zero(r);
return 1;
}
count = a->top + shift_words + 1;
if (count < shift_words)
return 0;
if (!bn_wexpand(r, count))
return 0;
src = a->d + a->top - 1;
dst = r->d + a->top + shift_words;
/* Handle right shift for top most word. */
*dst = (*src >> rshift) & rstride;
dst--;
/* Handle left shift and right shift for remaining words. */
while (src > a->d) {
*dst = *src << lshift | src[rstride] >> rshift;
src--;
dst--;
}
*dst = *src << lshift;
/* Zero any additional words resulting from the left shift. */
while (dst > r->d) {
dst--;
*dst = 0;
}
r->top = count;
bn_correct_top(r);
BN_set_negative(r, a->neg);
return 1;
}
static inline int
bn_rshift(BIGNUM *r, const BIGNUM *a, int n)
{
size_t count, shift_bits, shift_words;
size_t lshift, rshift;
ssize_t lstride;
BN_ULONG *dst, *src;
size_t i;
if (n < 0) {
BNerror(BN_R_INVALID_LENGTH);
return 0;
}
shift_bits = n;
/*
* Right bit shift, potentially across word boundaries.
*
* When shift is not an exact multiple of BN_BITS2, the top bits of
* the next word need to be left shifted and combined with the right
* shifted bits using bitwise OR. If shift is an exact multiple of
* BN_BITS2, the source for the left and right shifts are the same
* and the shifts become zero (which is effectively a memmove).
*/
shift_words = shift_bits / BN_BITS2;
rshift = shift_bits % BN_BITS2;
lshift = (BN_BITS2 - rshift) % BN_BITS2;
lstride = (lshift + rshift) / BN_BITS2;
if (a->top <= shift_words) {
BN_zero(r);
return 1;
}
count = a->top - shift_words;
if (!bn_wexpand(r, count))
return 0;
src = a->d + shift_words;
dst = r->d;
for (i = 1; i < count; i++) {
*dst = src[lstride] << lshift | *src >> rshift;
src++;
dst++;
}
*dst = *src >> rshift;
r->top = count;
bn_correct_top(r);
BN_set_negative(r, a->neg);
return 1;
}
int
BN_lshift1(BIGNUM *r, const BIGNUM *a)
{
return bn_lshift(r, a, 1);
}
int
BN_lshift(BIGNUM *r, const BIGNUM *a, int n)
{
return bn_lshift(r, a, n);
}
int
BN_rshift1(BIGNUM *r, const BIGNUM *a)
{
return bn_rshift(r, a, 1);
}
int
BN_rshift(BIGNUM *r, const BIGNUM *a, int n)
{
return bn_rshift(r, a, n);
}
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