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openssl/crypto/bn/bn_mod.c

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/*
* Copyright 1998-2021 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the Apache License 2.0 (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
#include "internal/cryptlib.h"
#include "bn_local.h"
int BN_nnmod(BIGNUM *r, const BIGNUM *m, const BIGNUM *d, BN_CTX *ctx)
{
/*
* like BN_mod, but returns non-negative remainder (i.e., 0 <= r < |d|
* always holds)
*/
if (!(BN_mod(r, m, d, ctx)))
return 0;
if (!r->neg)
return 1;
/* now -|d| < r < 0, so we have to set r := r + |d| */
return (d->neg ? BN_sub : BN_add) (r, r, d);
}
int BN_mod_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
BN_CTX *ctx)
{
if (!BN_add(r, a, b))
return 0;
return BN_nnmod(r, r, m, ctx);
}
/*
* BN_mod_add variant that may be used if both a and b are non-negative and
* less than m. The original algorithm was
*
* if (!BN_uadd(r, a, b))
* return 0;
* if (BN_ucmp(r, m) >= 0)
* return BN_usub(r, r, m);
*
* which is replaced with addition, subtracting modulus, and conditional
* move depending on whether or not subtraction borrowed.
*/
int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
const BIGNUM *m)
{
size_t i, ai, bi, mtop = m->top;
BN_ULONG storage[1024 / BN_BITS2];
BN_ULONG carry, temp, mask, *rp, *tp = storage;
const BN_ULONG *ap, *bp;
if (bn_wexpand(r, mtop) == NULL)
return 0;
if (mtop > sizeof(storage) / sizeof(storage[0])) {
tp = OPENSSL_malloc(mtop * sizeof(BN_ULONG));
if (tp == NULL) {
ERR_raise(ERR_LIB_BN, ERR_R_MALLOC_FAILURE);
return 0;
}
}
ap = a->d != NULL ? a->d : tp;
bp = b->d != NULL ? b->d : tp;
for (i = 0, ai = 0, bi = 0, carry = 0; i < mtop;) {
mask = (BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1));
temp = ((ap[ai] & mask) + carry) & BN_MASK2;
carry = (temp < carry);
mask = (BN_ULONG)0 - ((i - b->top) >> (8 * sizeof(i) - 1));
tp[i] = ((bp[bi] & mask) + temp) & BN_MASK2;
carry += (tp[i] < temp);
i++;
ai += (i - a->dmax) >> (8 * sizeof(i) - 1);
bi += (i - b->dmax) >> (8 * sizeof(i) - 1);
}
rp = r->d;
carry -= bn_sub_words(rp, tp, m->d, mtop);
for (i = 0; i < mtop; i++) {
rp[i] = (carry & tp[i]) | (~carry & rp[i]);
((volatile BN_ULONG *)tp)[i] = 0;
}
r->top = mtop;
r->flags |= BN_FLG_FIXED_TOP;
r->neg = 0;
if (tp != storage)
OPENSSL_free(tp);
return 1;
}
int BN_mod_add_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
const BIGNUM *m)
{
int ret = bn_mod_add_fixed_top(r, a, b, m);
if (ret)
bn_correct_top(r);
return ret;
}
int BN_mod_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
BN_CTX *ctx)
{
if (!BN_sub(r, a, b))
return 0;
return BN_nnmod(r, r, m, ctx);
}
/*
* BN_mod_sub variant that may be used if both a and b are non-negative,
* a is less than m, while b is of same bit width as m. It's implemented
* as subtraction followed by two conditional additions.
*
* 0 <= a < m
* 0 <= b < 2^w < 2*m
*
* after subtraction
*
* -2*m < r = a - b < m
*
* Thus it takes up to two conditional additions to make |r| positive.
*/
int bn_mod_sub_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
const BIGNUM *m)
{
size_t i, ai, bi, mtop = m->top;
BN_ULONG borrow, carry, ta, tb, mask, *rp;
const BN_ULONG *ap, *bp;
if (bn_wexpand(r, mtop) == NULL)
return 0;
rp = r->d;
ap = a->d != NULL ? a->d : rp;
bp = b->d != NULL ? b->d : rp;
for (i = 0, ai = 0, bi = 0, borrow = 0; i < mtop;) {
mask = (BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1));
ta = ap[ai] & mask;
mask = (BN_ULONG)0 - ((i - b->top) >> (8 * sizeof(i) - 1));
tb = bp[bi] & mask;
rp[i] = ta - tb - borrow;
if (ta != tb)
borrow = (ta < tb);
i++;
ai += (i - a->dmax) >> (8 * sizeof(i) - 1);
bi += (i - b->dmax) >> (8 * sizeof(i) - 1);
}
ap = m->d;
for (i = 0, mask = 0 - borrow, carry = 0; i < mtop; i++) {
ta = ((ap[i] & mask) + carry) & BN_MASK2;
carry = (ta < carry);
rp[i] = (rp[i] + ta) & BN_MASK2;
carry += (rp[i] < ta);
}
borrow -= carry;
for (i = 0, mask = 0 - borrow, carry = 0; i < mtop; i++) {
ta = ((ap[i] & mask) + carry) & BN_MASK2;
carry = (ta < carry);
rp[i] = (rp[i] + ta) & BN_MASK2;
carry += (rp[i] < ta);
}
r->top = mtop;
r->flags |= BN_FLG_FIXED_TOP;
r->neg = 0;
return 1;
}
/*
* BN_mod_sub variant that may be used if both a and b are non-negative and
* less than m
*/
int BN_mod_sub_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
const BIGNUM *m)
{
if (!BN_sub(r, a, b))
return 0;
if (r->neg)
return BN_add(r, r, m);
return 1;
}
/* slow but works */
int BN_mod_mul(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
BN_CTX *ctx)
{
BIGNUM *t;
int ret = 0;
bn_check_top(a);
bn_check_top(b);
bn_check_top(m);
BN_CTX_start(ctx);
if ((t = BN_CTX_get(ctx)) == NULL)
goto err;
if (a == b) {
if (!BN_sqr(t, a, ctx))
goto err;
} else {
if (!BN_mul(t, a, b, ctx))
goto err;
}
if (!BN_nnmod(r, t, m, ctx))
goto err;
bn_check_top(r);
ret = 1;
err:
BN_CTX_end(ctx);
return ret;
}
int BN_mod_sqr(BIGNUM *r, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx)
{
if (!BN_sqr(r, a, ctx))
return 0;
/* r->neg == 0, thus we don't need BN_nnmod */
return BN_mod(r, r, m, ctx);
}
int BN_mod_lshift1(BIGNUM *r, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx)
{
if (!BN_lshift1(r, a))
return 0;
bn_check_top(r);
return BN_nnmod(r, r, m, ctx);
}
/*
* BN_mod_lshift1 variant that may be used if a is non-negative and less than
* m
*/
int BN_mod_lshift1_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *m)
{
if (!BN_lshift1(r, a))
return 0;
bn_check_top(r);
if (BN_cmp(r, m) >= 0)
return BN_sub(r, r, m);
return 1;
}
int BN_mod_lshift(BIGNUM *r, const BIGNUM *a, int n, const BIGNUM *m,
BN_CTX *ctx)
{
BIGNUM *abs_m = NULL;
int ret;
if (!BN_nnmod(r, a, m, ctx))
return 0;
if (m->neg) {
abs_m = BN_dup(m);
if (abs_m == NULL)
return 0;
abs_m->neg = 0;
}
ret = BN_mod_lshift_quick(r, r, n, (abs_m ? abs_m : m));
bn_check_top(r);
BN_free(abs_m);
return ret;
}
/*
* BN_mod_lshift variant that may be used if a is non-negative and less than
* m
*/
int BN_mod_lshift_quick(BIGNUM *r, const BIGNUM *a, int n, const BIGNUM *m)
{
if (r != a) {
if (BN_copy(r, a) == NULL)
return 0;
}
while (n > 0) {
int max_shift;
/* 0 < r < m */
max_shift = BN_num_bits(m) - BN_num_bits(r);
/* max_shift >= 0 */
if (max_shift < 0) {
ERR_raise(ERR_LIB_BN, BN_R_INPUT_NOT_REDUCED);
return 0;
}
if (max_shift > n)
max_shift = n;
if (max_shift) {
if (!BN_lshift(r, r, max_shift))
return 0;
n -= max_shift;
} else {
if (!BN_lshift1(r, r))
return 0;
--n;
}
/* BN_num_bits(r) <= BN_num_bits(m) */
if (BN_cmp(r, m) >= 0) {
if (!BN_sub(r, r, m))
return 0;
}
}
bn_check_top(r);
return 1;
}
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