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// Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "internal.h"
#include "../../internal.h"
#include "../../limbs/limbs.h"
#include "../../limbs/limbs.inl"
OPENSSL_STATIC_ASSERT(BN_MONT_CTX_N0_LIMBS == 1 || BN_MONT_CTX_N0_LIMBS == 2,
"BN_MONT_CTX_N0_LIMBS value is invalid");
OPENSSL_STATIC_ASSERT(
sizeof(BN_ULONG) * BN_MONT_CTX_N0_LIMBS == sizeof(uint64_t),
"uint64_t is insufficient precision for n0");
int bn_from_montgomery_in_place(BN_ULONG r[], size_t num_r, BN_ULONG a[],
size_t num_a, const BN_ULONG n[],
size_t num_n,
const BN_ULONG n0_[BN_MONT_CTX_N0_LIMBS]) {
if (num_n == 0 || num_r != num_n || num_a != 2 * num_n) {
return 0;
}
// Add multiples of |n| to |r| until R = 2^(nl * BN_BITS2) divides it. On
// input, we had |r| < |n| * R, so now |r| < 2 * |n| * R. Note that |r|
// includes |carry| which is stored separately.
BN_ULONG n0 = n0_[0];
BN_ULONG carry = 0;
for (size_t i = 0; i < num_n; i++) {
BN_ULONG v = limbs_mul_add_limb(a + i, n, a[i] * n0, num_n);
v += carry + a[i + num_n];
carry |= (v != a[i + num_n]);
carry &= (v <= a[i + num_n]);
a[i + num_n] = v;
}
// Shift |num_n| words to divide by R. We have |a| < 2 * |n|. Note that |a|
// includes |carry| which is stored separately.
a += num_n;
// |a| thus requires at most one additional subtraction |n| to be reduced.
// Subtract |n| and select the answer in constant time.
BN_ULONG v = limbs_sub(r, a, n, num_n) - carry;
// |v| is one if |a| - |n| underflowed or zero if it did not. Note |v| cannot
// be -1. That would imply the subtraction did not fit in |num_n| words, and
// we know at most one subtraction is needed.
v = 0u - v;
for (size_t i = 0; i < num_n; i++) {
r[i] = constant_time_select_w(v, a[i], r[i]);
a[i] = 0;
}
return 1;
}
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