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https://github.com/cryb-to/cryb-to.git
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a09cdc6318
The count we passed to memcmp() in mpi_eq() and mpi_eq_abs() was actually the number of significant words in the MPI, rather than the number of bytes we wanted to compare. Multiply by 4 to get the correct value.
To make the intent of the code more apparent, introduce a private MPI_MSW() macro which evaluates to the number of significant words (or 1-based index of the most significant word). This also comes in handy in mpi_{add,sub,mul}_abs().
Add a couple of test cases which not only demonstrate the bug we fixed here but also demonstrate why we must compare whole words: on a big-endian machine, we would be comparing the unused upper bytes of the first and only word instead of the lower bytes which actually hold a value...
104 lines
3.2 KiB
C
104 lines
3.2 KiB
C
/*
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* Copyright (c) 2017 Dag-Erling Smørgrav
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. The name of the author may not be used to endorse or promote
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* products derived from this software without specific prior written
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* permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include "cryb/impl.h"
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#include <stddef.h>
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#include <stdint.h>
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#include <strings.h>
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#include <cryb/bitwise.h>
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#include <cryb/mpi.h>
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#include "cryb_mpi_impl.h"
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/*
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* Store the product of the absolutes values of A and B in X.
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*
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* Note: this is loosely based on HAC 14.2.3. However, the first
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* paragraph speaks of x and y having n and t digits respectively, with
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* the most significant digits being x[n - 1] and y[t - 1], but the rest
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* of the section refers to x and y having n + 1 and t + 1 digits. We
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* follow the former convention, but use a and b instead of n and t to
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* distance ourselves from this confusing text.
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*/
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int
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mpi_mul_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
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{
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cryb_mpi *P, T = CRYB_MPI_ZERO;
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uint64_t p;
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unsigned int a, b, i, j;
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uint32_t c;
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/*
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* Trivial cases: A and / or B is zero.
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*/
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if (A->msb == 0 || B->msb == 0) {
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mpi_zero(X);
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return (0);
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}
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/*
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* Unlike addition or subtraction, we can't multiply in place, so
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* if the destination is identical with one of the operands, we
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* will have to use temporary storage. Either way, make sure we
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* have enough room for the product.
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*/
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P = (X == A || X == B) ? &T : X;
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mpi_zero(P);
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a = MPI_MSW(A);
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b = MPI_MSW(B);
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if (mpi_grow(P, (a + b) * 32) != 0)
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return (-1);
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/*
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* Multiply...
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*/
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for (i = 0; i < b; ++i) {
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for (c = j = 0; j < a; ++j) {
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p = 1ULL * A->words[j] * B->words[i] + c;
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P->words[i + j] += p & 0xffffffffUL;
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c = p >> 32;
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}
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P->words[i + j] = c;
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}
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if (P->words[i = a + b - 1] == 0)
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--i;
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P->msb = i * 32 + flsl(P->words[i]);
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/*
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* If we used temporary storage, copy back and clean up.
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*/
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if (P == &T) {
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cryb_mpi_swap(X, P);
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cryb_mpi_destroy(P);
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}
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return (0);
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}
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