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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...
102 lines
2.9 KiB
C
102 lines
2.9 KiB
C
/*
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* Copyright (c) 2014-2017 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 absolute value of the difference of the absolute values of A
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* and B in X.
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*/
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int
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mpi_sub_abs(cryb_mpi *X, const cryb_mpi *A, const cryb_mpi *B)
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{
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const cryb_mpi *L, *G;
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unsigned int i;
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uint32_t c, cn;
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/*
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* Trivial cases: A and B are the same or equal or at least one of
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* them is zero.
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*/
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if (A == B || mpi_eq_abs(A, B)) {
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mpi_zero(X);
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return (0);
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}
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if (A->msb == 0) {
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if (X != B && mpi_copy(X, B) != 0)
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return (-1);
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X->neg = 0;
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return (0);
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}
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if (B->msb == 0) {
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if (X != A && mpi_copy(X, A) != 0)
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return (-1);
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X->neg = 0;
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return (0);
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}
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/* we want to subtract the smaller number from the larger */
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if (mpi_cmp_abs(A, B) < 0)
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L = A, G = B;
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else
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L = B, G = A;
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/* make sure X is large enough for the largest possible result */
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if (mpi_grow(X, G->msb) != 0)
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return (-1);
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/* subtract L from G word by word until we run out of L */
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for (c = i = 0; i < MPI_MSW(L); ++i) {
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cn = G->words[i] < c ||
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G->words[i] - c < L->words[i];
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X->words[i] = G->words[i] - L->words[i] - c;
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c = cn;
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}
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/* keep propagating carry */
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while (c) {
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cn = (G->words[i] < c);
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X->words[i] = G->words[i] - c;
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c = cn;
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++i;
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}
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while (X->words[i] == 0)
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--i;
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X->msb = i * 32 + flsl(X->words[i]);
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X->neg = 0;
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return (0);
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}
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