forked from ibphoenix/tomsfastmath
50f587c36f
Currently, the fp_sqr_comba_* functions do not fully clear the destination
number, but only overwrites the digits they care about. Eg: if
you call a comba4, it will overwrite the first 8 digits and leave
the others unchanged.
On the other hand, fp_mul_comba_* functions do *not* check incoming
unused digits (relying on the guarantee that they must be zero),
so they will happily compute the wrong result if those digits
are not empty. Testcase for a 32-bit system:
char buf[64];
fp_int num, num2, d;
memset(buf, 0xFF, sizeof(buf);
fp_read_unsigned_bin(&num, buf);
fp_set(&d, 1);
fp_sqr_comba_3(&d, &num);
// now num is { 0x1, 0x0, 0x0, 0x0, 0x0, 0x0,
// 0xFFFFFFFF, 0xFFFFFFFF ... }
// only first 6 digits have been written, but even
// if num.used is correctly set to 6, this can trigger
// bugs.
// Create a number larger than 6 digits
fp_2expt(&num2, 8*32+4);
fp_mul_comba_8(&num, &num2, &num2);
// wrong result has been computed, because the first 8
// digits of num have been read and multiplied
// even if num->used == 6, relying on the fact that
// they should be zero.
111 lines
2.5 KiB
C
111 lines
2.5 KiB
C
/* TomsFastMath, a fast ISO C bignum library.
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*
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* This project is meant to fill in where LibTomMath
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* falls short. That is speed ;-)
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*
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* This project is public domain and free for all purposes.
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*
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* Tom St Denis, tomstdenis@gmail.com
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*/
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/* Generates squaring comba code... it learns it knows our secrets! */
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#include <stdio.h>
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int main(int argc, char **argv)
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{
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int x, y, z, N, f;
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printf(
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"void fp_sqr_comba_small(fp_int *A, fp_int *B)\n"
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"{\n"
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" fp_digit *a, b[32], c0, c1, c2, sc0, sc1, sc2;\n"
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);
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printf(" switch (A->used) { \n");
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for (N = 1; N <= 16; N++) {
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printf(
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" case %d:\n"
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" a = A->dp;\n"
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" COMBA_START; \n"
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"\n"
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" /* clear carries */\n"
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" CLEAR_CARRY;\n"
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"\n"
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" /* output 0 */\n"
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" SQRADD(a[0],a[0]);\n"
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" COMBA_STORE(b[0]);\n", N);
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for (x = 1; x < N+N-1; x++) {
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printf(
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"\n /* output %d */\n"
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" CARRY_FORWARD;\n ", x);
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for (f = y = 0; y < N; y++) {
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for (z = 0; z < N; z++) {
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if (z != y && z + y == x && y <= z) {
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++f;
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}
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}
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}
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if (f <= 2) {
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for (y = 0; y < N; y++) {
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for (z = 0; z < N; z++) {
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if (y<=z && (y+z)==x) {
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if (y == z) {
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printf(" SQRADD(a[%d], a[%d]); ", y, y);
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} else {
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printf(" SQRADD2(a[%d], a[%d]); ", y, z);
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}
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}
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}
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}
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} else {
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// new method
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/* do evens first */
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f = 0;
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for (y = 0; y < N; y++) {
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for (z = 0; z < N; z++) {
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if (z != y && z + y == x && y <= z) {
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if (f == 0) {
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// first double
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printf("SQRADDSC(a[%d], a[%d]); ", y, z);
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f = 1;
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} else {
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printf("SQRADDAC(a[%d], a[%d]); ", y, z);
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}
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}
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}
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}
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// forward the carry
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printf("SQRADDDB; ");
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if ((x&1) == 0) {
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// add the square
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printf("SQRADD(a[%d], a[%d]); ", x/2, x/2);
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}
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}
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printf("\n COMBA_STORE(b[%d]);\n", x);
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}
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printf(" COMBA_STORE2(b[%d]);\n", N+N-1);
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printf(
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" COMBA_FINI;\n"
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"\n"
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" B->used = %d;\n"
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" B->sign = FP_ZPOS;\n"
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" memcpy(B->dp, b, %d * sizeof(fp_digit));\n"
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" memset(B->dp + %d, 0, (FP_SIZE - %d) * sizeof(fp_digit));\n"
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" fp_clamp(B);\n"
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" break;\n\n", N+N, N+N, N+N, N+N);
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}
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printf("}\n\n}\n");
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return 0;
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}
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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