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96 lines
2.3 KiB
C
96 lines
2.3 KiB
C
#include "ge.h"
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static void slide(signed char *r,const unsigned char *a)
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{
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int i;
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int b;
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int k;
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for (i = 0;i < 256;++i)
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r[i] = 1 & (a[i >> 3] >> (i & 7));
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for (i = 0;i < 256;++i)
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if (r[i]) {
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for (b = 1;b <= 6 && i + b < 256;++b) {
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if (r[i + b]) {
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if (r[i] + (r[i + b] << b) <= 15) {
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r[i] += r[i + b] << b; r[i + b] = 0;
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} else if (r[i] - (r[i + b] << b) >= -15) {
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r[i] -= r[i + b] << b;
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for (k = i + b;k < 256;++k) {
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if (!r[k]) {
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r[k] = 1;
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break;
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}
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r[k] = 0;
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}
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} else
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break;
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}
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}
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}
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}
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static ge_precomp Bi[8] = {
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#include "base2.h"
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} ;
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/*
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r = a * A + b * B
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where a = a[0]+256*a[1]+...+256^31 a[31].
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and b = b[0]+256*b[1]+...+256^31 b[31].
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B is the Ed25519 base point (x,4/5) with x positive.
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*/
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void ge_double_scalarmult_vartime(ge_p2 *r,const unsigned char *a,const ge_p3 *A,const unsigned char *b)
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{
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signed char aslide[256];
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signed char bslide[256];
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ge_cached Ai[8]; /* A,3A,5A,7A,9A,11A,13A,15A */
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ge_p1p1 t;
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ge_p3 u;
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ge_p3 A2;
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int i;
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slide(aslide,a);
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slide(bslide,b);
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ge_p3_to_cached(&Ai[0],A);
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ge_p3_dbl(&t,A); ge_p1p1_to_p3(&A2,&t);
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ge_add(&t,&A2,&Ai[0]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[1],&u);
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ge_add(&t,&A2,&Ai[1]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[2],&u);
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ge_add(&t,&A2,&Ai[2]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[3],&u);
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ge_add(&t,&A2,&Ai[3]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[4],&u);
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ge_add(&t,&A2,&Ai[4]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[5],&u);
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ge_add(&t,&A2,&Ai[5]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[6],&u);
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ge_add(&t,&A2,&Ai[6]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[7],&u);
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ge_p2_0(r);
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for (i = 255;i >= 0;--i) {
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if (aslide[i] || bslide[i]) break;
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}
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for (;i >= 0;--i) {
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ge_p2_dbl(&t,r);
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if (aslide[i] > 0) {
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ge_p1p1_to_p3(&u,&t);
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ge_add(&t,&u,&Ai[aslide[i]/2]);
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} else if (aslide[i] < 0) {
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ge_p1p1_to_p3(&u,&t);
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ge_sub(&t,&u,&Ai[(-aslide[i])/2]);
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}
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if (bslide[i] > 0) {
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ge_p1p1_to_p3(&u,&t);
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ge_madd(&t,&u,&Bi[bslide[i]/2]);
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} else if (bslide[i] < 0) {
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ge_p1p1_to_p3(&u,&t);
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ge_msub(&t,&u,&Bi[(-bslide[i])/2]);
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}
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ge_p1p1_to_p2(r,&t);
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}
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}
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