98 lines
2.5 KiB
C
98 lines
2.5 KiB
C
#include "fe25519.h"
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#include "sc25519.h"
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#include "ge25519.h"
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#define S1_SWINDOWSIZE 5
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#define PRE1_SIZE (1<<(S1_SWINDOWSIZE-2))
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#define S2_SWINDOWSIZE 7
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#define PRE2_SIZE (1<<(S2_SWINDOWSIZE-2))
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ge25519_niels pre2[PRE2_SIZE] = {
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#include "ge25519_base_slide_multiples.data"
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};
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static const fe25519 ec2d = {{1859910466990425, 932731440258426, 1072319116312658, 1815898335770999, 633789495995903}};
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static void setneutral(ge25519 *r)
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{
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fe25519_setint(&r->x,0);
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fe25519_setint(&r->y,1);
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fe25519_setint(&r->z,1);
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fe25519_setint(&r->t,0);
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}
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/* computes [s1]p1 + [s2]p2 */
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void ge25519_double_scalarmult_vartime(ge25519_p3 *r, const ge25519_p3 *p1, const sc25519 *s1, const sc25519 *s2)
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{
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signed char slide1[256], slide2[256];
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ge25519_pniels pre1[PRE1_SIZE], neg;
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ge25519_p3 d1;
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ge25519_p1p1 t;
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ge25519_niels nneg;
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fe25519 d;
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int i;
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sc25519_slide(slide1, s1, S1_SWINDOWSIZE);
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sc25519_slide(slide2, s2, S2_SWINDOWSIZE);
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/* precomputation */
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pre1[0] = *(ge25519_pniels *)p1;
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ge25519_dbl_p1p1(&t,(ge25519_p2 *)pre1); ge25519_p1p1_to_p3(&d1, &t);
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/* Convert pre[0] to projective Niels representation */
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d = pre1[0].ysubx;
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fe25519_sub(&pre1[0].ysubx, &pre1[0].xaddy, &pre1[0].ysubx);
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fe25519_add(&pre1[0].xaddy, &pre1[0].xaddy, &d);
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fe25519_mul(&pre1[0].t2d, &pre1[0].t2d, &ec2d);
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for(i=0;i<PRE1_SIZE-1;i++)
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{
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ge25519_pnielsadd_p1p1(&t, &d1, &pre1[i]); ge25519_p1p1_to_pniels(&pre1[i+1], &t);
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}
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setneutral(r);
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for (i = 255;i >= 0;--i) {
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if (slide1[i] || slide2[i]) goto firstbit;
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}
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for(;i>=0;i--)
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{
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firstbit:
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ge25519_dbl_p1p1(&t, (ge25519_p2 *)r);
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if(slide1[i]>0)
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{
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ge25519_p1p1_to_p3(r, &t);
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ge25519_pnielsadd_p1p1(&t, r, &pre1[slide1[i]/2]);
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}
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else if(slide1[i]<0)
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{
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ge25519_p1p1_to_p3(r, &t);
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neg = pre1[-slide1[i]/2];
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d = neg.ysubx;
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neg.ysubx = neg.xaddy;
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neg.xaddy = d;
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fe25519_neg(&neg.t2d, &neg.t2d);
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ge25519_pnielsadd_p1p1(&t, r, &neg);
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}
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if(slide2[i]>0)
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{
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ge25519_p1p1_to_p3(r, &t);
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ge25519_nielsadd_p1p1(&t, r, &pre2[slide2[i]/2]);
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}
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else if(slide2[i]<0)
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{
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ge25519_p1p1_to_p3(r, &t);
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nneg = pre2[-slide2[i]/2];
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d = nneg.ysubx;
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nneg.ysubx = nneg.xaddy;
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nneg.xaddy = d;
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fe25519_neg(&nneg.t2d, &nneg.t2d);
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ge25519_nielsadd_p1p1(&t, r, &nneg);
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}
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ge25519_p1p1_to_p2((ge25519_p2 *)r, &t);
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}
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}
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