You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
 
 
 
 
 
 

232 lines
8.6 KiB

#include <sstream>
#include <algorithm>
#include "num.h"
#include "group.h"
#include "ecmult.h"
// optimal for 128-bit and 256-bit exponents
#define WINDOW_A 5
// larger numbers may result in slightly better performance, at the cost of
// exponentially larger precomputed tables. WINDOW_G == 13 results in 640 KiB.
#define WINDOW_G 14
extern "C" {
/** Fill a table 'pre' with precomputed odd multiples of a. W determines the size of the table.
* pre will contains the values [1*a,3*a,5*a,...,(2^(w-1)-1)*a], so it needs place for
* 2^(w-2) entries.
*
* There are two versions of this function:
* - secp256k1_ecmult_precomp_wnaf_gej, which operates on group elements in jacobian notation,
* fast to precompute, but slower to use in later additions.
* - secp256k1_ecmult_precomp_wnaf_ge, which operates on group elements in affine notations,
* (much) slower to precompute, but a bit faster to use in later additions.
* To compute a*P + b*G, we use the jacobian version for P, and the affine version for G, as
* G is constant, so it only needs to be done once in advance.
*/
void static secp256k1_ecmult_table_precomp_gej(secp256k1_gej_t *pre, const secp256k1_gej_t *a, int w) {
pre[0] = *a;
secp256k1_gej_t d; secp256k1_gej_double(&d, &pre[0]);
for (int i=1; i<(1 << (w-2)); i++)
secp256k1_gej_add(&pre[i], &d, &pre[i-1]);
}
void static secp256k1_ecmult_table_precomp_ge(secp256k1_ge_t *pre, const secp256k1_ge_t *a, int w) {
pre[0] = *a;
secp256k1_gej_t x; secp256k1_gej_set_ge(&x, a);
secp256k1_gej_t d; secp256k1_gej_double(&d, &x);
for (int i=1; i<(1 << (w-2)); i++) {
secp256k1_gej_add_ge(&x, &d, &pre[i-1]);
secp256k1_ge_set_gej(&pre[i], &x);
}
}
/** The number of entries a table with precomputed multiples needs to have. */
#define ECMULT_TABLE_SIZE(w) (1 << ((w)-2))
/** The following two macro retrieves a particular odd multiple from a table
* of precomputed multiples. */
#define ECMULT_TABLE_GET(r,pre,n,w,neg) do { \
assert(((n) & 1) == 1); \
assert((n) >= -((1 << ((w)-1)) - 1)); \
assert((n) <= ((1 << ((w)-1)) - 1)); \
if ((n) > 0) \
*(r) = (pre)[((n)-1)/2]; \
else \
(neg)((r), &(pre)[(-(n)-1)/2]); \
} while(0)
#define ECMULT_TABLE_GET_GEJ(r,pre,n,w) ECMULT_TABLE_GET((r),(pre),(n),(w),secp256k1_gej_neg)
#define ECMULT_TABLE_GET_GE(r,pre,n,w) ECMULT_TABLE_GET((r),(pre),(n),(w),secp256k1_ge_neg)
typedef struct {
secp256k1_ge_t pre_g[ECMULT_TABLE_SIZE(WINDOW_G)]; // odd multiples of the generator
secp256k1_ge_t pre_g_128[ECMULT_TABLE_SIZE(WINDOW_G)]; // odd multiples of 2^128*generator
secp256k1_ge_t prec[64][16]; // prec[j][i] = 16^j * (i+1) * G
secp256k1_ge_t fin; // -(sum(prec[j][0], j=0..63))
} secp256k1_ecmult_consts_t;
static secp256k1_ecmult_consts_t *secp256k1_ecmult_consts = NULL;
static void secp256k1_ecmult_start(void) {
if (secp256k1_ecmult_consts != NULL)
return;
secp256k1_ecmult_consts_t *ret = (secp256k1_ecmult_consts_t*)malloc(sizeof(secp256k1_ecmult_consts_t));
secp256k1_ecmult_consts = ret;
// get the generator
const secp256k1_ge_t *g = &secp256k1_ge_consts->g;
// calculate 2^128*generator
secp256k1_gej_t g_128j; secp256k1_gej_set_ge(&g_128j, g);
for (int i=0; i<128; i++)
secp256k1_gej_double(&g_128j, &g_128j);
secp256k1_ge_t g_128; secp256k1_ge_set_gej(&g_128, &g_128j);
// precompute the tables with odd multiples
secp256k1_ecmult_table_precomp_ge(ret->pre_g, g, WINDOW_G);
secp256k1_ecmult_table_precomp_ge(ret->pre_g_128, &g_128, WINDOW_G);
// compute prec and fin
secp256k1_gej_t gg; secp256k1_gej_set_ge(&gg, g);
secp256k1_ge_t ad = *g;
secp256k1_gej_t fn; secp256k1_gej_set_infinity(&fn);
for (int j=0; j<64; j++) {
secp256k1_ge_set_gej(&ret->prec[j][0], &gg);
secp256k1_gej_add(&fn, &fn, &gg);
for (int i=1; i<16; i++) {
secp256k1_gej_add_ge(&gg, &gg, &ad);
secp256k1_ge_set_gej(&ret->prec[j][i], &gg);
}
ad = ret->prec[j][15];
}
secp256k1_ge_set_gej(&ret->fin, &fn);
secp256k1_ge_neg(&ret->fin, &ret->fin);
}
static void secp256k1_ecmult_stop(void) {
if (secp256k1_ecmult_consts == NULL)
return;
free(secp256k1_ecmult_consts);
secp256k1_ecmult_consts = NULL;
}
/** Convert a number to WNAF notation. The number becomes represented by sum(2^i * wnaf[i], i=0..bits),
* with the following guarantees:
* - each wnaf[i] is either 0, or an odd integer between -(1<<(w-1) - 1) and (1<<(w-1) - 1)
* - two non-zero entries in wnaf are separated by at least w-1 zeroes.
* - the index of the highest non-zero entry in wnaf (=return value-1) is at most bits, where
* bits is the number of bits necessary to represent the absolute value of the input.
*/
static int secp256k1_ecmult_wnaf(int *wnaf, const secp256k1_num_t *a, int w) {
int ret = 0;
int zeroes = 0;
secp256k1_num_t x;
secp256k1_num_init(&x);
secp256k1_num_copy(&x, a);
int sign = 1;
if (secp256k1_num_is_neg(&x)) {
sign = -1;
secp256k1_num_negate(&x);
}
while (!secp256k1_num_is_zero(&x)) {
while (!secp256k1_num_is_odd(&x)) {
zeroes++;
secp256k1_num_shift(&x, 1);
}
int word = secp256k1_num_shift(&x, w);
while (zeroes) {
wnaf[ret++] = 0;
zeroes--;
}
if (word & (1 << (w-1))) {
secp256k1_num_inc(&x);
wnaf[ret++] = sign * (word - (1 << w));
} else {
wnaf[ret++] = sign * word;
}
zeroes = w-1;
}
secp256k1_num_free(&x);
return ret;
}
void static secp256k1_ecmult_gen(secp256k1_gej_t *r, const secp256k1_num_t *gn) {
secp256k1_num_t n;
secp256k1_num_init(&n);
secp256k1_num_copy(&n, gn);
const secp256k1_ecmult_consts_t *c = secp256k1_ecmult_consts;
secp256k1_gej_set_ge(r, &c->prec[0][secp256k1_num_shift(&n, 4)]);
for (int j=1; j<64; j++)
secp256k1_gej_add_ge(r, r, &c->prec[j][secp256k1_num_shift(&n, 4)]);
secp256k1_num_free(&n);
secp256k1_gej_add_ge(r, r, &c->fin);
}
void static secp256k1_ecmult(secp256k1_gej_t *r, const secp256k1_gej_t *a, const secp256k1_num_t *na, const secp256k1_num_t *ng) {
const secp256k1_ecmult_consts_t *c = secp256k1_ecmult_consts;
secp256k1_num_t na_1, na_lam;
secp256k1_num_t ng_1, ng_128;
secp256k1_num_init(&na_1);
secp256k1_num_init(&na_lam);
secp256k1_num_init(&ng_1);
secp256k1_num_init(&ng_128);
// split na into na_1 and na_lam (where na = na_1 + na_lam*lambda, and na_1 and na_lam are ~128 bit)
secp256k1_gej_split_exp(&na_1, &na_lam, na);
// split ng into ng_1 and ng_128 (where gn = gn_1 + gn_128*2^128, and gn_1 and gn_128 are ~128 bit)
secp256k1_num_split(&ng_1, &ng_128, ng, 128);
// build wnaf representation for na_1, na_lam, ng_1, ng_128
int wnaf_na_1[129]; int bits_na_1 = secp256k1_ecmult_wnaf(wnaf_na_1, &na_1, WINDOW_A);
int wnaf_na_lam[129]; int bits_na_lam = secp256k1_ecmult_wnaf(wnaf_na_lam, &na_lam, WINDOW_A);
int wnaf_ng_1[129]; int bits_ng_1 = secp256k1_ecmult_wnaf(wnaf_ng_1, &ng_1, WINDOW_G);
int wnaf_ng_128[129]; int bits_ng_128 = secp256k1_ecmult_wnaf(wnaf_ng_128, &ng_128, WINDOW_G);
// calculate a_lam = a*lambda
secp256k1_gej_t a_lam; secp256k1_gej_mul_lambda(&a_lam, a);
// calculate odd multiples of a and a_lam
secp256k1_gej_t pre_a_1[ECMULT_TABLE_SIZE(WINDOW_A)], pre_a_lam[ECMULT_TABLE_SIZE(WINDOW_A)];
secp256k1_ecmult_table_precomp_gej(pre_a_1, a, WINDOW_A);
secp256k1_ecmult_table_precomp_gej(pre_a_lam, &a_lam, WINDOW_A);
int bits = std::max(std::max(bits_na_1, bits_na_lam), std::max(bits_ng_1, bits_ng_128));
secp256k1_gej_set_infinity(r);
secp256k1_gej_t tmpj;
secp256k1_ge_t tmpa;
for (int i=bits-1; i>=0; i--) {
secp256k1_gej_double(r, r);
int n;
if (i < bits_na_1 && (n = wnaf_na_1[i])) {
ECMULT_TABLE_GET_GEJ(&tmpj, pre_a_1, n, WINDOW_A);
secp256k1_gej_add(r, r, &tmpj);
}
if (i < bits_na_lam && (n = wnaf_na_lam[i])) {
ECMULT_TABLE_GET_GEJ(&tmpj, pre_a_lam, n, WINDOW_A);
secp256k1_gej_add(r, r, &tmpj);
}
if (i < bits_ng_1 && (n = wnaf_ng_1[i])) {
ECMULT_TABLE_GET_GE(&tmpa, c->pre_g, n, WINDOW_G);
secp256k1_gej_add_ge(r, r, &tmpa);
}
if (i < bits_ng_128 && (n = wnaf_ng_128[i])) {
ECMULT_TABLE_GET_GE(&tmpa, c->pre_g_128, n, WINDOW_G);
secp256k1_gej_add_ge(r, r, &tmpa);
}
}
secp256k1_num_free(&na_1);
secp256k1_num_free(&na_lam);
secp256k1_num_free(&ng_1);
secp256k1_num_free(&ng_128);
}
}