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Handling of special cases in EC arithmetic.

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This commit is contained in:
Ondrej Mikle 2014-07-04 15:12:58 +02:00
parent 82ed3f31db
commit da6a09880d
2 changed files with 64 additions and 0 deletions
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60
ecdsa.c
View File

@ -33,12 +33,38 @@
#include "ecdsa.h"
#include "base58.h"
// Set cp2 = cp1
void point_copy(const curve_point *cp1, curve_point *cp2)
{
memcpy(&(cp2->x), &(cp1->x), sizeof(bignum256));
memcpy(&(cp2->y), &(cp1->y), sizeof(bignum256));
}
// cp2 = cp1 + cp2
void point_add(const curve_point *cp1, curve_point *cp2)
{
int i;
uint32_t temp;
bignum256 lambda, inv, xr, yr;
if (point_is_infinity(cp1)) {
return;
}
if (point_is_infinity(cp2)) {
point_copy(cp1, cp2);
return;
}
if (point_is_equal(cp1, cp2)) {
point_double(cp2);
return;
}
if (point_is_negative_of(cp1, cp2)) {
// set to point at infinity
bn_zero(&(cp2->x));
bn_zero(&(cp2->y));
return;
}
bn_substract(&(cp2->x), &(cp1->x), &inv);
bn_inverse(&inv, &prime256k1);
bn_substract(&(cp2->y), &(cp1->y), &lambda);
@ -68,6 +94,11 @@ void point_double(curve_point *cp)
int i;
uint32_t temp;
bignum256 lambda, inverse_y, xr, yr;
if (point_is_infinity(cp)) {
return;
}
memcpy(&inverse_y, &(cp->y), sizeof(bignum256));
bn_inverse(&inverse_y, &prime256k1);
memcpy(&lambda, &three_over_two256k1, sizeof(bignum256));
@ -120,6 +151,35 @@ void point_multiply(const bignum256 *k, const curve_point *p, curve_point *res)
bn_mod(&(res->y), &prime256k1);
}
// return true iff p represent point at infinity
// both coords are zero in internal representation
int point_is_infinity(const curve_point *p)
{
return bn_is_zero(&(p->x)) && bn_is_zero(&(p->y));
}
// return true iff both points are equal
int point_is_equal(const curve_point *p, const curve_point *q)
{
return bn_is_equal(&(p->x), &(q->x)) && bn_is_equal(&(p->y), &(q->y));
}
// returns true iff p == -q
int point_is_negative_of(const curve_point *p, const curve_point *q)
{
// if P == (x, y), then -P would be (x, -y) on this curve
bignum256 y_added;
if (!bn_is_equal(&(p->x), &(q->x))) {
return 0;
}
memcpy(&y_added, &(p->y), sizeof(bignum256));
bn_addmod(&y_added, &(q->y), &prime256k1);
return bn_is_zero(&y_added);
}
// res = k * G
void scalar_multiply(const bignum256 *k, curve_point *res)
{

4
ecdsa.h
View File

@ -32,9 +32,13 @@
#define USE_RFC6979 1
#endif
void point_copy(const curve_point *cp1, curve_point *cp2);
void point_add(const curve_point *cp1, curve_point *cp2);
void point_double(curve_point *cp);
void point_multiply(const bignum256 *k, const curve_point *p, curve_point *res);
int point_is_infinity(const curve_point *p);
int point_is_equal(const curve_point *p, const curve_point *q);
int point_is_negative_of(const curve_point *p, const curve_point *q);
void scalar_multiply(const bignum256 *k, curve_point *res);
void uncompress_coords(uint8_t odd, const bignum256 *x, bignum256 *y);