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/* LibTomCrypt, modular cryptographic library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
#include "tomcrypt_private.h"
#ifdef LTC_MECC
/* http://crypto.stackexchange.com/questions/41468/point-at-infinity-for-jacobian-coordinates
* a point at infinity is any point (x,y,0) such that y^2 == x^3, except (0,0,0)
*/
int ltc_ecc_is_point_at_infinity(const ecc_point *P, void *modulus, int *retval)
{
int err;
void *x3, *y2;
/* trivial case */
if (!mp_iszero(P->z)) {
*retval = 0;
return CRYPT_OK;
}
/* point (0,0,0) is not at infinity */
if (mp_iszero(P->x) && mp_iszero(P->y)) {
*retval = 0;
return CRYPT_OK;
}
/* initialize */
if ((err = mp_init_multi(&x3, &y2, LTC_NULL)) != CRYPT_OK) goto done;
/* compute y^2 */
if ((err = mp_mulmod(P->y, P->y, modulus, y2)) != CRYPT_OK) goto cleanup;
/* compute x^3 */
if ((err = mp_mulmod(P->x, P->x, modulus, x3)) != CRYPT_OK) goto cleanup;
if ((err = mp_mulmod(P->x, x3, modulus, x3)) != CRYPT_OK) goto cleanup;
/* test y^2 == x^3 */
err = CRYPT_OK;
if ((mp_cmp(x3, y2) == LTC_MP_EQ) && !mp_iszero(y2)) {
*retval = 1;
} else {
*retval = 0;
}
cleanup:
mp_clear_multi(x3, y2, LTC_NULL);
done:
return err;
}
#endif
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