static int set_rand(void *a, int size)
{
LTC_ARGCHK(a != NULL);
fp_rand(a, size);
return CRYPT_OK;
}
void fp3_rand(fp3_t a) {
fp_rand(a[0]);
fp_rand(a[1]);
fp_rand(a[2]);
}
void fp2_rand(fp2_t a) {
fp_rand(a[0]);
fp_rand(a[1]);
}
int fp_srt(fp_t c, const fp_t a) {
bn_t e;
fp_t t0;
fp_t t1;
int r = 0;
bn_null(e);
fp_null(t0);
fp_null(t1);
TRY {
bn_new(e);
fp_new(t0);
fp_new(t1);
/* Make e = p. */
e->used = FP_DIGS;
dv_copy(e->dp, fp_prime_get(), FP_DIGS);
if (fp_prime_get_mod8() == 3 || fp_prime_get_mod8() == 7) {
/* Easy case, compute a^((p + 1)/4). */
bn_add_dig(e, e, 1);
bn_rsh(e, e, 2);
fp_exp(t0, a, e);
fp_sqr(t1, t0);
r = (fp_cmp(t1, a) == CMP_EQ);
fp_copy(c, t0);
} else {
int f = 0, m = 0;
/* First, check if there is a root. Compute t1 = a^((p - 1)/2). */
bn_rsh(e, e, 1);
fp_exp(t0, a, e);
if (fp_cmp_dig(t0, 1) != CMP_EQ) {
/* Nope, there is no square root. */
r = 0;
} else {
r = 1;
/* Find a quadratic non-residue modulo p, that is a number t2
* such that (t2 | p) = t2^((p - 1)/2)!= 1. */
do {
fp_rand(t1);
fp_exp(t0, t1, e);
} while (fp_cmp_dig(t0, 1) == CMP_EQ);
/* Write p - 1 as (e * 2^f), odd e. */
bn_lsh(e, e, 1);
while (bn_is_even(e)) {
bn_rsh(e, e, 1);
f++;
}
/* Compute t2 = t2^e. */
fp_exp(t1, t1, e);
/* Compute t1 = a^e, c = a^((e + 1)/2) = a^(e/2 + 1), odd e. */
bn_rsh(e, e, 1);
fp_exp(t0, a, e);
fp_mul(e->dp, t0, a);
fp_sqr(t0, t0);
fp_mul(t0, t0, a);
fp_copy(c, e->dp);
while (1) {
if (fp_cmp_dig(t0, 1) == CMP_EQ) {
break;
}
fp_copy(e->dp, t0);
for (m = 0; (m < f) && (fp_cmp_dig(t0, 1) != CMP_EQ); m++) {
fp_sqr(t0, t0);
}
fp_copy(t0, e->dp);
for (int i = 0; i < f - m - 1; i++) {
fp_sqr(t1, t1);
}
fp_mul(c, c, t1);
fp_sqr(t1, t1);
fp_mul(t0, t0, t1);
f = m;
}
}
}
}
CATCH_ANY {
THROW(ERR_CAUGHT);
}
FINALLY {
bn_free(e);
fp_free(t0);
fp_free(t1);
}
return r;
}
