// Given q, t such that #E(F_q) = q - t + 1, compute #E(F_q^k).
void pbc_mpz_curve_order_extn(mpz_t res, mpz_t q, mpz_t t, int k) {
mpz_t z;
mpz_t tk;
mpz_init(z);
mpz_init(tk);
mpz_pow_ui(z, q, k);
mpz_add_ui(z, z, 1);
pbc_mpz_trace_n(tk, q, t, k);
mpz_sub(z, z, tk);
mpz_set(res, z);
mpz_clear(z);
mpz_clear(tk);
}
void pbc_param_init_f_gen(pbc_param_t p, int bits) {
f_init(p);
f_param_ptr fp = p->data;
//36 is a 6-bit number
int xbit = (bits - 6) / 4;
//TODO: use binary search to find smallest appropriate x
mpz_t x, t;
mpz_ptr q = fp->q;
mpz_ptr r = fp->r;
mpz_ptr b = fp->b;
field_t Fq, Fq2, Fq2x;
element_t e1;
element_t f;
field_t c;
element_t P;
mpz_init(x);
mpz_init(t);
mpz_setbit(x, xbit);
for (;;) {
mpz_mul(t, x, x);
mpz_mul_ui(t, t, 6);
mpz_add_ui(t, t, 1);
tryminusx(q, x);
mpz_sub(r, q, t);
mpz_add_ui(r, r, 1);
if (mpz_probab_prime_p(q, 10) && mpz_probab_prime_p(r, 10)) break;
tryplusx(q, x);
mpz_sub(r, q, t);
mpz_add_ui(r, r, 1);
if (mpz_probab_prime_p(q, 10) && mpz_probab_prime_p(r, 10)) break;
mpz_add_ui(x, x, 1);
}
field_init_fp(Fq, q);
element_init(e1, Fq);
for (;;) {
element_random(e1);
field_init_curve_b(c, e1, r, NULL);
element_init(P, c);
element_random(P);
element_mul_mpz(P, P, r);
if (element_is0(P)) break;
element_clear(P);
field_clear(c);
}
element_to_mpz(b, e1);
element_clear(e1);
field_init_quadratic(Fq2, Fq);
element_to_mpz(fp->beta, field_get_nqr(Fq));
field_init_poly(Fq2x, Fq2);
element_init(f, Fq2x);
// Find an irreducible polynomial of the form f = x^6 + alpha.
// Call poly_set_coeff1() first so we can use element_item() for the other
// coefficients.
poly_set_coeff1(f, 6);
for (;;) {
element_random(element_item(f, 0));
if (poly_is_irred(f)) break;
}
//extend F_q^2 using f = x^6 + alpha
//see if sextic twist contains a subgroup of order r
//if not, it's the wrong twist: replace alpha with alpha^5
{
field_t ctest;
element_t Ptest;
mpz_t z0, z1;
mpz_init(z0);
mpz_init(z1);
element_init(e1, Fq2);
element_set_mpz(e1, fp->b);
element_mul(e1, e1, element_item(f, 0));
element_neg(e1, e1);
field_init_curve_b(ctest, e1, r, NULL);
element_init(Ptest, ctest);
element_random(Ptest);
//I'm not sure what the #E'(F_q^2) is, but
//it definitely divides n_12 = #E(F_q^12). It contains a
//subgroup of order r if and only if
//(n_12 / r^2)P != O for some (in fact most) P in E'(F_q^6)
mpz_pow_ui(z0, q, 12);
mpz_add_ui(z0, z0, 1);
pbc_mpz_trace_n(z1, q, t, 12);
mpz_sub(z1, z0, z1);
mpz_mul(z0, r, r);
mpz_divexact(z1, z1, z0);
element_mul_mpz(Ptest, Ptest, z1);
if (element_is0(Ptest)) {
mpz_set_ui(z0, 5);
element_pow_mpz(element_item(f, 0), element_item(f, 0), z0);
}
element_clear(e1);
element_clear(Ptest);
field_clear(ctest);
mpz_clear(z0);
mpz_clear(z1);
}
element_to_mpz(fp->alpha0, element_x(element_item(f, 0)));
element_to_mpz(fp->alpha1, element_y(element_item(f, 0)));
element_clear(f);
field_clear(Fq2x);
field_clear(Fq2);
field_clear(Fq);
mpz_clear(t);
mpz_clear(x);
}
// Computes a curve and sets fp to the field it is defined over using the
// complex multiplication method, where cm holds the appropriate information
// (e.g. discriminant, field order).
static void compute_cm_curve(d_param_ptr param, pbc_cm_ptr cm) {
element_t hp, root;
field_t fp, fpx;
field_t cc;
field_init_fp(fp, cm->q);
field_init_poly(fpx, fp);
element_init(hp, fpx);
mpz_t *coefflist;
int n = (int)pbc_hilbert(&coefflist, cm->D);
// Temporarily set the coefficient of x^{n-1} to 1 so hp has degree n - 1,
// allowing us to use poly_coeff().
poly_set_coeff1(hp, n - 1);
int i;
for (i = 0; i < n; i++) {
element_set_mpz(element_item(hp, i), coefflist[i]);
}
pbc_hilbert_free(coefflist, n);
// TODO: Remove x = 0, 1728 roots.
// TODO: What if there are no roots?
//printf("hp ");
//element_out_str(stdout, 0, hp);
//printf("\n");
element_init(root, fp);
poly_findroot(root, hp);
//printf("root = ");
//element_out_str(stdout, 0, root);
//printf("\n");
element_clear(hp);
field_clear(fpx);
// The root is the j-invariant of the desired curve.
field_init_curve_j(cc, root, cm->n, NULL);
element_clear(root);
// We may need to twist it.
{
// Pick a random point P and twist the curve if it has the wrong order.
element_t P;
element_init(P, cc);
element_random(P);
element_mul_mpz(P, P, cm->n);
if (!element_is0(P)) field_reinit_curve_twist(cc);
element_clear(P);
}
mpz_set(param->q, cm->q);
mpz_set(param->n, cm->n);
mpz_set(param->h, cm->h);
mpz_set(param->r, cm->r);
element_to_mpz(param->a, curve_field_a_coeff(cc));
element_to_mpz(param->b, curve_field_b_coeff(cc));
param->k = cm->k;
{
mpz_t z;
mpz_init(z);
// Compute order of curve in F_q^k.
// n = q - t + 1 hence t = q - n + 1
mpz_sub(z, param->q, param->n);
mpz_add_ui(z, z, 1);
pbc_mpz_trace_n(z, param->q, z, param->k);
mpz_pow_ui(param->nk, param->q, param->k);
mpz_sub_ui(z, z, 1);
mpz_sub(param->nk, param->nk, z);
mpz_mul(z, param->r, param->r);
mpz_divexact(param->hk, param->nk, z);
mpz_clear(z);
}
field_clear(cc);
field_clear(fp);
}
