/* Computes scalar*point and stores the result in r.
* point can not equal r.
* Uses a modified algorithm 2P of
* Lopez, J. and Dahab, R. "Fast multiplication on elliptic curves over
* GF(2^m) without precomputation" (CHES '99, LNCS 1717).
*
* To protect against side-channel attack the function uses constant time swap,
* avoiding conditional branches.
*/
static int ec_GF2m_montgomery_point_multiply(const EC_GROUP *group, EC_POINT *r, const BIGNUM *scalar,
const EC_POINT *point, BN_CTX *ctx)
{
BIGNUM *x1, *x2, *z1, *z2;
int ret = 0, i;
BN_ULONG mask,word;
if (r == point)
{
ECerr(EC_F_EC_GF2M_MONTGOMERY_POINT_MULTIPLY, EC_R_INVALID_ARGUMENT);
return 0;
}
/* if result should be point at infinity */
if ((scalar == NULL) || BN_is_zero(scalar) || (point == NULL) ||
EC_POINT_is_at_infinity(group, point))
{
return EC_POINT_set_to_infinity(group, r);
}
/* only support affine coordinates */
if (!point->Z_is_one) return 0;
/* Since point_multiply is static we can guarantee that ctx != NULL. */
BN_CTX_start(ctx);
x1 = BN_CTX_get(ctx);
z1 = BN_CTX_get(ctx);
if (z1 == NULL) goto err;
x2 = &r->X;
z2 = &r->Y;
bn_wexpand(x1, group->field.top);
bn_wexpand(z1, group->field.top);
bn_wexpand(x2, group->field.top);
bn_wexpand(z2, group->field.top);
if (!BN_GF2m_mod_arr(x1, &point->X, group->poly)) goto err; /* x1 = x */
if (!BN_one(z1)) goto err; /* z1 = 1 */
if (!group->meth->field_sqr(group, z2, x1, ctx)) goto err; /* z2 = x1^2 = x^2 */
if (!group->meth->field_sqr(group, x2, z2, ctx)) goto err;
if (!BN_GF2m_add(x2, x2, &group->b)) goto err; /* x2 = x^4 + b */
/* find top most bit and go one past it */
i = scalar->top - 1;
mask = BN_TBIT;
word = scalar->d[i];
while (!(word & mask)) mask >>= 1;
mask >>= 1;
/* if top most bit was at word break, go to next word */
if (!mask)
{
i--;
mask = BN_TBIT;
}
for (; i >= 0; i--)
{
word = scalar->d[i];
while (mask)
{
BN_consttime_swap(word & mask, x1, x2, group->field.top);
BN_consttime_swap(word & mask, z1, z2, group->field.top);
if (!gf2m_Madd(group, &point->X, x2, z2, x1, z1, ctx)) goto err;
if (!gf2m_Mdouble(group, x1, z1, ctx)) goto err;
BN_consttime_swap(word & mask, x1, x2, group->field.top);
BN_consttime_swap(word & mask, z1, z2, group->field.top);
mask >>= 1;
}
mask = BN_TBIT;
}
/* convert out of "projective" coordinates */
i = gf2m_Mxy(group, &point->X, &point->Y, x1, z1, x2, z2, ctx);
if (i == 0) goto err;
else if (i == 1)
{
if (!EC_POINT_set_to_infinity(group, r)) goto err;
}
else
{
if (!BN_one(&r->Z)) goto err;
r->Z_is_one = 1;
}
/* GF(2^m) field elements should always have BIGNUM::neg = 0 */
BN_set_negative(&r->X, 0);
BN_set_negative(&r->Y, 0);
ret = 1;
err:
BN_CTX_end(ctx);
return ret;
}
/* Computes R = nP based on algorithm 2P of Lopex, J. and Dahab, R. "Fast
* multiplication on elliptic curves over GF(2^m) without
* precomputation". Elliptic curve points P and R can be identical. Uses
* Montgomery projective coordinates. */
mp_err
ec_GF2m_pt_mul_mont(const mp_int *n, const mp_int *px, const mp_int *py,
mp_int *rx, mp_int *ry, const ECGroup *group)
{
mp_err res = MP_OKAY;
mp_int x1, x2, z1, z2;
int i, j;
mp_digit top_bit, mask;
MP_DIGITS(&x1) = 0;
MP_DIGITS(&x2) = 0;
MP_DIGITS(&z1) = 0;
MP_DIGITS(&z2) = 0;
MP_CHECKOK(mp_init(&x1, FLAG(n)));
MP_CHECKOK(mp_init(&x2, FLAG(n)));
MP_CHECKOK(mp_init(&z1, FLAG(n)));
MP_CHECKOK(mp_init(&z2, FLAG(n)));
/* if result should be point at infinity */
if ((mp_cmp_z(n) == 0) || (ec_GF2m_pt_is_inf_aff(px, py) == MP_YES)) {
MP_CHECKOK(ec_GF2m_pt_set_inf_aff(rx, ry));
goto CLEANUP;
}
MP_CHECKOK(mp_copy(px, &x1)); /* x1 = px */
MP_CHECKOK(mp_set_int(&z1, 1)); /* z1 = 1 */
MP_CHECKOK(group->meth->field_sqr(&x1, &z2, group->meth)); /* z2 =
* x1^2 =
* px^2 */
MP_CHECKOK(group->meth->field_sqr(&z2, &x2, group->meth));
MP_CHECKOK(group->meth->field_add(&x2, &group->curveb, &x2, group->meth)); /* x2
* =
* px^4
* +
* b
*/
/* find top-most bit and go one past it */
i = MP_USED(n) - 1;
j = MP_DIGIT_BIT - 1;
top_bit = 1;
top_bit <<= MP_DIGIT_BIT - 1;
mask = top_bit;
while (!(MP_DIGITS(n)[i] & mask)) {
mask >>= 1;
j--;
}
mask >>= 1;
j--;
/* if top most bit was at word break, go to next word */
if (!mask) {
i--;
j = MP_DIGIT_BIT - 1;
mask = top_bit;
}
for (; i >= 0; i--) {
for (; j >= 0; j--) {
if (MP_DIGITS(n)[i] & mask) {
MP_CHECKOK(gf2m_Madd(px, &x1, &z1, &x2, &z2, group, FLAG(n)));
MP_CHECKOK(gf2m_Mdouble(&x2, &z2, group, FLAG(n)));
} else {
MP_CHECKOK(gf2m_Madd(px, &x2, &z2, &x1, &z1, group, FLAG(n)));
MP_CHECKOK(gf2m_Mdouble(&x1, &z1, group, FLAG(n)));
}
mask >>= 1;
}
j = MP_DIGIT_BIT - 1;
mask = top_bit;
}
/* convert out of "projective" coordinates */
i = gf2m_Mxy(px, py, &x1, &z1, &x2, &z2, group);
if (i == 0) {
res = MP_BADARG;
goto CLEANUP;
} else if (i == 1) {
MP_CHECKOK(ec_GF2m_pt_set_inf_aff(rx, ry));
} else {
MP_CHECKOK(mp_copy(&x2, rx));
MP_CHECKOK(mp_copy(&z2, ry));
}
CLEANUP:
mp_clear(&x1);
mp_clear(&x2);
mp_clear(&z1);
mp_clear(&z2);
return res;
}
