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Eliminate the prej array from ecmult_strauss_wnaf.

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This commit is contained in:
Russell O'Connor
2021-02-26 15:18:50 -05:00
parent c9da1baad1
commit e5c18892db
4 changed files with 80 additions and 49 deletions
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74
src/ecmult_impl.h
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@@ -56,14 +56,23 @@
#define ECMULT_MAX_POINTS_PER_BATCH 5000000
/** Fill a table 'prej' with precomputed odd multiples of a. Prej will contain
* the values [1*a,3*a,...,(2*n-1)*a], so it space for n values. zr[0] will
* contain prej[0].z / a.z. The other zr[i] values = prej[i].z / prej[i-1].z.
* Prej's Z values are undefined, except for the last value.
/** Fill a table 'pre_a' with precomputed odd multiples of a.
* pre_a will contain [1*a,3*a,...,(2*n-1)*a], so it needs space for n group elements.
* zr needs space for n field elements.
*
* Although pre_a is an array of _ge rather than _gej, it actually represents elements
* in Jacobian coordinates with their z coordinates omitted. The omitted z-coordinates
* can be recovered using z and zr. Using the notation z(b) to represent the omitted
* z coordinate of b:
* - z(pre_a[n-1]) = 'z'
* - z(pre_a[i-1]) = z(pre_a[i]) / zr[i] for n > i > 0
*
* Lastly the zr[0] value, which isn't used above, is set so that:
* - a.z = z(pre_a[0]) / zr[0]
*/
static void secp256k1_ecmult_odd_multiples_table(int n, secp256k1_gej *prej, secp256k1_fe *zr, const secp256k1_gej *a) {
secp256k1_gej d;
secp256k1_ge a_ge, d_ge;
static void secp256k1_ecmult_odd_multiples_table(int n, secp256k1_ge *pre_a, secp256k1_fe *zr, secp256k1_fe *z, const secp256k1_gej *a) {
secp256k1_gej d, ai;
secp256k1_ge d_ge;
int i;
VERIFY_CHECK(!a->infinity);
@@ -71,29 +80,38 @@ static void secp256k1_ecmult_odd_multiples_table(int n, secp256k1_gej *prej, sec
secp256k1_gej_double_var(&d, a, NULL);
/*
* Perform the additions on an isomorphism where 'd' is affine: drop the z coordinate
* of 'd', and scale the 1P starting value's x/y coordinates without changing its z.
* Perform the additions using an isomorphic curve Y^2 = X^3 + 7*C^6 where C := d.z.
* The isomorphism, phi, maps a secp256k1 point (x, y) to the point (x*C^2, y*C^3) on the other curve.
* In Jacobian coordinates phi maps (x, y, z) to (x*C^2, y*C^3, z) or, equivalently to (x, y, z/C).
*
* phi(x, y, z) = (x*C^2, y*C^3, z) = (x, y, z/C)
* d_ge := phi(d) = (d.x, d.y, 1)
* ai := phi(a) = (a.x*C^2, a.y*C^3, a.z)
*
* The group addition functions work correctly on these isomorphic curves.
* In particular phi(d) is easy to represent in affine coordinates under this isomorphism.
* This lets us use the faster secp256k1_gej_add_ge_var group addition function that we wouldn't be able to use otherwise.
*/
d_ge.x = d.x;
d_ge.y = d.y;
d_ge.infinity = 0;
secp256k1_ge_set_gej_zinv(&a_ge, a, &d.z);
prej[0].x = a_ge.x;
prej[0].y = a_ge.y;
prej[0].z = a->z;
prej[0].infinity = 0;
secp256k1_ge_set_xy(&d_ge, &d.x, &d.y);
secp256k1_ge_set_gej_zinv(&pre_a[0], a, &d.z);
secp256k1_gej_set_ge(&ai, &pre_a[0]);
ai.z = a->z;
/* pre_a[0] is the point (a.x*C^2, a.y*C^3, a.z*C) which is equvalent to a.
* Set zr[0] to C, which is the ratio between the omitted z(pre_a[0]) value and a.z.
*/
zr[0] = d.z;
for (i = 1; i < n; i++) {
secp256k1_gej_add_ge_var(&prej[i], &prej[i-1], &d_ge, &zr[i]);
secp256k1_gej_add_ge_var(&ai, &ai, &d_ge, &zr[i]);
secp256k1_ge_set_xy(&pre_a[i], &ai.x, &ai.y);
}
/*
* Each point in 'prej' has a z coordinate too small by a factor of 'd.z'. Only
* the final point's z coordinate is actually used though, so just update that.
/* Multiply the last z-coordinate by C to undo the isomorphism.
* Since the z-coordinates of the pre_a values are implied by the zr array of z-coordinate ratios,
* undoing the isomorphism here undoes the isomorphism for all pre_a values.
*/
secp256k1_fe_mul(&prej[n-1].z, &prej[n-1].z, &d.z);
secp256k1_fe_mul(z, &ai.z, &d.z);
}
/** The following two macro retrieves a particular odd multiple from a table
@@ -246,18 +264,18 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
*/
if (no > 0) {
/* Compute the odd multiples in Jacobian form. */
secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), state->prej, state->zr, &a[state->ps[0].input_pos]);
secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), state->pre_a, state->zr, &Z, &a[state->ps[0].input_pos]);
for (np = 1; np < no; ++np) {
secp256k1_gej tmp = a[state->ps[np].input_pos];
#ifdef VERIFY
secp256k1_fe_normalize_var(&(state->prej[(np - 1) * ECMULT_TABLE_SIZE(WINDOW_A) + ECMULT_TABLE_SIZE(WINDOW_A) - 1].z));
secp256k1_fe_normalize_var(&Z);
#endif
secp256k1_gej_rescale(&tmp, &(state->prej[(np - 1) * ECMULT_TABLE_SIZE(WINDOW_A) + ECMULT_TABLE_SIZE(WINDOW_A) - 1].z));
secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), state->prej + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &tmp);
secp256k1_gej_rescale(&tmp, &Z);
secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), state->pre_a + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &Z, &tmp);
secp256k1_fe_mul(state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &(a[state->ps[np].input_pos].z));
}
/* Bring them to the same Z denominator. */
secp256k1_ge_globalz_set_table_gej(ECMULT_TABLE_SIZE(WINDOW_A) * no, state->pre_a, &Z, state->prej, state->zr);
secp256k1_ge_table_set_globalz(ECMULT_TABLE_SIZE(WINDOW_A) * no, state->pre_a, state->zr);
} else {
secp256k1_fe_set_int(&Z, 1);
}