From e5c18892db69b5db44d282225ab4fea788af8035 Mon Sep 17 00:00:00 2001 From: Russell O'Connor Date: Fri, 26 Feb 2021 15:18:50 -0500 Subject: [PATCH] Eliminate the prej array from ecmult_strauss_wnaf. --- src/ecmult_const_impl.h | 7 ++-- src/ecmult_impl.h | 74 +++++++++++++++++++++++++---------------- src/group.h | 33 +++++++++++++----- src/group_impl.h | 15 ++++----- 4 files changed, 80 insertions(+), 49 deletions(-) diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h index 3d198dcedead8..12dbcc6c5b692 100644 --- a/src/ecmult_const_impl.h +++ b/src/ecmult_const_impl.h @@ -19,13 +19,10 @@ * It only operates on tables sized for WINDOW_A wnaf multiples. */ static void secp256k1_ecmult_odd_multiples_table_globalz_windowa(secp256k1_ge *pre, secp256k1_fe *globalz, const secp256k1_gej *a) { - secp256k1_gej prej[ECMULT_TABLE_SIZE(WINDOW_A)]; secp256k1_fe zr[ECMULT_TABLE_SIZE(WINDOW_A)]; - /* Compute the odd multiples in Jacobian form. */ - secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), prej, zr, a); - /* Bring them to the same Z denominator. */ - secp256k1_ge_globalz_set_table_gej(ECMULT_TABLE_SIZE(WINDOW_A), pre, globalz, prej, zr); + secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), pre, zr, globalz, a); + secp256k1_ge_table_set_globalz(ECMULT_TABLE_SIZE(WINDOW_A), pre, zr); } /* This is like `ECMULT_TABLE_GET_GE` but is constant time */ diff --git a/src/ecmult_impl.h b/src/ecmult_impl.h index 52af941fb1062..f3b581395660c 100644 --- a/src/ecmult_impl.h +++ b/src/ecmult_impl.h @@ -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); } diff --git a/src/group.h b/src/group.h index 0b55ad73c6d4a..bb7dae1cf781e 100644 --- a/src/group.h +++ b/src/group.h @@ -9,7 +9,10 @@ #include "field.h" -/** A group element of the secp256k1 curve, in affine coordinates. */ +/** A group element in affine coordinates on the secp256k1 curve, + * or occasionally on an isomorphic curve of the form y^2 = x^3 + 7*t^6. + * Note: For exhaustive test mode, secp256k1 is replaced by a small subgroup of a different curve. + */ typedef struct { secp256k1_fe x; secp256k1_fe y; @@ -19,7 +22,9 @@ typedef struct { #define SECP256K1_GE_CONST(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) {SECP256K1_FE_CONST((a),(b),(c),(d),(e),(f),(g),(h)), SECP256K1_FE_CONST((i),(j),(k),(l),(m),(n),(o),(p)), 0} #define SECP256K1_GE_CONST_INFINITY {SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 0), SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 0), 1} -/** A group element of the secp256k1 curve, in jacobian coordinates. */ +/** A group element of the secp256k1 curve, in jacobian coordinates. + * Note: For exhastive test mode, sepc256k1 is replaced by a small subgroup of a different curve. + */ typedef struct { secp256k1_fe x; /* actual X: x/z^2 */ secp256k1_fe y; /* actual Y: y/z^3 */ @@ -64,12 +69,24 @@ static void secp256k1_ge_set_gej_var(secp256k1_ge *r, secp256k1_gej *a); /** Set a batch of group elements equal to the inputs given in jacobian coordinates */ static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a, size_t len); -/** Bring a batch inputs given in jacobian coordinates (with known z-ratios) to - * the same global z "denominator". zr must contain the known z-ratios such - * that mul(a[i].z, zr[i+1]) == a[i+1].z. zr[0] is ignored. The x and y - * coordinates of the result are stored in r, the common z coordinate is - * stored in globalz. */ -static void secp256k1_ge_globalz_set_table_gej(size_t len, secp256k1_ge *r, secp256k1_fe *globalz, const secp256k1_gej *a, const secp256k1_fe *zr); +/** Bring a batch of inputs to the same global z "denominator", based on ratios between + * (omitted) z coordinates of adjacent elements. + * + * Although the elements a[i] are _ge rather than _gej, they actually represent elements + * in Jacobian coordinates with their z coordinates omitted. + * + * Using the notation z(b) to represent the omitted z coordinate of b, the array zr of + * z coordinate ratios must satisfy zr[i] == z(a[i]) / z(a[i-1]) for 0 < 'i' < len. + * The zr[0] value is unused. + * + * This function adjusts the coordinates of 'a' in place so that for all 'i', z(a[i]) == z(a[len-1]). + * In other words, the initial value of z(a[len-1]) becomes the global z "denominator". Only the + * a[i].x and a[i].y coordinates are explicitly modified; the adjustment of the omitted z coordinate is + * implicit. + * + * The coordinates of the final element a[len-1] are not changed. + */ +static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const secp256k1_fe *zr); /** Set a group element (affine) equal to the point at infinity. */ static void secp256k1_ge_set_infinity(secp256k1_ge *r); diff --git a/src/group_impl.h b/src/group_impl.h index 1bb859efbf7e4..1d5cfa2805940 100644 --- a/src/group_impl.h +++ b/src/group_impl.h @@ -161,27 +161,26 @@ static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a } } -static void secp256k1_ge_globalz_set_table_gej(size_t len, secp256k1_ge *r, secp256k1_fe *globalz, const secp256k1_gej *a, const secp256k1_fe *zr) { +static void secp256k1_ge_table_set_globalz(size_t len, secp256k1_ge *a, const secp256k1_fe *zr) { size_t i = len - 1; secp256k1_fe zs; if (len > 0) { - /* The z of the final point gives us the "global Z" for the table. */ - r[i].x = a[i].x; - r[i].y = a[i].y; /* Ensure all y values are in weak normal form for fast negation of points */ - secp256k1_fe_normalize_weak(&r[i].y); - *globalz = a[i].z; - r[i].infinity = 0; + secp256k1_fe_normalize_weak(&a[i].y); zs = zr[i]; /* Work our way backwards, using the z-ratios to scale the x/y values. */ while (i > 0) { + secp256k1_gej tmpa; if (i != len - 1) { secp256k1_fe_mul(&zs, &zs, &zr[i]); } i--; - secp256k1_ge_set_gej_zinv(&r[i], &a[i], &zs); + tmpa.x = a[i].x; + tmpa.y = a[i].y; + tmpa.infinity = 0; + secp256k1_ge_set_gej_zinv(&a[i], &tmpa, &zs); } } }