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Optimization: track f,g limb count and pass to new variable-time update_fg_var

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The magnitude of the f and g variables generally goes down as the algorithm
progresses. Make use of this by keeping tracking how many limbs are used, and
when the number becomes small enough, make use of this to reduce the complexity
of arithmetic on them.

Refactored by: Pieter Wuille <pieter@wuille.net>
This commit is contained in:
Peter Dettman
2020-12-15 18:17:19 -08:00
committed by Pieter Wuille
parent b306935ac1
commit ebc1af700f
2 changed files with 207 additions and 109 deletions
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159
src/modinv32_impl.h
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@@ -24,25 +24,25 @@
static const secp256k1_modinv32_signed30 SECP256K1_SIGNED30_ONE = {{1}};
/* Compute a*factor and put it in r. All but the top limb in r will be in range [0,2^30). */
static void secp256k1_modinv32_mul_30(secp256k1_modinv32_signed30 *r, const secp256k1_modinv32_signed30 *a, int32_t factor) {
static void secp256k1_modinv32_mul_30(secp256k1_modinv32_signed30 *r, const secp256k1_modinv32_signed30 *a, int alen, int32_t factor) {
const int32_t M30 = (int32_t)(UINT32_MAX >> 2);
int64_t c = 0;
int i;
for (i = 0; i < 8; ++i) {
c += (int64_t)a->v[i] * factor;
if (i < alen) c += (int64_t)a->v[i] * factor;
r->v[i] = (int32_t)c & M30; c >>= 30;
}
c += (int64_t)a->v[8] * factor;
if (8 < alen) c += (int64_t)a->v[8] * factor;
VERIFY_CHECK(c == (int32_t)c);
r->v[8] = (int32_t)c;
}
/* Return -1 for a<b*factor, 0 for a==b*factor, 1 for a>b*factor. */
static int secp256k1_modinv32_mul_cmp_30(const secp256k1_modinv32_signed30 *a, const secp256k1_modinv32_signed30 *b, int32_t factor) {
/* Return -1 for a<b*factor, 0 for a==b*factor, 1 for a>b*factor. A consists of alen limbs; b has 9. */
static int secp256k1_modinv32_mul_cmp_30(const secp256k1_modinv32_signed30 *a, int alen, const secp256k1_modinv32_signed30 *b, int32_t factor) {
int i;
secp256k1_modinv32_signed30 am, bm;
secp256k1_modinv32_mul_30(&am, a, 1); /* Normalize all but the top limb of a. */
secp256k1_modinv32_mul_30(&bm, b, factor);
secp256k1_modinv32_mul_30(&am, a, alen, 1); /* Normalize all but the top limb of a. */
secp256k1_modinv32_mul_30(&bm, b, 9, factor);
for (i = 0; i < 8; ++i) {
/* Verify that all but the top limb of a and b are normalized. */
VERIFY_CHECK(am.v[i] >> 30 == 0);
@@ -73,8 +73,8 @@ static void secp256k1_modinv32_normalize_30(secp256k1_modinv32_signed30 *r, int3
VERIFY_CHECK(r->v[i] >= -M30);
VERIFY_CHECK(r->v[i] <= M30);
}
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, &modinfo->modulus, 1) < 0); /* r < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, 1) < 0); /* r < modulus */
#endif
/* In a first step, add the modulus if the input is negative, and then negate if requested.
@@ -154,8 +154,8 @@ static void secp256k1_modinv32_normalize_30(secp256k1_modinv32_signed30 *r, int3
VERIFY_CHECK(r6 >> 30 == 0);
VERIFY_CHECK(r7 >> 30 == 0);
VERIFY_CHECK(r8 >> 30 == 0);
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, &modinfo->modulus, 0) >= 0); /* r >= 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, &modinfo->modulus, 1) < 0); /* r < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, 0) >= 0); /* r >= 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(r, 9, &modinfo->modulus, 1) < 0); /* r < modulus */
#endif
}
@@ -331,10 +331,10 @@ static void secp256k1_modinv32_update_de_30(secp256k1_modinv32_signed30 *d, secp
int64_t cd, ce;
int i;
#ifdef VERIFY
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, &modinfo->modulus, 1) < 0); /* d < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, &modinfo->modulus, 1) < 0); /* e < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, 9, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, 9, &modinfo->modulus, 1) < 0); /* d < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, 1) < 0); /* e < modulus */
VERIFY_CHECK((labs(u) + labs(v)) >= 0); /* |u|+|v| doesn't overflow */
VERIFY_CHECK((labs(q) + labs(r)) >= 0); /* |q|+|r| doesn't overflow */
VERIFY_CHECK((labs(u) + labs(v)) <= M30 + 1); /* |u|+|v| <= 2^30 */
@@ -375,10 +375,10 @@ static void secp256k1_modinv32_update_de_30(secp256k1_modinv32_signed30 *d, secp
d->v[8] = (int32_t)cd;
e->v[8] = (int32_t)ce;
#ifdef VERIFY
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, &modinfo->modulus, 1) < 0); /* d < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, &modinfo->modulus, 1) < 0); /* e < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, 9, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(d, 9, &modinfo->modulus, 1) < 0); /* d < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(e, 9, &modinfo->modulus, 1) < 0); /* e < modulus */
#endif
}
@@ -415,6 +415,42 @@ static void secp256k1_modinv32_update_fg_30(secp256k1_modinv32_signed30 *f, secp
g->v[8] = (int32_t)cg;
}
/* Compute (t/2^30) * [f, g], where t is a transition matrix for 30 divsteps.
*
* Version that operates on a variable number of limbs in f and g.
*
* This implements the update_fg function from the explanation in modinv64_impl.h.
*/
static void secp256k1_modinv32_update_fg_30_var(int len, secp256k1_modinv32_signed30 *f, secp256k1_modinv32_signed30 *g, const secp256k1_modinv32_trans2x2 *t) {
const int32_t M30 = (int32_t)(UINT32_MAX >> 2);
const int32_t u = t->u, v = t->v, q = t->q, r = t->r;
int32_t fi, gi;
int64_t cf, cg;
int i;
VERIFY_CHECK(len > 0);
/* Start computing t*[f,g]. */
fi = f->v[0];
gi = g->v[0];
cf = (int64_t)u * fi + (int64_t)v * gi;
cg = (int64_t)q * fi + (int64_t)r * gi;
/* Verify that the bottom 62 bits of the result are zero, and then throw them away. */
VERIFY_CHECK(((int32_t)cf & M30) == 0); cf >>= 30;
VERIFY_CHECK(((int32_t)cg & M30) == 0); cg >>= 30;
/* Now iteratively compute limb i=1..len of t*[f,g], and store them in output limb i-1 (shifting
* down by 30 bits). */
for (i = 1; i < len; ++i) {
fi = f->v[i];
gi = g->v[i];
cf += (int64_t)u * fi + (int64_t)v * gi;
cg += (int64_t)q * fi + (int64_t)r * gi;
f->v[i - 1] = (int32_t)cf & M30; cf >>= 30;
g->v[i - 1] = (int32_t)cg & M30; cg >>= 30;
}
/* What remains is limb (len) of t*[f,g]; store it as output limb (len-1). */
f->v[len - 1] = (int32_t)cf;
g->v[len - 1] = (int32_t)cg;
}
/* Compute the inverse of x modulo modinfo->modulus, and replace x with it (constant time in x). */
static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo) {
/* Start with d=0, e=1, f=modulus, g=x, eta=-1. */
@@ -434,17 +470,17 @@ static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_m
secp256k1_modinv32_update_de_30(&d, &e, &t, modinfo);
/* Update f,g using that transition matrix. */
#ifdef VERIFY
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, 1) < 0); /* g < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &modinfo->modulus, 1) < 0); /* g < modulus */
#endif
secp256k1_modinv32_update_fg_30(&f, &g, &t);
#ifdef VERIFY
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, 1) < 0); /* g < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &modinfo->modulus, 1) < 0); /* g < modulus */
#endif
}
@@ -453,14 +489,14 @@ static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_m
* values i.e. +/- 1, and d now contains +/- the modular inverse. */
#ifdef VERIFY
/* g == 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &SECP256K1_SIGNED30_ONE, 0) == 0);
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &SECP256K1_SIGNED30_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) == 0)));
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, 9, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, -1) == 0)));
#endif
/* Optionally negate d, normalize to [0,modulus), and return it. */
@@ -478,9 +514,9 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256
#ifdef VERIFY
int i = 0;
#endif
int j;
int j, len = 9;
int32_t eta = -1;
int32_t cond;
int32_t cond, fn, gn;
/* Do iterations of 30 divsteps each until g=0. */
while (1) {
@@ -491,28 +527,41 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256
secp256k1_modinv32_update_de_30(&d, &e, &t, modinfo);
/* Update f,g using that transition matrix. */
#ifdef VERIFY
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, 1) < 0); /* g < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &modinfo->modulus, 1) < 0); /* g < modulus */
#endif
secp256k1_modinv32_update_fg_30(&f, &g, &t);
secp256k1_modinv32_update_fg_30_var(len, &f, &g, &t);
/* If the bottom limb of g is 0, there is a chance g=0. */
if (g.v[0] == 0) {
cond = 0;
/* Check if the other limbs are also 0. */
for (j = 1; j < 9; ++j) {
/* Check if all other limbs are also 0. */
for (j = 1; j < len; ++j) {
cond |= g.v[j];
}
/* If so, we're done. */
if (cond == 0) break;
}
/* Determine if len>1 and limb (len-1) of both f and g is 0 or -1. */
fn = f.v[len - 1];
gn = g.v[len - 1];
cond = ((int32_t)len - 2) >> 31;
cond |= fn ^ (fn >> 31);
cond |= gn ^ (gn >> 31);
/* If so, reduce length, propagating the sign of f and g's top limb into the one below. */
if (cond == 0) {
f.v[len - 2] |= (uint32_t)fn << 30;
g.v[len - 2] |= (uint32_t)gn << 30;
--len;
}
#ifdef VERIFY
VERIFY_CHECK(++i < 25); /* We should never need more than 25*30 = 750 divsteps */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &modinfo->modulus, 1) < 0); /* g < modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &modinfo->modulus, 1) < 0); /* g < modulus */
#endif
}
@@ -520,18 +569,18 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256
* the initial f, g values i.e. +/- 1, and d now contains +/- the modular inverse. */
#ifdef VERIFY
/* g == 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, &SECP256K1_SIGNED30_ONE, 0) == 0);
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &SECP256K1_SIGNED30_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, &modinfo->modulus, -1) == 0)));
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, len, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, -1) == 0)));
#endif
/* Optionally negate d, normalize to [0,modulus), and return it. */
secp256k1_modinv32_normalize_30(&d, f.v[8], modinfo);
secp256k1_modinv32_normalize_30(&d, f.v[len - 1], modinfo);
*x = d;
}