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muun-recovery/vendor/github.com/muun/libwallet/musig/modinv32_impl.h
Santiago Lezica 58d843ad79 Release v2.2.0
2021-11-12 19:11:09 -03:00

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/***********************************************************************
* Copyright (c) 2020 Peter Dettman *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
**********************************************************************/
#ifndef SECP256K1_MODINV32_IMPL_H
#define SECP256K1_MODINV32_IMPL_H
#include "modinv32.h"
#include "util.h"
#include <stdlib.h>
/* This file implements modular inversion based on the paper "Fast constant-time gcd computation and
* modular inversion" by Daniel J. Bernstein and Bo-Yin Yang.
*
* For an explanation of the algorithm, see doc/safegcd_implementation.md. This file contains an
* implementation for N=30, using 30-bit signed limbs represented as int32_t.
*/
#ifdef VERIFY
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, 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) {
if (i < alen) c += (int64_t)a->v[i] * factor;
r->v[i] = (int32_t)c & M30; c >>= 30;
}
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. 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, 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);
VERIFY_CHECK(bm.v[i] >> 30 == 0);
}
for (i = 8; i >= 0; --i) {
if (am.v[i] < bm.v[i]) return -1;
if (am.v[i] > bm.v[i]) return 1;
}
return 0;
}
#endif
/* Take as input a signed30 number in range (-2*modulus,modulus), and add a multiple of the modulus
* to it to bring it to range [0,modulus). If sign < 0, the input will also be negated in the
* process. The input must have limbs in range (-2^30,2^30). The output will have limbs in range
* [0,2^30). */
static void secp256k1_modinv32_normalize_30(secp256k1_modinv32_signed30 *r, int32_t sign, const secp256k1_modinv32_modinfo *modinfo) {
const int32_t M30 = (int32_t)(UINT32_MAX >> 2);
int32_t r0 = r->v[0], r1 = r->v[1], r2 = r->v[2], r3 = r->v[3], r4 = r->v[4],
r5 = r->v[5], r6 = r->v[6], r7 = r->v[7], r8 = r->v[8];
int32_t cond_add, cond_negate;
#ifdef VERIFY
/* Verify that all limbs are in range (-2^30,2^30). */
int i;
for (i = 0; i < 9; ++i) {
VERIFY_CHECK(r->v[i] >= -M30);
VERIFY_CHECK(r->v[i] <= M30);
}
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.
* This brings r from range (-2*modulus,modulus) to range (-modulus,modulus). As all input
* limbs are in range (-2^30,2^30), this cannot overflow an int32_t. Note that the right
* shifts below are signed sign-extending shifts (see assumptions.h for tests that that is
* indeed the behavior of the right shift operator). */
cond_add = r8 >> 31;
r0 += modinfo->modulus.v[0] & cond_add;
r1 += modinfo->modulus.v[1] & cond_add;
r2 += modinfo->modulus.v[2] & cond_add;
r3 += modinfo->modulus.v[3] & cond_add;
r4 += modinfo->modulus.v[4] & cond_add;
r5 += modinfo->modulus.v[5] & cond_add;
r6 += modinfo->modulus.v[6] & cond_add;
r7 += modinfo->modulus.v[7] & cond_add;
r8 += modinfo->modulus.v[8] & cond_add;
cond_negate = sign >> 31;
r0 = (r0 ^ cond_negate) - cond_negate;
r1 = (r1 ^ cond_negate) - cond_negate;
r2 = (r2 ^ cond_negate) - cond_negate;
r3 = (r3 ^ cond_negate) - cond_negate;
r4 = (r4 ^ cond_negate) - cond_negate;
r5 = (r5 ^ cond_negate) - cond_negate;
r6 = (r6 ^ cond_negate) - cond_negate;
r7 = (r7 ^ cond_negate) - cond_negate;
r8 = (r8 ^ cond_negate) - cond_negate;
/* Propagate the top bits, to bring limbs back to range (-2^30,2^30). */
r1 += r0 >> 30; r0 &= M30;
r2 += r1 >> 30; r1 &= M30;
r3 += r2 >> 30; r2 &= M30;
r4 += r3 >> 30; r3 &= M30;
r5 += r4 >> 30; r4 &= M30;
r6 += r5 >> 30; r5 &= M30;
r7 += r6 >> 30; r6 &= M30;
r8 += r7 >> 30; r7 &= M30;
/* In a second step add the modulus again if the result is still negative, bringing r to range
* [0,modulus). */
cond_add = r8 >> 31;
r0 += modinfo->modulus.v[0] & cond_add;
r1 += modinfo->modulus.v[1] & cond_add;
r2 += modinfo->modulus.v[2] & cond_add;
r3 += modinfo->modulus.v[3] & cond_add;
r4 += modinfo->modulus.v[4] & cond_add;
r5 += modinfo->modulus.v[5] & cond_add;
r6 += modinfo->modulus.v[6] & cond_add;
r7 += modinfo->modulus.v[7] & cond_add;
r8 += modinfo->modulus.v[8] & cond_add;
/* And propagate again. */
r1 += r0 >> 30; r0 &= M30;
r2 += r1 >> 30; r1 &= M30;
r3 += r2 >> 30; r2 &= M30;
r4 += r3 >> 30; r3 &= M30;
r5 += r4 >> 30; r4 &= M30;
r6 += r5 >> 30; r5 &= M30;
r7 += r6 >> 30; r6 &= M30;
r8 += r7 >> 30; r7 &= M30;
r->v[0] = r0;
r->v[1] = r1;
r->v[2] = r2;
r->v[3] = r3;
r->v[4] = r4;
r->v[5] = r5;
r->v[6] = r6;
r->v[7] = r7;
r->v[8] = r8;
#ifdef VERIFY
VERIFY_CHECK(r0 >> 30 == 0);
VERIFY_CHECK(r1 >> 30 == 0);
VERIFY_CHECK(r2 >> 30 == 0);
VERIFY_CHECK(r3 >> 30 == 0);
VERIFY_CHECK(r4 >> 30 == 0);
VERIFY_CHECK(r5 >> 30 == 0);
VERIFY_CHECK(r6 >> 30 == 0);
VERIFY_CHECK(r7 >> 30 == 0);
VERIFY_CHECK(r8 >> 30 == 0);
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
}
/* Data type for transition matrices (see section 3 of explanation).
*
* t = [ u v ]
* [ q r ]
*/
typedef struct {
int32_t u, v, q, r;
} secp256k1_modinv32_trans2x2;
/* Compute the transition matrix and zeta for 30 divsteps.
*
* Input: zeta: initial zeta
* f0: bottom limb of initial f
* g0: bottom limb of initial g
* Output: t: transition matrix
* Return: final zeta
*
* Implements the divsteps_n_matrix function from the explanation.
*/
static int32_t secp256k1_modinv32_divsteps_30(int32_t zeta, uint32_t f0, uint32_t g0, secp256k1_modinv32_trans2x2 *t) {
/* u,v,q,r are the elements of the transformation matrix being built up,
* starting with the identity matrix. Semantically they are signed integers
* in range [-2^30,2^30], but here represented as unsigned mod 2^32. This
* permits left shifting (which is UB for negative numbers). The range
* being inside [-2^31,2^31) means that casting to signed works correctly.
*/
uint32_t u = 1, v = 0, q = 0, r = 1;
uint32_t c1, c2, f = f0, g = g0, x, y, z;
int i;
for (i = 0; i < 30; ++i) {
VERIFY_CHECK((f & 1) == 1); /* f must always be odd */
VERIFY_CHECK((u * f0 + v * g0) == f << i);
VERIFY_CHECK((q * f0 + r * g0) == g << i);
/* Compute conditional masks for (zeta < 0) and for (g & 1). */
c1 = zeta >> 31;
c2 = -(g & 1);
/* Compute x,y,z, conditionally negated versions of f,u,v. */
x = (f ^ c1) - c1;
y = (u ^ c1) - c1;
z = (v ^ c1) - c1;
/* Conditionally add x,y,z to g,q,r. */
g += x & c2;
q += y & c2;
r += z & c2;
/* In what follows, c1 is a condition mask for (zeta < 0) and (g & 1). */
c1 &= c2;
/* Conditionally change zeta into -zeta-2 or zeta-1. */
zeta = (zeta ^ c1) - 1;
/* Conditionally add g,q,r to f,u,v. */
f += g & c1;
u += q & c1;
v += r & c1;
/* Shifts */
g >>= 1;
u <<= 1;
v <<= 1;
/* Bounds on zeta that follow from the bounds on iteration count (max 20*30 divsteps). */
VERIFY_CHECK(zeta >= -601 && zeta <= 601);
}
/* Return data in t and return value. */
t->u = (int32_t)u;
t->v = (int32_t)v;
t->q = (int32_t)q;
t->r = (int32_t)r;
/* The determinant of t must be a power of two. This guarantees that multiplication with t
* does not change the gcd of f and g, apart from adding a power-of-2 factor to it (which
* will be divided out again). As each divstep's individual matrix has determinant 2, the
* aggregate of 30 of them will have determinant 2^30. */
VERIFY_CHECK((int64_t)t->u * t->r - (int64_t)t->v * t->q == ((int64_t)1) << 30);
return zeta;
}
/* Compute the transition matrix and eta for 30 divsteps (variable time).
*
* Input: eta: initial eta
* f0: bottom limb of initial f
* g0: bottom limb of initial g
* Output: t: transition matrix
* Return: final eta
*
* Implements the divsteps_n_matrix_var function from the explanation.
*/
static int32_t secp256k1_modinv32_divsteps_30_var(int32_t eta, uint32_t f0, uint32_t g0, secp256k1_modinv32_trans2x2 *t) {
/* inv256[i] = -(2*i+1)^-1 (mod 256) */
static const uint8_t inv256[128] = {
0xFF, 0x55, 0x33, 0x49, 0xC7, 0x5D, 0x3B, 0x11, 0x0F, 0xE5, 0xC3, 0x59,
0xD7, 0xED, 0xCB, 0x21, 0x1F, 0x75, 0x53, 0x69, 0xE7, 0x7D, 0x5B, 0x31,
0x2F, 0x05, 0xE3, 0x79, 0xF7, 0x0D, 0xEB, 0x41, 0x3F, 0x95, 0x73, 0x89,
0x07, 0x9D, 0x7B, 0x51, 0x4F, 0x25, 0x03, 0x99, 0x17, 0x2D, 0x0B, 0x61,
0x5F, 0xB5, 0x93, 0xA9, 0x27, 0xBD, 0x9B, 0x71, 0x6F, 0x45, 0x23, 0xB9,
0x37, 0x4D, 0x2B, 0x81, 0x7F, 0xD5, 0xB3, 0xC9, 0x47, 0xDD, 0xBB, 0x91,
0x8F, 0x65, 0x43, 0xD9, 0x57, 0x6D, 0x4B, 0xA1, 0x9F, 0xF5, 0xD3, 0xE9,
0x67, 0xFD, 0xDB, 0xB1, 0xAF, 0x85, 0x63, 0xF9, 0x77, 0x8D, 0x6B, 0xC1,
0xBF, 0x15, 0xF3, 0x09, 0x87, 0x1D, 0xFB, 0xD1, 0xCF, 0xA5, 0x83, 0x19,
0x97, 0xAD, 0x8B, 0xE1, 0xDF, 0x35, 0x13, 0x29, 0xA7, 0x3D, 0x1B, 0xF1,
0xEF, 0xC5, 0xA3, 0x39, 0xB7, 0xCD, 0xAB, 0x01
};
/* Transformation matrix; see comments in secp256k1_modinv32_divsteps_30. */
uint32_t u = 1, v = 0, q = 0, r = 1;
uint32_t f = f0, g = g0, m;
uint16_t w;
int i = 30, limit, zeros;
for (;;) {
/* Use a sentinel bit to count zeros only up to i. */
zeros = secp256k1_ctz32_var(g | (UINT32_MAX << i));
/* Perform zeros divsteps at once; they all just divide g by two. */
g >>= zeros;
u <<= zeros;
v <<= zeros;
eta -= zeros;
i -= zeros;
/* We're done once we've done 30 divsteps. */
if (i == 0) break;
VERIFY_CHECK((f & 1) == 1);
VERIFY_CHECK((g & 1) == 1);
VERIFY_CHECK((u * f0 + v * g0) == f << (30 - i));
VERIFY_CHECK((q * f0 + r * g0) == g << (30 - i));
/* Bounds on eta that follow from the bounds on iteration count (max 25*30 divsteps). */
VERIFY_CHECK(eta >= -751 && eta <= 751);
/* If eta is negative, negate it and replace f,g with g,-f. */
if (eta < 0) {
uint32_t tmp;
eta = -eta;
tmp = f; f = g; g = -tmp;
tmp = u; u = q; q = -tmp;
tmp = v; v = r; r = -tmp;
}
/* eta is now >= 0. In what follows we're going to cancel out the bottom bits of g. No more
* than i can be cancelled out (as we'd be done before that point), and no more than eta+1
* can be done as its sign will flip once that happens. */
limit = ((int)eta + 1) > i ? i : ((int)eta + 1);
/* m is a mask for the bottom min(limit, 8) bits (our table only supports 8 bits). */
VERIFY_CHECK(limit > 0 && limit <= 30);
m = (UINT32_MAX >> (32 - limit)) & 255U;
/* Find what multiple of f must be added to g to cancel its bottom min(limit, 8) bits. */
w = (g * inv256[(f >> 1) & 127]) & m;
/* Do so. */
g += f * w;
q += u * w;
r += v * w;
VERIFY_CHECK((g & m) == 0);
}
/* Return data in t and return value. */
t->u = (int32_t)u;
t->v = (int32_t)v;
t->q = (int32_t)q;
t->r = (int32_t)r;
/* The determinant of t must be a power of two. This guarantees that multiplication with t
* does not change the gcd of f and g, apart from adding a power-of-2 factor to it (which
* will be divided out again). As each divstep's individual matrix has determinant 2, the
* aggregate of 30 of them will have determinant 2^30. */
VERIFY_CHECK((int64_t)t->u * t->r - (int64_t)t->v * t->q == ((int64_t)1) << 30);
return eta;
}
/* Compute (t/2^30) * [d, e] mod modulus, where t is a transition matrix for 30 divsteps.
*
* On input and output, d and e are in range (-2*modulus,modulus). All output limbs will be in range
* (-2^30,2^30).
*
* This implements the update_de function from the explanation.
*/
static void secp256k1_modinv32_update_de_30(secp256k1_modinv32_signed30 *d, secp256k1_modinv32_signed30 *e, const secp256k1_modinv32_trans2x2 *t, const secp256k1_modinv32_modinfo* modinfo) {
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 di, ei, md, me, sd, se;
int64_t cd, ce;
int i;
#ifdef VERIFY
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 */
VERIFY_CHECK((labs(q) + labs(r)) <= M30 + 1); /* |q|+|r| <= 2^30 */
#endif
/* [md,me] start as zero; plus [u,q] if d is negative; plus [v,r] if e is negative. */
sd = d->v[8] >> 31;
se = e->v[8] >> 31;
md = (u & sd) + (v & se);
me = (q & sd) + (r & se);
/* Begin computing t*[d,e]. */
di = d->v[0];
ei = e->v[0];
cd = (int64_t)u * di + (int64_t)v * ei;
ce = (int64_t)q * di + (int64_t)r * ei;
/* Correct md,me so that t*[d,e]+modulus*[md,me] has 30 zero bottom bits. */
md -= (modinfo->modulus_inv30 * (uint32_t)cd + md) & M30;
me -= (modinfo->modulus_inv30 * (uint32_t)ce + me) & M30;
/* Update the beginning of computation for t*[d,e]+modulus*[md,me] now md,me are known. */
cd += (int64_t)modinfo->modulus.v[0] * md;
ce += (int64_t)modinfo->modulus.v[0] * me;
/* Verify that the low 30 bits of the computation are indeed zero, and then throw them away. */
VERIFY_CHECK(((int32_t)cd & M30) == 0); cd >>= 30;
VERIFY_CHECK(((int32_t)ce & M30) == 0); ce >>= 30;
/* Now iteratively compute limb i=1..8 of t*[d,e]+modulus*[md,me], and store them in output
* limb i-1 (shifting down by 30 bits). */
for (i = 1; i < 9; ++i) {
di = d->v[i];
ei = e->v[i];
cd += (int64_t)u * di + (int64_t)v * ei;
ce += (int64_t)q * di + (int64_t)r * ei;
cd += (int64_t)modinfo->modulus.v[i] * md;
ce += (int64_t)modinfo->modulus.v[i] * me;
d->v[i - 1] = (int32_t)cd & M30; cd >>= 30;
e->v[i - 1] = (int32_t)ce & M30; ce >>= 30;
}
/* What remains is limb 9 of t*[d,e]+modulus*[md,me]; store it as output limb 8. */
d->v[8] = (int32_t)cd;
e->v[8] = (int32_t)ce;
#ifdef VERIFY
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
}
/* Compute (t/2^30) * [f, g], where t is a transition matrix for 30 divsteps.
*
* This implements the update_fg function from the explanation.
*/
static void secp256k1_modinv32_update_fg_30(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;
/* 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 30 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..8 of t*[f,g], and store them in output limb i-1 (shifting
* down by 30 bits). */
for (i = 1; i < 9; ++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 9 of t*[f,g]; store it as output limb 8. */
f->v[8] = (int32_t)cf;
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, zeta=-1. */
secp256k1_modinv32_signed30 d = {{0}};
secp256k1_modinv32_signed30 e = {{1}};
secp256k1_modinv32_signed30 f = modinfo->modulus;
secp256k1_modinv32_signed30 g = *x;
int i;
int32_t zeta = -1; /* zeta = -(delta+1/2); delta is initially 1/2. */
/* Do 20 iterations of 30 divsteps each = 600 divsteps. 590 suffices for 256-bit inputs. */
for (i = 0; i < 20; ++i) {
/* Compute transition matrix and new zeta after 30 divsteps. */
secp256k1_modinv32_trans2x2 t;
zeta = secp256k1_modinv32_divsteps_30(zeta, f.v[0], g.v[0], &t);
/* Update d,e using that transition matrix. */
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, 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, 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
}
/* At this point sufficient iterations have been performed that g must have reached 0
* and (if g was not originally 0) f must now equal +/- GCD of 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, 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, 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. */
secp256k1_modinv32_normalize_30(&d, f.v[8], modinfo);
*x = d;
}
/* Compute the inverse of x modulo modinfo->modulus, and replace x with it (variable time). */
static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo) {
/* Start with d=0, e=1, f=modulus, g=x, eta=-1. */
secp256k1_modinv32_signed30 d = {{0, 0, 0, 0, 0, 0, 0, 0, 0}};
secp256k1_modinv32_signed30 e = {{1, 0, 0, 0, 0, 0, 0, 0, 0}};
secp256k1_modinv32_signed30 f = modinfo->modulus;
secp256k1_modinv32_signed30 g = *x;
#ifdef VERIFY
int i = 0;
#endif
int j, len = 9;
int32_t eta = -1; /* eta = -delta; delta is initially 1 (faster for the variable-time code) */
int32_t cond, fn, gn;
/* Do iterations of 30 divsteps each until g=0. */
while (1) {
/* Compute transition matrix and new eta after 30 divsteps. */
secp256k1_modinv32_trans2x2 t;
eta = secp256k1_modinv32_divsteps_30_var(eta, f.v[0], g.v[0], &t);
/* Update d,e using that transition matrix. */
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, 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_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 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, 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
}
/* At this point g is 0 and (if g was not originally 0) f must now equal +/- GCD of
* 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, 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, 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[len - 1], modinfo);
*x = d;
}
#endif /* SECP256K1_MODINV32_IMPL_H */
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