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src/secp256k1/src/modinv32_impl.h
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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. | |||||
*/ | |||||
/* 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; | |||||
/* 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; | |||||
} | |||||
/* 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 eta for 30 divsteps. | |||||
* | |||||
* 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 function from the explanation. | |||||
*/ | |||||
static int32_t secp256k1_modinv32_divsteps_30(int32_t eta, 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 (eta < 0) and for (g & 1). */ | |||||
c1 = eta >> 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 (eta < 0) and (g & 1). */ | |||||
c1 &= c2; | |||||
/* Conditionally negate eta, and unconditionally subtract 1. */ | |||||
eta = (eta ^ c1) - (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; | |||||
} | |||||
/* 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; | |||||
return eta; | |||||
} | |||||
/* 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)); | |||||
/* 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). */ | |||||
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; | |||||
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; | |||||
/* [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; | |||||
} | |||||
/* 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 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. */ | |||||
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 eta = -1; | |||||
/* Do 25 iterations of 30 divsteps each = 750 divsteps. 724 suffices for 256-bit inputs. */ | |||||
for (i = 0; i < 25; ++i) { | |||||
/* Compute transition matrix and new eta after 30 divsteps. */ | |||||
secp256k1_modinv32_trans2x2 t; | |||||
eta = secp256k1_modinv32_divsteps_30(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. */ | |||||
secp256k1_modinv32_update_fg_30(&f, &g, &t); | |||||
} | |||||
/* 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. */ | |||||
VERIFY_CHECK((g.v[0] | g.v[1] | g.v[2] | g.v[3] | g.v[4] | g.v[5] | g.v[6] | g.v[7] | g.v[8]) == 0); | |||||
/* 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; | |||||
int j; | |||||
int32_t eta = -1; | |||||
int32_t cond; | |||||
/* 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. */ | |||||
secp256k1_modinv32_update_fg_30(&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) { | |||||
cond |= g.v[j]; | |||||
} | |||||
/* If so, we're done. */ | |||||
if (cond == 0) break; | |||||
} | |||||
} | |||||
/* 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. */ | |||||
/* Optionally negate d, normalize to [0,modulus), and return it. */ | |||||
secp256k1_modinv32_normalize_30(&d, f.v[8], modinfo); | |||||
*x = d; | |||||
} | |||||
#endif /* SECP256K1_MODINV32_IMPL_H */ |