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modinv64_impl.h
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1 /***********************************************************************
2  * Copyright (c) 2020 Peter Dettman *
3  * Distributed under the MIT software license, see the accompanying *
4  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
5  **********************************************************************/
6 
7 #ifndef SECP256K1_MODINV64_IMPL_H
8 #define SECP256K1_MODINV64_IMPL_H
9 
10 #include "modinv64.h"
11 
12 #include "util.h"
13 
14 /* This file implements modular inversion based on the paper "Fast constant-time gcd computation and
15  * modular inversion" by Daniel J. Bernstein and Bo-Yin Yang.
16  *
17  * For an explanation of the algorithm, see doc/safegcd_implementation.md. This file contains an
18  * implementation for N=62, using 62-bit signed limbs represented as int64_t.
19  */
20 
21 #ifdef VERIFY
22 /* Helper function to compute the absolute value of an int64_t.
23  * (we don't use abs/labs/llabs as it depends on the int sizes). */
24 static int64_t secp256k1_modinv64_abs(int64_t v) {
25  VERIFY_CHECK(v > INT64_MIN);
26  if (v < 0) return -v;
27  return v;
28 }
29 
30 static const secp256k1_modinv64_signed62 SECP256K1_SIGNED62_ONE = {{1}};
31 
32 /* Compute a*factor and put it in r. All but the top limb in r will be in range [0,2^62). */
33 static void secp256k1_modinv64_mul_62(secp256k1_modinv64_signed62 *r, const secp256k1_modinv64_signed62 *a, int alen, int64_t factor) {
34  const int64_t M62 = (int64_t)(UINT64_MAX >> 2);
35  int128_t c = 0;
36  int i;
37  for (i = 0; i < 4; ++i) {
38  if (i < alen) c += (int128_t)a->v[i] * factor;
39  r->v[i] = (int64_t)c & M62; c >>= 62;
40  }
41  if (4 < alen) c += (int128_t)a->v[4] * factor;
42  VERIFY_CHECK(c == (int64_t)c);
43  r->v[4] = (int64_t)c;
44 }
45 
46 /* Return -1 for a<b*factor, 0 for a==b*factor, 1 for a>b*factor. A has alen limbs; b has 5. */
47 static int secp256k1_modinv64_mul_cmp_62(const secp256k1_modinv64_signed62 *a, int alen, const secp256k1_modinv64_signed62 *b, int64_t factor) {
48  int i;
50  secp256k1_modinv64_mul_62(&am, a, alen, 1); /* Normalize all but the top limb of a. */
51  secp256k1_modinv64_mul_62(&bm, b, 5, factor);
52  for (i = 0; i < 4; ++i) {
53  /* Verify that all but the top limb of a and b are normalized. */
54  VERIFY_CHECK(am.v[i] >> 62 == 0);
55  VERIFY_CHECK(bm.v[i] >> 62 == 0);
56  }
57  for (i = 4; i >= 0; --i) {
58  if (am.v[i] < bm.v[i]) return -1;
59  if (am.v[i] > bm.v[i]) return 1;
60  }
61  return 0;
62 }
63 #endif
64 
65 /* Take as input a signed62 number in range (-2*modulus,modulus), and add a multiple of the modulus
66  * to it to bring it to range [0,modulus). If sign < 0, the input will also be negated in the
67  * process. The input must have limbs in range (-2^62,2^62). The output will have limbs in range
68  * [0,2^62). */
70  const int64_t M62 = (int64_t)(UINT64_MAX >> 2);
71  int64_t r0 = r->v[0], r1 = r->v[1], r2 = r->v[2], r3 = r->v[3], r4 = r->v[4];
72  int64_t cond_add, cond_negate;
73 
74 #ifdef VERIFY
75  /* Verify that all limbs are in range (-2^62,2^62). */
76  int i;
77  for (i = 0; i < 5; ++i) {
78  VERIFY_CHECK(r->v[i] >= -M62);
79  VERIFY_CHECK(r->v[i] <= M62);
80  }
81  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, -2) > 0); /* r > -2*modulus */
82  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, 1) < 0); /* r < modulus */
83 #endif
84 
85  /* In a first step, add the modulus if the input is negative, and then negate if requested.
86  * This brings r from range (-2*modulus,modulus) to range (-modulus,modulus). As all input
87  * limbs are in range (-2^62,2^62), this cannot overflow an int64_t. Note that the right
88  * shifts below are signed sign-extending shifts (see assumptions.h for tests that that is
89  * indeed the behavior of the right shift operator). */
90  cond_add = r4 >> 63;
91  r0 += modinfo->modulus.v[0] & cond_add;
92  r1 += modinfo->modulus.v[1] & cond_add;
93  r2 += modinfo->modulus.v[2] & cond_add;
94  r3 += modinfo->modulus.v[3] & cond_add;
95  r4 += modinfo->modulus.v[4] & cond_add;
96  cond_negate = sign >> 63;
97  r0 = (r0 ^ cond_negate) - cond_negate;
98  r1 = (r1 ^ cond_negate) - cond_negate;
99  r2 = (r2 ^ cond_negate) - cond_negate;
100  r3 = (r3 ^ cond_negate) - cond_negate;
101  r4 = (r4 ^ cond_negate) - cond_negate;
102  /* Propagate the top bits, to bring limbs back to range (-2^62,2^62). */
103  r1 += r0 >> 62; r0 &= M62;
104  r2 += r1 >> 62; r1 &= M62;
105  r3 += r2 >> 62; r2 &= M62;
106  r4 += r3 >> 62; r3 &= M62;
107 
108  /* In a second step add the modulus again if the result is still negative, bringing
109  * r to range [0,modulus). */
110  cond_add = r4 >> 63;
111  r0 += modinfo->modulus.v[0] & cond_add;
112  r1 += modinfo->modulus.v[1] & cond_add;
113  r2 += modinfo->modulus.v[2] & cond_add;
114  r3 += modinfo->modulus.v[3] & cond_add;
115  r4 += modinfo->modulus.v[4] & cond_add;
116  /* And propagate again. */
117  r1 += r0 >> 62; r0 &= M62;
118  r2 += r1 >> 62; r1 &= M62;
119  r3 += r2 >> 62; r2 &= M62;
120  r4 += r3 >> 62; r3 &= M62;
121 
122  r->v[0] = r0;
123  r->v[1] = r1;
124  r->v[2] = r2;
125  r->v[3] = r3;
126  r->v[4] = r4;
127 
128 #ifdef VERIFY
129  VERIFY_CHECK(r0 >> 62 == 0);
130  VERIFY_CHECK(r1 >> 62 == 0);
131  VERIFY_CHECK(r2 >> 62 == 0);
132  VERIFY_CHECK(r3 >> 62 == 0);
133  VERIFY_CHECK(r4 >> 62 == 0);
134  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, 0) >= 0); /* r >= 0 */
135  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(r, 5, &modinfo->modulus, 1) < 0); /* r < modulus */
136 #endif
137 }
138 
139 /* Data type for transition matrices (see section 3 of explanation).
140  *
141  * t = [ u v ]
142  * [ q r ]
143  */
144 typedef struct {
145  int64_t u, v, q, r;
147 
148 /* Compute the transition matrix and eta for 62 divsteps.
149  *
150  * Input: eta: initial eta
151  * f0: bottom limb of initial f
152  * g0: bottom limb of initial g
153  * Output: t: transition matrix
154  * Return: final eta
155  *
156  * Implements the divsteps_n_matrix function from the explanation.
157  */
158 static int64_t secp256k1_modinv64_divsteps_62(int64_t eta, uint64_t f0, uint64_t g0, secp256k1_modinv64_trans2x2 *t) {
159  /* u,v,q,r are the elements of the transformation matrix being built up,
160  * starting with the identity matrix. Semantically they are signed integers
161  * in range [-2^62,2^62], but here represented as unsigned mod 2^64. This
162  * permits left shifting (which is UB for negative numbers). The range
163  * being inside [-2^63,2^63) means that casting to signed works correctly.
164  */
165  uint64_t u = 1, v = 0, q = 0, r = 1;
166  uint64_t c1, c2, f = f0, g = g0, x, y, z;
167  int i;
168 
169  for (i = 0; i < 62; ++i) {
170  VERIFY_CHECK((f & 1) == 1); /* f must always be odd */
171  VERIFY_CHECK((u * f0 + v * g0) == f << i);
172  VERIFY_CHECK((q * f0 + r * g0) == g << i);
173  /* Compute conditional masks for (eta < 0) and for (g & 1). */
174  c1 = eta >> 63;
175  c2 = -(g & 1);
176  /* Compute x,y,z, conditionally negated versions of f,u,v. */
177  x = (f ^ c1) - c1;
178  y = (u ^ c1) - c1;
179  z = (v ^ c1) - c1;
180  /* Conditionally add x,y,z to g,q,r. */
181  g += x & c2;
182  q += y & c2;
183  r += z & c2;
184  /* In what follows, c1 is a condition mask for (eta < 0) and (g & 1). */
185  c1 &= c2;
186  /* Conditionally negate eta, and unconditionally subtract 1. */
187  eta = (eta ^ c1) - (c1 + 1);
188  /* Conditionally add g,q,r to f,u,v. */
189  f += g & c1;
190  u += q & c1;
191  v += r & c1;
192  /* Shifts */
193  g >>= 1;
194  u <<= 1;
195  v <<= 1;
196  /* Bounds on eta that follow from the bounds on iteration count (max 12*62 divsteps). */
197  VERIFY_CHECK(eta >= -745 && eta <= 745);
198  }
199  /* Return data in t and return value. */
200  t->u = (int64_t)u;
201  t->v = (int64_t)v;
202  t->q = (int64_t)q;
203  t->r = (int64_t)r;
204  /* The determinant of t must be a power of two. This guarantees that multiplication with t
205  * does not change the gcd of f and g, apart from adding a power-of-2 factor to it (which
206  * will be divided out again). As each divstep's individual matrix has determinant 2, the
207  * aggregate of 62 of them will have determinant 2^62. */
208  VERIFY_CHECK((int128_t)t->u * t->r - (int128_t)t->v * t->q == ((int128_t)1) << 62);
209  return eta;
210 }
211 
212 /* Compute the transition matrix and eta for 62 divsteps (variable time).
213  *
214  * Input: eta: initial eta
215  * f0: bottom limb of initial f
216  * g0: bottom limb of initial g
217  * Output: t: transition matrix
218  * Return: final eta
219  *
220  * Implements the divsteps_n_matrix_var function from the explanation.
221  */
222 static int64_t secp256k1_modinv64_divsteps_62_var(int64_t eta, uint64_t f0, uint64_t g0, secp256k1_modinv64_trans2x2 *t) {
223  /* Transformation matrix; see comments in secp256k1_modinv64_divsteps_62. */
224  uint64_t u = 1, v = 0, q = 0, r = 1;
225  uint64_t f = f0, g = g0, m;
226  uint32_t w;
227  int i = 62, limit, zeros;
228 
229  for (;;) {
230  /* Use a sentinel bit to count zeros only up to i. */
231  zeros = secp256k1_ctz64_var(g | (UINT64_MAX << i));
232  /* Perform zeros divsteps at once; they all just divide g by two. */
233  g >>= zeros;
234  u <<= zeros;
235  v <<= zeros;
236  eta -= zeros;
237  i -= zeros;
238  /* We're done once we've done 62 divsteps. */
239  if (i == 0) break;
240  VERIFY_CHECK((f & 1) == 1);
241  VERIFY_CHECK((g & 1) == 1);
242  VERIFY_CHECK((u * f0 + v * g0) == f << (62 - i));
243  VERIFY_CHECK((q * f0 + r * g0) == g << (62 - i));
244  /* Bounds on eta that follow from the bounds on iteration count (max 12*62 divsteps). */
245  VERIFY_CHECK(eta >= -745 && eta <= 745);
246  /* If eta is negative, negate it and replace f,g with g,-f. */
247  if (eta < 0) {
248  uint64_t tmp;
249  eta = -eta;
250  tmp = f; f = g; g = -tmp;
251  tmp = u; u = q; q = -tmp;
252  tmp = v; v = r; r = -tmp;
253  /* Use a formula to cancel out up to 6 bits of g. Also, no more than i can be cancelled
254  * out (as we'd be done before that point), and no more than eta+1 can be done as its
255  * will flip again once that happens. */
256  limit = ((int)eta + 1) > i ? i : ((int)eta + 1);
257  VERIFY_CHECK(limit > 0 && limit <= 62);
258  /* m is a mask for the bottom min(limit, 6) bits. */
259  m = (UINT64_MAX >> (64 - limit)) & 63U;
260  /* Find what multiple of f must be added to g to cancel its bottom min(limit, 6)
261  * bits. */
262  w = (f * g * (f * f - 2)) & m;
263  } else {
264  /* In this branch, use a simpler formula that only lets us cancel up to 4 bits of g, as
265  * eta tends to be smaller here. */
266  limit = ((int)eta + 1) > i ? i : ((int)eta + 1);
267  VERIFY_CHECK(limit > 0 && limit <= 62);
268  /* m is a mask for the bottom min(limit, 4) bits. */
269  m = (UINT64_MAX >> (64 - limit)) & 15U;
270  /* Find what multiple of f must be added to g to cancel its bottom min(limit, 4)
271  * bits. */
272  w = f + (((f + 1) & 4) << 1);
273  w = (-w * g) & m;
274  }
275  g += f * w;
276  q += u * w;
277  r += v * w;
278  VERIFY_CHECK((g & m) == 0);
279  }
280  /* Return data in t and return value. */
281  t->u = (int64_t)u;
282  t->v = (int64_t)v;
283  t->q = (int64_t)q;
284  t->r = (int64_t)r;
285  /* The determinant of t must be a power of two. This guarantees that multiplication with t
286  * does not change the gcd of f and g, apart from adding a power-of-2 factor to it (which
287  * will be divided out again). As each divstep's individual matrix has determinant 2, the
288  * aggregate of 62 of them will have determinant 2^62. */
289  VERIFY_CHECK((int128_t)t->u * t->r - (int128_t)t->v * t->q == ((int128_t)1) << 62);
290  return eta;
291 }
292 
293 /* Compute (t/2^62) * [d, e] mod modulus, where t is a transition matrix for 62 divsteps.
294  *
295  * On input and output, d and e are in range (-2*modulus,modulus). All output limbs will be in range
296  * (-2^62,2^62).
297  *
298  * This implements the update_de function from the explanation.
299  */
301  const int64_t M62 = (int64_t)(UINT64_MAX >> 2);
302  const int64_t d0 = d->v[0], d1 = d->v[1], d2 = d->v[2], d3 = d->v[3], d4 = d->v[4];
303  const int64_t e0 = e->v[0], e1 = e->v[1], e2 = e->v[2], e3 = e->v[3], e4 = e->v[4];
304  const int64_t u = t->u, v = t->v, q = t->q, r = t->r;
305  int64_t md, me, sd, se;
306  int128_t cd, ce;
307 #ifdef VERIFY
308  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(d, 5, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
309  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(d, 5, &modinfo->modulus, 1) < 0); /* d < modulus */
310  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
311  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, 1) < 0); /* e < modulus */
312  VERIFY_CHECK((secp256k1_modinv64_abs(u) + secp256k1_modinv64_abs(v)) >= 0); /* |u|+|v| doesn't overflow */
313  VERIFY_CHECK((secp256k1_modinv64_abs(q) + secp256k1_modinv64_abs(r)) >= 0); /* |q|+|r| doesn't overflow */
314  VERIFY_CHECK((secp256k1_modinv64_abs(u) + secp256k1_modinv64_abs(v)) <= M62 + 1); /* |u|+|v| <= 2^62 */
315  VERIFY_CHECK((secp256k1_modinv64_abs(q) + secp256k1_modinv64_abs(r)) <= M62 + 1); /* |q|+|r| <= 2^62 */
316 #endif
317  /* [md,me] start as zero; plus [u,q] if d is negative; plus [v,r] if e is negative. */
318  sd = d4 >> 63;
319  se = e4 >> 63;
320  md = (u & sd) + (v & se);
321  me = (q & sd) + (r & se);
322  /* Begin computing t*[d,e]. */
323  cd = (int128_t)u * d0 + (int128_t)v * e0;
324  ce = (int128_t)q * d0 + (int128_t)r * e0;
325  /* Correct md,me so that t*[d,e]+modulus*[md,me] has 62 zero bottom bits. */
326  md -= (modinfo->modulus_inv62 * (uint64_t)cd + md) & M62;
327  me -= (modinfo->modulus_inv62 * (uint64_t)ce + me) & M62;
328  /* Update the beginning of computation for t*[d,e]+modulus*[md,me] now md,me are known. */
329  cd += (int128_t)modinfo->modulus.v[0] * md;
330  ce += (int128_t)modinfo->modulus.v[0] * me;
331  /* Verify that the low 62 bits of the computation are indeed zero, and then throw them away. */
332  VERIFY_CHECK(((int64_t)cd & M62) == 0); cd >>= 62;
333  VERIFY_CHECK(((int64_t)ce & M62) == 0); ce >>= 62;
334  /* Compute limb 1 of t*[d,e]+modulus*[md,me], and store it as output limb 0 (= down shift). */
335  cd += (int128_t)u * d1 + (int128_t)v * e1;
336  ce += (int128_t)q * d1 + (int128_t)r * e1;
337  if (modinfo->modulus.v[1]) { /* Optimize for the case where limb of modulus is zero. */
338  cd += (int128_t)modinfo->modulus.v[1] * md;
339  ce += (int128_t)modinfo->modulus.v[1] * me;
340  }
341  d->v[0] = (int64_t)cd & M62; cd >>= 62;
342  e->v[0] = (int64_t)ce & M62; ce >>= 62;
343  /* Compute limb 2 of t*[d,e]+modulus*[md,me], and store it as output limb 1. */
344  cd += (int128_t)u * d2 + (int128_t)v * e2;
345  ce += (int128_t)q * d2 + (int128_t)r * e2;
346  if (modinfo->modulus.v[2]) { /* Optimize for the case where limb of modulus is zero. */
347  cd += (int128_t)modinfo->modulus.v[2] * md;
348  ce += (int128_t)modinfo->modulus.v[2] * me;
349  }
350  d->v[1] = (int64_t)cd & M62; cd >>= 62;
351  e->v[1] = (int64_t)ce & M62; ce >>= 62;
352  /* Compute limb 3 of t*[d,e]+modulus*[md,me], and store it as output limb 2. */
353  cd += (int128_t)u * d3 + (int128_t)v * e3;
354  ce += (int128_t)q * d3 + (int128_t)r * e3;
355  if (modinfo->modulus.v[3]) { /* Optimize for the case where limb of modulus is zero. */
356  cd += (int128_t)modinfo->modulus.v[3] * md;
357  ce += (int128_t)modinfo->modulus.v[3] * me;
358  }
359  d->v[2] = (int64_t)cd & M62; cd >>= 62;
360  e->v[2] = (int64_t)ce & M62; ce >>= 62;
361  /* Compute limb 4 of t*[d,e]+modulus*[md,me], and store it as output limb 3. */
362  cd += (int128_t)u * d4 + (int128_t)v * e4;
363  ce += (int128_t)q * d4 + (int128_t)r * e4;
364  cd += (int128_t)modinfo->modulus.v[4] * md;
365  ce += (int128_t)modinfo->modulus.v[4] * me;
366  d->v[3] = (int64_t)cd & M62; cd >>= 62;
367  e->v[3] = (int64_t)ce & M62; ce >>= 62;
368  /* What remains is limb 5 of t*[d,e]+modulus*[md,me]; store it as output limb 4. */
369  d->v[4] = (int64_t)cd;
370  e->v[4] = (int64_t)ce;
371 #ifdef VERIFY
372  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(d, 5, &modinfo->modulus, -2) > 0); /* d > -2*modulus */
373  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(d, 5, &modinfo->modulus, 1) < 0); /* d < modulus */
374  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, -2) > 0); /* e > -2*modulus */
375  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(e, 5, &modinfo->modulus, 1) < 0); /* e < modulus */
376 #endif
377 }
378 
379 /* Compute (t/2^62) * [f, g], where t is a transition matrix for 62 divsteps.
380  *
381  * This implements the update_fg function from the explanation.
382  */
384  const int64_t M62 = (int64_t)(UINT64_MAX >> 2);
385  const int64_t f0 = f->v[0], f1 = f->v[1], f2 = f->v[2], f3 = f->v[3], f4 = f->v[4];
386  const int64_t g0 = g->v[0], g1 = g->v[1], g2 = g->v[2], g3 = g->v[3], g4 = g->v[4];
387  const int64_t u = t->u, v = t->v, q = t->q, r = t->r;
388  int128_t cf, cg;
389  /* Start computing t*[f,g]. */
390  cf = (int128_t)u * f0 + (int128_t)v * g0;
391  cg = (int128_t)q * f0 + (int128_t)r * g0;
392  /* Verify that the bottom 62 bits of the result are zero, and then throw them away. */
393  VERIFY_CHECK(((int64_t)cf & M62) == 0); cf >>= 62;
394  VERIFY_CHECK(((int64_t)cg & M62) == 0); cg >>= 62;
395  /* Compute limb 1 of t*[f,g], and store it as output limb 0 (= down shift). */
396  cf += (int128_t)u * f1 + (int128_t)v * g1;
397  cg += (int128_t)q * f1 + (int128_t)r * g1;
398  f->v[0] = (int64_t)cf & M62; cf >>= 62;
399  g->v[0] = (int64_t)cg & M62; cg >>= 62;
400  /* Compute limb 2 of t*[f,g], and store it as output limb 1. */
401  cf += (int128_t)u * f2 + (int128_t)v * g2;
402  cg += (int128_t)q * f2 + (int128_t)r * g2;
403  f->v[1] = (int64_t)cf & M62; cf >>= 62;
404  g->v[1] = (int64_t)cg & M62; cg >>= 62;
405  /* Compute limb 3 of t*[f,g], and store it as output limb 2. */
406  cf += (int128_t)u * f3 + (int128_t)v * g3;
407  cg += (int128_t)q * f3 + (int128_t)r * g3;
408  f->v[2] = (int64_t)cf & M62; cf >>= 62;
409  g->v[2] = (int64_t)cg & M62; cg >>= 62;
410  /* Compute limb 4 of t*[f,g], and store it as output limb 3. */
411  cf += (int128_t)u * f4 + (int128_t)v * g4;
412  cg += (int128_t)q * f4 + (int128_t)r * g4;
413  f->v[3] = (int64_t)cf & M62; cf >>= 62;
414  g->v[3] = (int64_t)cg & M62; cg >>= 62;
415  /* What remains is limb 5 of t*[f,g]; store it as output limb 4. */
416  f->v[4] = (int64_t)cf;
417  g->v[4] = (int64_t)cg;
418 }
419 
420 /* Compute (t/2^62) * [f, g], where t is a transition matrix for 62 divsteps.
421  *
422  * Version that operates on a variable number of limbs in f and g.
423  *
424  * This implements the update_fg function from the explanation.
425  */
427  const int64_t M62 = (int64_t)(UINT64_MAX >> 2);
428  const int64_t u = t->u, v = t->v, q = t->q, r = t->r;
429  int64_t fi, gi;
430  int128_t cf, cg;
431  int i;
432  VERIFY_CHECK(len > 0);
433  /* Start computing t*[f,g]. */
434  fi = f->v[0];
435  gi = g->v[0];
436  cf = (int128_t)u * fi + (int128_t)v * gi;
437  cg = (int128_t)q * fi + (int128_t)r * gi;
438  /* Verify that the bottom 62 bits of the result are zero, and then throw them away. */
439  VERIFY_CHECK(((int64_t)cf & M62) == 0); cf >>= 62;
440  VERIFY_CHECK(((int64_t)cg & M62) == 0); cg >>= 62;
441  /* Now iteratively compute limb i=1..len of t*[f,g], and store them in output limb i-1 (shifting
442  * down by 62 bits). */
443  for (i = 1; i < len; ++i) {
444  fi = f->v[i];
445  gi = g->v[i];
446  cf += (int128_t)u * fi + (int128_t)v * gi;
447  cg += (int128_t)q * fi + (int128_t)r * gi;
448  f->v[i - 1] = (int64_t)cf & M62; cf >>= 62;
449  g->v[i - 1] = (int64_t)cg & M62; cg >>= 62;
450  }
451  /* What remains is limb (len) of t*[f,g]; store it as output limb (len-1). */
452  f->v[len - 1] = (int64_t)cf;
453  g->v[len - 1] = (int64_t)cg;
454 }
455 
456 /* Compute the inverse of x modulo modinfo->modulus, and replace x with it (constant time in x). */
458  /* Start with d=0, e=1, f=modulus, g=x, eta=-1. */
459  secp256k1_modinv64_signed62 d = {{0, 0, 0, 0, 0}};
460  secp256k1_modinv64_signed62 e = {{1, 0, 0, 0, 0}};
463  int i;
464  int64_t eta = -1;
465 
466  /* Do 12 iterations of 62 divsteps each = 744 divsteps. 724 suffices for 256-bit inputs. */
467  for (i = 0; i < 12; ++i) {
468  /* Compute transition matrix and new eta after 62 divsteps. */
470  eta = secp256k1_modinv64_divsteps_62(eta, f.v[0], g.v[0], &t);
471  /* Update d,e using that transition matrix. */
472  secp256k1_modinv64_update_de_62(&d, &e, &t, modinfo);
473  /* Update f,g using that transition matrix. */
474 #ifdef VERIFY
475  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, -1) > 0); /* f > -modulus */
476  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, 1) <= 0); /* f <= modulus */
477  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &modinfo->modulus, -1) > 0); /* g > -modulus */
478  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &modinfo->modulus, 1) < 0); /* g < modulus */
479 #endif
481 #ifdef VERIFY
482  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, -1) > 0); /* f > -modulus */
483  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, 1) <= 0); /* f <= modulus */
484  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &modinfo->modulus, -1) > 0); /* g > -modulus */
485  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &modinfo->modulus, 1) < 0); /* g < modulus */
486 #endif
487  }
488 
489  /* At this point sufficient iterations have been performed that g must have reached 0
490  * and (if g was not originally 0) f must now equal +/- GCD of the initial f, g
491  * values i.e. +/- 1, and d now contains +/- the modular inverse. */
492 #ifdef VERIFY
493  /* g == 0 */
494  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &SECP256K1_SIGNED62_ONE, 0) == 0);
495  /* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
496  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &SECP256K1_SIGNED62_ONE, -1) == 0 ||
497  secp256k1_modinv64_mul_cmp_62(&f, 5, &SECP256K1_SIGNED62_ONE, 1) == 0 ||
498  (secp256k1_modinv64_mul_cmp_62(x, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
499  secp256k1_modinv64_mul_cmp_62(&d, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
500  (secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, 1) == 0 ||
501  secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, -1) == 0)));
502 #endif
503 
504  /* Optionally negate d, normalize to [0,modulus), and return it. */
505  secp256k1_modinv64_normalize_62(&d, f.v[4], modinfo);
506  *x = d;
507 }
508 
509 /* Compute the inverse of x modulo modinfo->modulus, and replace x with it (variable time). */
511  /* Start with d=0, e=1, f=modulus, g=x, eta=-1. */
512  secp256k1_modinv64_signed62 d = {{0, 0, 0, 0, 0}};
513  secp256k1_modinv64_signed62 e = {{1, 0, 0, 0, 0}};
516 #ifdef VERIFY
517  int i = 0;
518 #endif
519  int j, len = 5;
520  int64_t eta = -1;
521  int64_t cond, fn, gn;
522 
523  /* Do iterations of 62 divsteps each until g=0. */
524  while (1) {
525  /* Compute transition matrix and new eta after 62 divsteps. */
527  eta = secp256k1_modinv64_divsteps_62_var(eta, f.v[0], g.v[0], &t);
528  /* Update d,e using that transition matrix. */
529  secp256k1_modinv64_update_de_62(&d, &e, &t, modinfo);
530  /* Update f,g using that transition matrix. */
531 #ifdef VERIFY
532  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
533  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
534  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */
535  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &modinfo->modulus, 1) < 0); /* g < modulus */
536 #endif
537  secp256k1_modinv64_update_fg_62_var(len, &f, &g, &t);
538  /* If the bottom limb of g is zero, there is a chance that g=0. */
539  if (g.v[0] == 0) {
540  cond = 0;
541  /* Check if the other limbs are also 0. */
542  for (j = 1; j < len; ++j) {
543  cond |= g.v[j];
544  }
545  /* If so, we're done. */
546  if (cond == 0) break;
547  }
548 
549  /* Determine if len>1 and limb (len-1) of both f and g is 0 or -1. */
550  fn = f.v[len - 1];
551  gn = g.v[len - 1];
552  cond = ((int64_t)len - 2) >> 63;
553  cond |= fn ^ (fn >> 63);
554  cond |= gn ^ (gn >> 63);
555  /* If so, reduce length, propagating the sign of f and g's top limb into the one below. */
556  if (cond == 0) {
557  f.v[len - 2] |= (uint64_t)fn << 62;
558  g.v[len - 2] |= (uint64_t)gn << 62;
559  --len;
560  }
561 #ifdef VERIFY
562  VERIFY_CHECK(++i < 12); /* We should never need more than 12*62 = 744 divsteps */
563  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */
564  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) <= 0); /* f <= modulus */
565  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &modinfo->modulus, -1) > 0); /* g > -modulus */
566  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &modinfo->modulus, 1) < 0); /* g < modulus */
567 #endif
568  }
569 
570  /* At this point g is 0 and (if g was not originally 0) f must now equal +/- GCD of
571  * the initial f, g values i.e. +/- 1, and d now contains +/- the modular inverse. */
572 #ifdef VERIFY
573  /* g == 0 */
574  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &SECP256K1_SIGNED62_ONE, 0) == 0);
575  /* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
576  VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &SECP256K1_SIGNED62_ONE, -1) == 0 ||
577  secp256k1_modinv64_mul_cmp_62(&f, len, &SECP256K1_SIGNED62_ONE, 1) == 0 ||
578  (secp256k1_modinv64_mul_cmp_62(x, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
579  secp256k1_modinv64_mul_cmp_62(&d, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
580  (secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) == 0 ||
581  secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) == 0)));
582 #endif
583 
584  /* Optionally negate d, normalize to [0,modulus), and return it. */
585  secp256k1_modinv64_normalize_62(&d, f.v[len - 1], modinfo);
586  *x = d;
587 }
588 
589 #endif /* SECP256K1_MODINV64_IMPL_H */
secp256k1_modinv64_trans2x2
Definition: modinv64_impl.h:144
secp256k1_modinv64_normalize_62
static void secp256k1_modinv64_normalize_62(secp256k1_modinv64_signed62 *r, int64_t sign, const secp256k1_modinv64_modinfo *modinfo)
Definition: modinv64_impl.h:69
VERIFY_CHECK
#define VERIFY_CHECK(cond)
Definition: util.h:68
secp256k1_modinv64_update_fg_62_var
static void secp256k1_modinv64_update_fg_62_var(int len, secp256k1_modinv64_signed62 *f, secp256k1_modinv64_signed62 *g, const secp256k1_modinv64_trans2x2 *t)
Definition: modinv64_impl.h:426
secp256k1_modinv64_divsteps_62
static int64_t secp256k1_modinv64_divsteps_62(int64_t eta, uint64_t f0, uint64_t g0, secp256k1_modinv64_trans2x2 *t)
Definition: modinv64_impl.h:158
secp256k1_modinv64_update_fg_62
static void secp256k1_modinv64_update_fg_62(secp256k1_modinv64_signed62 *f, secp256k1_modinv64_signed62 *g, const secp256k1_modinv64_trans2x2 *t)
Definition: modinv64_impl.h:383
util.h
secp256k1_modinv64
static void secp256k1_modinv64(secp256k1_modinv64_signed62 *x, const secp256k1_modinv64_modinfo *modinfo)
Definition: modinv64_impl.h:457
secp256k1_ctz64_var
static SECP256K1_INLINE int secp256k1_ctz64_var(uint64_t x)
Definition: util.h:327
secp256k1_modinv64_trans2x2::q
int64_t q
Definition: modinv64_impl.h:145
secp256k1_modinv64_trans2x2::u
int64_t u
Definition: modinv64_impl.h:145
secp256k1_modinv64_var
static void secp256k1_modinv64_var(secp256k1_modinv64_signed62 *x, const secp256k1_modinv64_modinfo *modinfo)
Definition: modinv64_impl.h:510
secp256k1_modinv64_modinfo::modulus_inv62
uint64_t modulus_inv62
Definition: modinv64.h:32
secp256k1_modinv64_update_de_62
static void secp256k1_modinv64_update_de_62(secp256k1_modinv64_signed62 *d, secp256k1_modinv64_signed62 *e, const secp256k1_modinv64_trans2x2 *t, const secp256k1_modinv64_modinfo *modinfo)
Definition: modinv64_impl.h:300
secp256k1_modinv64_modinfo
Definition: modinv64.h:27
secp256k1_modinv64_modinfo::modulus
secp256k1_modinv64_signed62 modulus
Definition: modinv64.h:29
modinv64.h
secp256k1_modinv64_divsteps_62_var
static int64_t secp256k1_modinv64_divsteps_62_var(int64_t eta, uint64_t f0, uint64_t g0, secp256k1_modinv64_trans2x2 *t)
Definition: modinv64_impl.h:222
secp256k1_modinv64_signed62
Definition: modinv64.h:23
secp256k1_modinv64_trans2x2::v
int64_t v
Definition: modinv64_impl.h:145
secp256k1_modinv64_trans2x2::r
int64_t r
Definition: modinv64_impl.h:145
secp256k1_modinv64_signed62::v
int64_t v[5]
Definition: modinv64.h:28