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0bcb8184 MW |
1 | /* |
2 | * fgoldi.c: arithmetic modulo 2^448 - 2^224 - 1 | |
3 | */ | |
4 | /* | |
5 | * This file is Free Software. It has been modified to as part of its | |
6 | * incorporation into secnet. | |
7 | * | |
8 | * Copyright 2017 Mark Wooding | |
9 | * | |
10 | * You may redistribute this file and/or modify it under the terms of | |
11 | * the permissive licence shown below. | |
12 | * | |
13 | * You may redistribute secnet as a whole and/or modify it under the | |
14 | * terms of the GNU General Public License as published by the Free | |
15 | * Software Foundation; either version 3, or (at your option) any | |
16 | * later version. | |
17 | * | |
18 | * This program is distributed in the hope that it will be useful, | |
19 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
20 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
21 | * GNU General Public License for more details. | |
22 | * | |
23 | * You should have received a copy of the GNU General Public License | |
24 | * along with this program; if not, see | |
25 | * https://www.gnu.org/licenses/gpl.html. | |
26 | */ | |
27 | /* | |
28 | * Imported from Catacomb, and modified for Secnet (2017-04-30): | |
29 | * | |
30 | * * Use `fake-mLib-bits.h' in place of the real <mLib/bits.h>. | |
31 | * | |
32 | * * Remove the 16/32-bit implementation, since C99 always has 64-bit | |
33 | * arithmetic. | |
34 | * | |
35 | * * Remove the test rig code: a replacement is in a separate source file. | |
36 | * | |
37 | * The file's original comment headers are preserved below. | |
38 | */ | |
b7a5ecfc MW |
39 | /* -*-c-*- |
40 | * | |
41 | * Arithmetic in the Goldilocks field GF(2^448 - 2^224 - 1) | |
42 | * | |
43 | * (c) 2017 Straylight/Edgeware | |
44 | */ | |
45 | ||
46 | /*----- Licensing notice --------------------------------------------------* | |
47 | * | |
48 | * This file is part of Catacomb. | |
49 | * | |
50 | * Catacomb is free software; you can redistribute it and/or modify | |
51 | * it under the terms of the GNU Library General Public License as | |
52 | * published by the Free Software Foundation; either version 2 of the | |
53 | * License, or (at your option) any later version. | |
54 | * | |
55 | * Catacomb is distributed in the hope that it will be useful, | |
56 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
57 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
58 | * GNU Library General Public License for more details. | |
59 | * | |
60 | * You should have received a copy of the GNU Library General Public | |
61 | * License along with Catacomb; if not, write to the Free | |
62 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, | |
63 | * MA 02111-1307, USA. | |
64 | */ | |
65 | ||
66 | /*----- Header files ------------------------------------------------------*/ | |
67 | ||
b7a5ecfc MW |
68 | #include "fgoldi.h" |
69 | ||
70 | /*----- Basic setup -------------------------------------------------------* | |
71 | * | |
72 | * Let φ = 2^224; then p = φ^2 - φ - 1, and, in GF(p), we have φ^2 = φ + 1 | |
73 | * (hence the name). | |
74 | */ | |
75 | ||
b7a5ecfc MW |
76 | /* We represent an element of GF(p) as 16 28-bit signed integer pieces x_i: |
77 | * x = SUM_{0<=i<16} x_i 2^(28i). | |
78 | */ | |
79 | ||
80 | typedef int32 piece; typedef int64 dblpiece; | |
81 | typedef uint32 upiece; typedef uint64 udblpiece; | |
82 | #define PIECEWD(i) 28 | |
83 | #define NPIECE 16 | |
84 | #define P p28 | |
85 | ||
86 | #define B28 0x10000000u | |
87 | #define B27 0x08000000u | |
88 | #define M28 0x0fffffffu | |
89 | #define M27 0x07ffffffu | |
90 | #define M32 0xffffffffu | |
91 | ||
b7a5ecfc MW |
92 | /*----- Debugging machinery -----------------------------------------------*/ |
93 | ||
0bcb8184 | 94 | #if defined(FGOLDI_DEBUG) |
b7a5ecfc MW |
95 | |
96 | #include <stdio.h> | |
97 | ||
98 | #include "mp.h" | |
99 | #include "mptext.h" | |
100 | ||
101 | static mp *get_pgoldi(void) | |
102 | { | |
103 | mp *p = MP_NEW, *t = MP_NEW; | |
104 | ||
105 | p = mp_setbit(p, MP_ZERO, 448); | |
106 | t = mp_setbit(t, MP_ZERO, 224); | |
107 | p = mp_sub(p, p, t); | |
108 | p = mp_sub(p, p, MP_ONE); | |
109 | mp_drop(t); | |
110 | return (p); | |
111 | } | |
112 | ||
113 | DEF_FDUMP(fdump, piece, PIECEWD, NPIECE, 56, get_pgoldi()) | |
114 | ||
115 | #endif | |
116 | ||
117 | /*----- Loading and storing -----------------------------------------------*/ | |
118 | ||
119 | /* --- @fgoldi_load@ --- * | |
120 | * | |
121 | * Arguments: @fgoldi *z@ = where to store the result | |
122 | * @const octet xv[56]@ = source to read | |
123 | * | |
124 | * Returns: --- | |
125 | * | |
126 | * Use: Reads an element of %$\gf{2^{448} - 2^{224} - 19}$% in | |
127 | * external representation from @xv@ and stores it in @z@. | |
128 | * | |
129 | * External representation is little-endian base-256. Some | |
130 | * elements have multiple encodings, which are not produced by | |
131 | * correct software; use of noncanonical encodings is not an | |
132 | * error, and toleration of them is considered a performance | |
133 | * feature. | |
134 | */ | |
135 | ||
136 | void fgoldi_load(fgoldi *z, const octet xv[56]) | |
137 | { | |
b7a5ecfc MW |
138 | unsigned i; |
139 | uint32 xw[14]; | |
140 | piece b, c; | |
141 | ||
142 | /* First, read the input value as words. */ | |
143 | for (i = 0; i < 14; i++) xw[i] = LOAD32_L(xv + 4*i); | |
144 | ||
145 | /* Extract unsigned 28-bit pieces from the words. */ | |
146 | z->P[ 0] = (xw[ 0] >> 0)&M28; | |
147 | z->P[ 7] = (xw[ 6] >> 4)&M28; | |
148 | z->P[ 8] = (xw[ 7] >> 0)&M28; | |
149 | z->P[15] = (xw[13] >> 4)&M28; | |
150 | for (i = 1; i < 7; i++) { | |
151 | z->P[i + 0] = ((xw[i + 0] << (4*i)) | (xw[i - 1] >> (32 - 4*i)))&M28; | |
152 | z->P[i + 8] = ((xw[i + 7] << (4*i)) | (xw[i + 6] >> (32 - 4*i)))&M28; | |
153 | } | |
154 | ||
155 | /* Convert the nonnegative pieces into a balanced signed representation, so | |
156 | * each piece ends up in the interval |z_i| <= 2^27. For each piece, if | |
157 | * its top bit is set, lend a bit leftwards; in the case of z_15, reduce | |
158 | * this bit by adding it onto z_0 and z_8, since this is the φ^2 bit, and | |
159 | * φ^2 = φ + 1. We delay this carry until after all of the pieces have | |
160 | * been balanced. If we don't do this, then we have to do a more expensive | |
161 | * test for nonzeroness to decide whether to lend a bit leftwards rather | |
162 | * than just testing a single bit. | |
163 | * | |
164 | * Note that we don't try for a canonical representation here: both upper | |
165 | * and lower bounds are achievable. | |
166 | */ | |
167 | b = z->P[15]&B27; z->P[15] -= b << 1; c = b >> 27; | |
168 | for (i = NPIECE - 1; i--; ) | |
169 | { b = z->P[i]&B27; z->P[i] -= b << 1; z->P[i + 1] += b >> 27; } | |
170 | z->P[0] += c; z->P[8] += c; | |
b7a5ecfc MW |
171 | } |
172 | ||
173 | /* --- @fgoldi_store@ --- * | |
174 | * | |
175 | * Arguments: @octet zv[56]@ = where to write the result | |
176 | * @const fgoldi *x@ = the field element to write | |
177 | * | |
178 | * Returns: --- | |
179 | * | |
180 | * Use: Stores a field element in the given octet vector in external | |
181 | * representation. A canonical encoding is always stored. | |
182 | */ | |
183 | ||
184 | void fgoldi_store(octet zv[56], const fgoldi *x) | |
185 | { | |
b7a5ecfc MW |
186 | piece y[NPIECE], yy[NPIECE], c, d; |
187 | uint32 u, v; | |
188 | mask32 m; | |
189 | unsigned i; | |
190 | ||
191 | for (i = 0; i < NPIECE; i++) y[i] = x->P[i]; | |
192 | ||
193 | /* First, propagate the carries. By the end of this, we'll have all of the | |
194 | * the pieces canonically sized and positive, and maybe there'll be | |
195 | * (signed) carry out. The carry c is in { -1, 0, +1 }, and the remaining | |
196 | * value will be in the half-open interval [0, φ^2). The whole represented | |
197 | * value is then y + φ^2 c. | |
198 | * | |
199 | * Assume that we start out with |y_i| <= 2^30. We start off by cutting | |
200 | * off and reducing the carry c_15 from the topmost piece, y_15. This | |
201 | * leaves 0 <= y_15 < 2^28; and we'll have |c_15| <= 4. We'll add this | |
202 | * onto y_0 and y_8, and propagate the carries. It's very clear that we'll | |
203 | * end up with |y + (φ + 1) c_15 - φ^2/2| << φ^2. | |
204 | * | |
205 | * Here, the y_i are signed, so we must be cautious about bithacking them. | |
206 | */ | |
207 | c = ASR(piece, y[15], 28); y[15] = (upiece)y[15]&M28; y[8] += c; | |
208 | for (i = 0; i < NPIECE; i++) | |
209 | { y[i] += c; c = ASR(piece, y[i], 28); y[i] = (upiece)y[i]&M28; } | |
210 | ||
211 | /* Now we have a slightly fiddly job to do. If c = +1, or if c = 0 and | |
212 | * y >= p, then we should subtract p from the whole value; if c = -1 then | |
213 | * we should add p; and otherwise we should do nothing. | |
214 | * | |
215 | * But conditional behaviour is bad, m'kay. So here's what we do instead. | |
216 | * | |
217 | * The first job is to sort out what we wanted to do. If c = -1 then we | |
218 | * want to (a) invert the constant addend and (b) feed in a carry-in; | |
219 | * otherwise, we don't. | |
220 | */ | |
221 | m = SIGN(c)&M28; | |
222 | d = m&1; | |
223 | ||
224 | /* Now do the addition/subtraction. Remember that all of the y_i are | |
225 | * nonnegative, so shifting and masking are safe and easy. | |
226 | */ | |
227 | d += y[0] + (1 ^ m); yy[0] = d&M28; d >>= 28; | |
228 | for (i = 1; i < 8; i++) | |
229 | { d += y[i] + m; yy[i] = d&M28; d >>= 28; } | |
230 | d += y[8] + (1 ^ m); yy[8] = d&M28; d >>= 28; | |
231 | for (i = 9; i < 16; i++) | |
232 | { d += y[i] + m; yy[i] = d&M28; d >>= 28; } | |
233 | ||
234 | /* The final carry-out is in d; since we only did addition, and the y_i are | |
235 | * nonnegative, then d is in { 0, 1 }. We want to keep y', rather than y, | |
236 | * if (a) c /= 0 (in which case we know that the old value was | |
237 | * unsatisfactory), or (b) if d = 1 (in which case, if c = 0, we know that | |
238 | * the subtraction didn't cause a borrow, so we must be in the case where | |
239 | * p <= y < φ^2. | |
240 | */ | |
241 | m = NONZEROP(c) | ~NONZEROP(d - 1); | |
242 | for (i = 0; i < NPIECE; i++) y[i] = (yy[i]&m) | (y[i]&~m); | |
243 | ||
244 | /* Extract 32-bit words from the value. */ | |
245 | for (i = 0; i < 7; i++) { | |
246 | u = ((y[i + 0] >> (4*i)) | ((uint32)y[i + 1] << (28 - 4*i)))&M32; | |
247 | v = ((y[i + 8] >> (4*i)) | ((uint32)y[i + 9] << (28 - 4*i)))&M32; | |
248 | STORE32_L(zv + 4*i, u); | |
249 | STORE32_L(zv + 4*i + 28, v); | |
250 | } | |
b7a5ecfc MW |
251 | } |
252 | ||
253 | /* --- @fgoldi_set@ --- * | |
254 | * | |
255 | * Arguments: @fgoldi *z@ = where to write the result | |
256 | * @int a@ = a small-ish constant | |
257 | * | |
258 | * Returns: --- | |
259 | * | |
260 | * Use: Sets @z@ to equal @a@. | |
261 | */ | |
262 | ||
263 | void fgoldi_set(fgoldi *x, int a) | |
264 | { | |
265 | unsigned i; | |
266 | ||
267 | x->P[0] = a; | |
268 | for (i = 1; i < NPIECE; i++) x->P[i] = 0; | |
269 | } | |
270 | ||
271 | /*----- Basic arithmetic --------------------------------------------------*/ | |
272 | ||
273 | /* --- @fgoldi_add@ --- * | |
274 | * | |
275 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@ or @y@) | |
276 | * @const fgoldi *x, *y@ = two operands | |
277 | * | |
278 | * Returns: --- | |
279 | * | |
280 | * Use: Set @z@ to the sum %$x + y$%. | |
281 | */ | |
282 | ||
283 | void fgoldi_add(fgoldi *z, const fgoldi *x, const fgoldi *y) | |
284 | { | |
285 | unsigned i; | |
286 | for (i = 0; i < NPIECE; i++) z->P[i] = x->P[i] + y->P[i]; | |
287 | } | |
288 | ||
289 | /* --- @fgoldi_sub@ --- * | |
290 | * | |
291 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@ or @y@) | |
292 | * @const fgoldi *x, *y@ = two operands | |
293 | * | |
294 | * Returns: --- | |
295 | * | |
296 | * Use: Set @z@ to the difference %$x - y$%. | |
297 | */ | |
298 | ||
299 | void fgoldi_sub(fgoldi *z, const fgoldi *x, const fgoldi *y) | |
300 | { | |
301 | unsigned i; | |
302 | for (i = 0; i < NPIECE; i++) z->P[i] = x->P[i] - y->P[i]; | |
303 | } | |
304 | ||
305 | /*----- Constant-time utilities -------------------------------------------*/ | |
306 | ||
307 | /* --- @fgoldi_condswap@ --- * | |
308 | * | |
309 | * Arguments: @fgoldi *x, *y@ = two operands | |
310 | * @uint32 m@ = a mask | |
311 | * | |
312 | * Returns: --- | |
313 | * | |
314 | * Use: If @m@ is zero, do nothing; if @m@ is all-bits-set, then | |
315 | * exchange @x@ and @y@. If @m@ has some other value, then | |
316 | * scramble @x@ and @y@ in an unhelpful way. | |
317 | */ | |
318 | ||
319 | void fgoldi_condswap(fgoldi *x, fgoldi *y, uint32 m) | |
320 | { | |
321 | unsigned i; | |
322 | mask32 mm = FIX_MASK32(m); | |
323 | ||
324 | for (i = 0; i < NPIECE; i++) CONDSWAP(x->P[i], y->P[i], mm); | |
325 | } | |
326 | ||
327 | /*----- Multiplication ----------------------------------------------------*/ | |
328 | ||
b7a5ecfc MW |
329 | /* Let B = 2^63 - 1 be the largest value such that +B and -B can be |
330 | * represented in a double-precision piece. On entry, it must be the case | |
331 | * that |X_i| <= M <= B - 2^27 for some M. If this is the case, then, on | |
332 | * exit, we will have |Z_i| <= 2^27 + M/2^27. | |
333 | */ | |
334 | #define CARRY_REDUCE(z, x) do { \ | |
335 | dblpiece _t[NPIECE], _c; \ | |
336 | unsigned _i; \ | |
337 | \ | |
338 | /* Bias the input pieces. This keeps the carries and so on centred \ | |
339 | * around zero rather than biased positive. \ | |
340 | */ \ | |
341 | for (_i = 0; _i < NPIECE; _i++) _t[_i] = (x)[_i] + B27; \ | |
342 | \ | |
343 | /* Calculate the reduced pieces. Careful with the bithacking. */ \ | |
344 | _c = ASR(dblpiece, _t[15], 28); \ | |
345 | (z)[0] = (dblpiece)((udblpiece)_t[0]&M28) - B27 + _c; \ | |
346 | for (_i = 1; _i < NPIECE; _i++) { \ | |
347 | (z)[_i] = (dblpiece)((udblpiece)_t[_i]&M28) - B27 + \ | |
348 | ASR(dblpiece, _t[_i - 1], 28); \ | |
349 | } \ | |
350 | (z)[8] += _c; \ | |
351 | } while (0) | |
352 | ||
b7a5ecfc MW |
353 | /* --- @fgoldi_mulconst@ --- * |
354 | * | |
355 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@) | |
356 | * @const fgoldi *x@ = an operand | |
357 | * @long a@ = a small-ish constant; %$|a| < 2^{20}$%. | |
358 | * | |
359 | * Returns: --- | |
360 | * | |
361 | * Use: Set @z@ to the product %$a x$%. | |
362 | */ | |
363 | ||
364 | void fgoldi_mulconst(fgoldi *z, const fgoldi *x, long a) | |
365 | { | |
366 | unsigned i; | |
367 | dblpiece zz[NPIECE], aa = a; | |
368 | ||
369 | for (i = 0; i < NPIECE; i++) zz[i] = aa*x->P[i]; | |
b7a5ecfc | 370 | CARRY_REDUCE(z->P, zz); |
b7a5ecfc MW |
371 | } |
372 | ||
373 | /* --- @fgoldi_mul@ --- * | |
374 | * | |
375 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@ or @y@) | |
376 | * @const fgoldi *x, *y@ = two operands | |
377 | * | |
378 | * Returns: --- | |
379 | * | |
380 | * Use: Set @z@ to the product %$x y$%. | |
381 | */ | |
382 | ||
383 | void fgoldi_mul(fgoldi *z, const fgoldi *x, const fgoldi *y) | |
384 | { | |
385 | dblpiece zz[NPIECE], u[NPIECE]; | |
386 | piece ab[NPIECE/2], cd[NPIECE/2]; | |
387 | const piece | |
388 | *a = x->P + NPIECE/2, *b = x->P, | |
389 | *c = y->P + NPIECE/2, *d = y->P; | |
390 | unsigned i, j; | |
391 | ||
b7a5ecfc MW |
392 | # define M(x,i, y,j) ((dblpiece)(x)[i]*(y)[j]) |
393 | ||
b7a5ecfc MW |
394 | /* Behold the magic. |
395 | * | |
396 | * Write x = a φ + b, and y = c φ + d. Then x y = a c φ^2 + | |
397 | * (a d + b c) φ + b d. Karatsuba and Ofman observed that a d + b c = | |
398 | * (a + b) (c + d) - a c - b d, saving a multiplication, and Hamburg chose | |
399 | * the prime p so that φ^2 = φ + 1. So | |
400 | * | |
401 | * x y = ((a + b) (c + d) - b d) φ + a c + b d | |
402 | */ | |
403 | ||
404 | for (i = 0; i < NPIECE; i++) zz[i] = 0; | |
405 | ||
406 | /* Our first job will be to calculate (1 - φ) b d, and write the result | |
407 | * into z. As we do this, an interesting thing will happen. Write | |
408 | * b d = u φ + v; then (1 - φ) b d = u φ + v - u φ^2 - v φ = (1 - φ) v - u. | |
409 | * So, what we do is to write the product end-swapped and negated, and then | |
410 | * we'll subtract the (negated, remember) high half from the low half. | |
411 | */ | |
412 | for (i = 0; i < NPIECE/2; i++) { | |
413 | for (j = 0; j < NPIECE/2 - i; j++) | |
414 | zz[i + j + NPIECE/2] -= M(b,i, d,j); | |
415 | for (; j < NPIECE/2; j++) | |
416 | zz[i + j - NPIECE/2] -= M(b,i, d,j); | |
417 | } | |
418 | for (i = 0; i < NPIECE/2; i++) | |
419 | zz[i] -= zz[i + NPIECE/2]; | |
420 | ||
421 | /* Next, we add on a c. There are no surprises here. */ | |
422 | for (i = 0; i < NPIECE/2; i++) | |
423 | for (j = 0; j < NPIECE/2; j++) | |
424 | zz[i + j] += M(a,i, c,j); | |
425 | ||
426 | /* Now, calculate a + b and c + d. */ | |
427 | for (i = 0; i < NPIECE/2; i++) | |
428 | { ab[i] = a[i] + b[i]; cd[i] = c[i] + d[i]; } | |
429 | ||
430 | /* Finally (for the multiplication) we must add on (a + b) (c + d) φ. | |
431 | * Write (a + b) (c + d) as u φ + v; then we actually want u φ^2 + v φ = | |
432 | * v φ + (1 + φ) u. We'll store u in a temporary place and add it on | |
433 | * twice. | |
434 | */ | |
435 | for (i = 0; i < NPIECE; i++) u[i] = 0; | |
436 | for (i = 0; i < NPIECE/2; i++) { | |
437 | for (j = 0; j < NPIECE/2 - i; j++) | |
438 | zz[i + j + NPIECE/2] += M(ab,i, cd,j); | |
439 | for (; j < NPIECE/2; j++) | |
440 | u[i + j - NPIECE/2] += M(ab,i, cd,j); | |
441 | } | |
442 | for (i = 0; i < NPIECE/2; i++) | |
443 | { zz[i] += u[i]; zz[i + NPIECE/2] += u[i]; } | |
444 | ||
445 | #undef M | |
446 | ||
b7a5ecfc MW |
447 | /* That wraps it up for the multiplication. Let's figure out some bounds. |
448 | * Fortunately, Karatsuba is a polynomial identity, so all of the pieces | |
449 | * end up the way they'd be if we'd done the thing the easy way, which | |
450 | * simplifies the analysis. Suppose we started with |x_i|, |y_i| <= 9/5 | |
451 | * 2^28. The overheads in the result are given by the coefficients of | |
452 | * | |
453 | * ((u^16 - 1)/(u - 1))^2 mod u^16 - u^8 - 1 | |
454 | * | |
455 | * the greatest of which is 38. So |z_i| <= 38*81/25*2^56 < 2^63. | |
456 | * | |
457 | * Anyway, a round of `CARRY_REDUCE' will leave us with |z_i| < 2^27 + | |
458 | * 2^36; and a second round will leave us with |z_i| < 2^27 + 512. | |
459 | */ | |
460 | for (i = 0; i < 2; i++) CARRY_REDUCE(zz, zz); | |
461 | for (i = 0; i < NPIECE; i++) z->P[i] = zz[i]; | |
b7a5ecfc MW |
462 | } |
463 | ||
464 | /* --- @fgoldi_sqr@ --- * | |
465 | * | |
466 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@ or @y@) | |
467 | * @const fgoldi *x@ = an operand | |
468 | * | |
469 | * Returns: --- | |
470 | * | |
471 | * Use: Set @z@ to the square %$x^2$%. | |
472 | */ | |
473 | ||
474 | void fgoldi_sqr(fgoldi *z, const fgoldi *x) | |
475 | { | |
b7a5ecfc MW |
476 | dblpiece zz[NPIECE], u[NPIECE]; |
477 | piece ab[NPIECE]; | |
478 | const piece *a = x->P + NPIECE/2, *b = x->P; | |
479 | unsigned i, j; | |
480 | ||
481 | # define M(x,i, y,j) ((dblpiece)(x)[i]*(y)[j]) | |
482 | ||
483 | /* The magic is basically the same as `fgoldi_mul' above. We write | |
484 | * x = a φ + b and use Karatsuba and the special prime shape. This time, | |
485 | * we have | |
486 | * | |
487 | * x^2 = ((a + b)^2 - b^2) φ + a^2 + b^2 | |
488 | */ | |
489 | ||
490 | for (i = 0; i < NPIECE; i++) zz[i] = 0; | |
491 | ||
492 | /* Our first job will be to calculate (1 - φ) b^2, and write the result | |
493 | * into z. Again, this interacts pleasantly with the prime shape. | |
494 | */ | |
495 | for (i = 0; i < NPIECE/4; i++) { | |
496 | zz[2*i + NPIECE/2] -= M(b,i, b,i); | |
497 | for (j = i + 1; j < NPIECE/2 - i; j++) | |
498 | zz[i + j + NPIECE/2] -= 2*M(b,i, b,j); | |
499 | for (; j < NPIECE/2; j++) | |
500 | zz[i + j - NPIECE/2] -= 2*M(b,i, b,j); | |
501 | } | |
502 | for (; i < NPIECE/2; i++) { | |
503 | zz[2*i - NPIECE/2] -= M(b,i, b,i); | |
504 | for (j = i + 1; j < NPIECE/2; j++) | |
505 | zz[i + j - NPIECE/2] -= 2*M(b,i, b,j); | |
506 | } | |
507 | for (i = 0; i < NPIECE/2; i++) | |
508 | zz[i] -= zz[i + NPIECE/2]; | |
509 | ||
510 | /* Next, we add on a^2. There are no surprises here. */ | |
511 | for (i = 0; i < NPIECE/2; i++) { | |
512 | zz[2*i] += M(a,i, a,i); | |
513 | for (j = i + 1; j < NPIECE/2; j++) | |
514 | zz[i + j] += 2*M(a,i, a,j); | |
515 | } | |
516 | ||
517 | /* Now, calculate a + b. */ | |
518 | for (i = 0; i < NPIECE/2; i++) | |
519 | ab[i] = a[i] + b[i]; | |
520 | ||
521 | /* Finally (for the multiplication) we must add on (a + b)^2 φ. | |
522 | * Write (a + b)^2 as u φ + v; then we actually want (u + v) φ + u. We'll | |
523 | * store u in a temporary place and add it on twice. | |
524 | */ | |
525 | for (i = 0; i < NPIECE; i++) u[i] = 0; | |
526 | for (i = 0; i < NPIECE/4; i++) { | |
527 | zz[2*i + NPIECE/2] += M(ab,i, ab,i); | |
528 | for (j = i + 1; j < NPIECE/2 - i; j++) | |
529 | zz[i + j + NPIECE/2] += 2*M(ab,i, ab,j); | |
530 | for (; j < NPIECE/2; j++) | |
531 | u[i + j - NPIECE/2] += 2*M(ab,i, ab,j); | |
532 | } | |
533 | for (; i < NPIECE/2; i++) { | |
534 | u[2*i - NPIECE/2] += M(ab,i, ab,i); | |
535 | for (j = i + 1; j < NPIECE/2; j++) | |
536 | u[i + j - NPIECE/2] += 2*M(ab,i, ab,j); | |
537 | } | |
538 | for (i = 0; i < NPIECE/2; i++) | |
539 | { zz[i] += u[i]; zz[i + NPIECE/2] += u[i]; } | |
540 | ||
541 | #undef M | |
542 | ||
543 | /* Finally, carrying. */ | |
544 | for (i = 0; i < 2; i++) CARRY_REDUCE(zz, zz); | |
545 | for (i = 0; i < NPIECE; i++) z->P[i] = zz[i]; | |
546 | ||
b7a5ecfc MW |
547 | } |
548 | ||
549 | /*----- More advanced operations ------------------------------------------*/ | |
550 | ||
551 | /* --- @fgoldi_inv@ --- * | |
552 | * | |
553 | * Arguments: @fgoldi *z@ = where to put the result (may alias @x@) | |
554 | * @const fgoldi *x@ = an operand | |
555 | * | |
556 | * Returns: --- | |
557 | * | |
558 | * Use: Stores in @z@ the multiplicative inverse %$x^{-1}$%. If | |
559 | * %$x = 0$% then @z@ is set to zero. This is considered a | |
560 | * feature. | |
561 | */ | |
562 | ||
563 | void fgoldi_inv(fgoldi *z, const fgoldi *x) | |
564 | { | |
565 | fgoldi t, u; | |
566 | unsigned i; | |
567 | ||
568 | #define SQRN(z, x, n) do { \ | |
569 | fgoldi_sqr((z), (x)); \ | |
570 | for (i = 1; i < (n); i++) fgoldi_sqr((z), (z)); \ | |
571 | } while (0) | |
572 | ||
573 | /* Calculate x^-1 = x^(p - 2) = x^(2^448 - 2^224 - 3), which also handles | |
574 | * x = 0 as intended. The addition chain is home-made. | |
575 | */ /* step | value */ | |
576 | fgoldi_sqr(&u, x); /* 1 | 2 */ | |
577 | fgoldi_mul(&t, &u, x); /* 2 | 3 */ | |
578 | SQRN(&u, &t, 2); /* 4 | 12 */ | |
579 | fgoldi_mul(&t, &u, &t); /* 5 | 15 */ | |
580 | SQRN(&u, &t, 4); /* 9 | 240 */ | |
581 | fgoldi_mul(&u, &u, &t); /* 10 | 255 = 2^8 - 1 */ | |
582 | SQRN(&u, &u, 4); /* 14 | 2^12 - 16 */ | |
583 | fgoldi_mul(&t, &u, &t); /* 15 | 2^12 - 1 */ | |
584 | SQRN(&u, &t, 12); /* 27 | 2^24 - 2^12 */ | |
585 | fgoldi_mul(&u, &u, &t); /* 28 | 2^24 - 1 */ | |
586 | SQRN(&u, &u, 12); /* 40 | 2^36 - 2^12 */ | |
587 | fgoldi_mul(&t, &u, &t); /* 41 | 2^36 - 1 */ | |
588 | fgoldi_sqr(&t, &t); /* 42 | 2^37 - 2 */ | |
589 | fgoldi_mul(&t, &t, x); /* 43 | 2^37 - 1 */ | |
590 | SQRN(&u, &t, 37); /* 80 | 2^74 - 2^37 */ | |
591 | fgoldi_mul(&u, &u, &t); /* 81 | 2^74 - 1 */ | |
592 | SQRN(&u, &u, 37); /* 118 | 2^111 - 2^37 */ | |
593 | fgoldi_mul(&t, &u, &t); /* 119 | 2^111 - 1 */ | |
594 | SQRN(&u, &t, 111); /* 230 | 2^222 - 2^111 */ | |
595 | fgoldi_mul(&t, &u, &t); /* 231 | 2^222 - 1 */ | |
596 | fgoldi_sqr(&u, &t); /* 232 | 2^223 - 2 */ | |
597 | fgoldi_mul(&u, &u, x); /* 233 | 2^223 - 1 */ | |
598 | SQRN(&u, &u, 223); /* 456 | 2^446 - 2^223 */ | |
599 | fgoldi_mul(&t, &u, &t); /* 457 | 2^446 - 2^222 - 1 */ | |
600 | SQRN(&t, &t, 2); /* 459 | 2^448 - 2^224 - 4 */ | |
601 | fgoldi_mul(z, &t, x); /* 460 | 2^448 - 2^224 - 3 */ | |
602 | ||
603 | #undef SQRN | |
604 | } | |
605 | ||
b7a5ecfc | 606 | /*----- That's all, folks -------------------------------------------------*/ |