3 * $Id: mpbarrett.c,v 1.7 2001/04/19 18:25:26 mdw Exp $
5 * Barrett modular reduction
7 * (c) 1999 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
14 * Catacomb is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Library General Public License as
16 * published by the Free Software Foundation; either version 2 of the
17 * License, or (at your option) any later version.
19 * Catacomb is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
24 * You should have received a copy of the GNU Library General Public
25 * License along with Catacomb; if not, write to the Free
26 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
30 /*----- Revision history --------------------------------------------------*
32 * $Log: mpbarrett.c,v $
33 * Revision 1.7 2001/04/19 18:25:26 mdw
34 * Use sliding-window exponentiation.
36 * Revision 1.6 2000/10/08 12:03:44 mdw
37 * (mpbarrett_reduce): Cope with negative numbers.
39 * Revision 1.5 2000/07/29 17:04:33 mdw
40 * Change to use left-to-right bitwise exponentiation. This will improve
41 * performance when the base is small.
43 * Revision 1.4 2000/06/17 11:45:09 mdw
44 * Major memory management overhaul. Added arena support. Use the secure
45 * arena for secret integers. Replace and improve the MP management macros
46 * (e.g., replace MP_MODIFY by MP_DEST).
48 * Revision 1.3 1999/12/12 15:08:52 mdw
49 * Don't bother shifting %$q$% in @mpbarrett_reduce@, just skip the least
52 * Revision 1.2 1999/12/11 01:50:56 mdw
53 * Improve initialization slightly.
55 * Revision 1.1 1999/12/10 23:21:59 mdw
56 * Barrett reduction support: works with even moduli.
60 /*----- Header files ------------------------------------------------------*/
63 #include "mpbarrett.h"
65 /*----- Main code ---------------------------------------------------------*/
67 /* --- @mpbarrett_create@ --- *
69 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
70 * @mp *m@ = modulus to work to
75 * Use: Initializes a Barrett reduction context ready for use.
78 void mpbarrett_create(mpbarrett *mb, mp *m)
82 /* --- Validate the arguments --- */
84 assert(((void)"Barrett modulus must be positive", (m->f & MP_NEG) == 0));
86 /* --- Compute %$\mu$% --- */
91 b = mp_new(2 * mb->k + 1, 0);
92 MPX_ZERO(b->v, b->vl - 1);
98 /* --- @mpbarrett_destroy@ --- *
100 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
104 * Use: Destroys a Barrett reduction context releasing any resources
108 void mpbarrett_destroy(mpbarrett *mb)
114 /* --- @mpbarrett_reduce@ --- *
116 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
117 * @mp *d@ = destination for result
118 * @mp *m@ = number to reduce
120 * Returns: The residue of @m@ modulo the number in the reduction
123 * Use: Performs an efficient modular reduction.
126 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
131 /* --- Special case if @m@ is too small --- */
140 /* --- First stage --- */
144 mp_build(&qq, m->v + (k - 1), m->vl);
145 q = mp_mul(MP_NEW, &qq, mb->mu);
146 if (MP_LEN(q) <= k) {
154 /* --- Second stage --- */
161 if (MP_LEN(m) <= k + 1)
165 r = mp_new(k + 1, (q->f | mb->m->f) & MP_BURN);
166 mpx_umul(r->v, r->vl, q->v + k + 1, q->vl, mb->m->v, mb->m->vl);
167 MP_DEST(d, k + 1, r->f);
168 mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
169 d->f = (m->f | r->f) & (MP_BURN | MP_NEG);
175 /* --- Final stage --- */
178 while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
179 mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
181 /* --- Fix up the sign --- */
184 mpx_usub(d->v, d->vl, mb->m->v, mb->m->vl, d->v, d->vl);
192 /* --- @mpbarrett_exp@ --- *
194 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
195 * @mp *d@ = fake destination
199 * Returns: Result, %$a^e \bmod m$%.
203 #define TABSZ (1 << (WINSZ - 1))
205 #define THRESH (((MPW_BITS / WINSZ) << 2) + 1)
207 static mp *exp_simple(mpbarrett *mb, mp *d, mp *a, mp *e)
211 mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
218 while (!MP_RBIT(&sc))
221 /* --- Do the main body of the work --- */
227 y = mp_sqr(spare, x);
228 y = mpbarrett_reduce(mb, y, y);
233 mp *y = mp_mul(spare, x, a);
234 y = mpbarrett_reduce(mb, y, y);
246 /* --- Do a final round of squaring --- */
251 y = mp_sqr(spare, x);
252 y = mpbarrett_reduce(mb, y, y);
266 mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
270 mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
275 /* --- Do we bother? --- */
280 if (MP_LEN(e) < THRESH) {
282 return (exp_simple(mb, d, a, e));
285 /* --- Do the precomputation --- */
287 a2 = mp_sqr(MP_NEW, a);
288 a2 = mpbarrett_reduce(mb, a2, a2);
289 tab = xmalloc(TABSZ * sizeof(mp *));
291 for (i = 1; i < TABSZ; i++) {
292 mp *x = mp_mul(MP_NEW, tab[i - 1], a2);
293 tab[i] = mpbarrett_reduce(mb, x, x);
298 /* --- Skip top-end zero bits --- *
300 * If the initial step worked, there must be a set bit somewhere, so keep
301 * stepping until I find it.
305 while (!MP_RBIT(&sc))
308 /* --- Now for the main work --- */
314 /* --- The next bit is set, so read a window index --- *
316 * Reset @i@ to zero and increment @sq@. Then, until either I read
317 * @WINSZ@ bits or I run out of bits, scan in a bit: if it's clear, bump
318 * the @z@ counter; if it's set, push a set bit into @i@, shift it over
319 * by @z@ bits, bump @sq@ by @z + 1@ and clear @z@. By the end of this
320 * palaver, @i@ is an index to the precomputed value in @tab@.
327 if (l >= WINSZ || !MP_RSTEP(&sc))
332 i = ((i << 1) | 1) << z;
338 /* --- Do the squaring --- *
340 * Remember that @sq@ carries over from the zero-skipping stuff below.
345 y = mp_sqr(spare, x);
346 y = mpbarrett_reduce(mb, y, y);
351 /* --- Do the multiply --- */
353 { mp *y = mp_mul(spare, x, tab[i]); spare = x;
354 x = mpbarrett_reduce(mb, y, y); }
356 /* --- Now grind along through the rest of the bits --- */
368 /* --- Do a final round of squaring --- */
373 y = mp_sqr(spare, x);
374 y = mpbarrett_reduce(mb, y, y);
381 for (i = 0; i < TABSZ; i++)
390 /*----- Test rig ----------------------------------------------------------*/
394 static int vmod(dstr *v)
396 mp *x = *(mp **)v[0].buf;
397 mp *n = *(mp **)v[1].buf;
398 mp *r = *(mp **)v[2].buf;
403 mpbarrett_create(&mb, n);
404 s = mpbarrett_reduce(&mb, MP_NEW, x);
407 fputs("\n*** barrett reduction failure\n", stderr);
408 fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
409 fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
410 fputs("r = ", stderr); mp_writefile(r, stderr, 10); fputc('\n', stderr);
411 fputs("s = ", stderr); mp_writefile(s, stderr, 10); fputc('\n', stderr);
415 mpbarrett_destroy(&mb);
420 assert(mparena_count(MPARENA_GLOBAL) == 0);
424 static int vexp(dstr *v)
426 mp *m = *(mp **)v[0].buf;
427 mp *a = *(mp **)v[1].buf;
428 mp *b = *(mp **)v[2].buf;
429 mp *r = *(mp **)v[3].buf;
434 mpbarrett_create(&mb, m);
436 mr = mpbarrett_exp(&mb, MP_NEW, a, b);
439 fputs("\n*** barrett modexp failed", stderr);
440 fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
441 fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
442 fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
443 fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
444 fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
454 mpbarrett_destroy(&mb);
455 assert(mparena_count(MPARENA_GLOBAL) == 0);
459 static test_chunk tests[] = {
460 { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
461 { "mpbarrett-exp", vexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
465 int main(int argc, char *argv[])
468 test_run(argc, argv, tests, SRCDIR "/tests/mpbarrett");
474 /*----- That's all, folks -------------------------------------------------*/