3 * $Id: mpbarrett.c,v 1.2 1999/12/11 01:50:56 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 1.2 1999/12/11 01:50:56 mdw
34 * Improve initialization slightly.
36 * Revision 1.1 1999/12/10 23:21:59 mdw
37 * Barrett reduction support: works with even moduli.
41 /*----- Header files ------------------------------------------------------*/
44 #include "mpbarrett.h"
46 /*----- Main code ---------------------------------------------------------*/
48 /* --- @mpbarrett_create@ --- *
50 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
51 * @mp *m@ = modulus to work to
56 * Use: Initializes a Barrett reduction context ready for use.
59 void mpbarrett_create(mpbarrett *mb, mp *m)
63 /* --- Validate the arguments --- */
65 assert(((void)"Barrett modulus must be positive", (m->f & MP_NEG) == 0));
67 /* --- Compute %$\mu$% --- */
72 b = mp_create(2 * mb->k + 1);
73 MPX_ZERO(b->v, b->vl - 1);
79 /* --- @mpbarrett_destroy@ --- *
81 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
85 * Use: Destroys a Barrett reduction context releasing any resources
89 void mpbarrett_destroy(mpbarrett *mb)
95 /* --- @mpbarrett_reduce@ --- *
97 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
98 * @mp *d@ = destination for result
99 * @mp *m@ = number to reduce
101 * Returns: The residue of @m@ modulo the number in the reduction
104 * Use: Performs an efficient modular reduction. The argument is
105 * assumed to be positive.
108 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
113 /* --- Special case if @m@ is too small --- */
121 /* --- First stage --- */
125 mp_build(&qq, m->v + (k - 1), m->vl);
126 q = mp_mul(MP_NEW, &qq, mb->mu);
127 q = mp_lsr(q, q, MPW_BITS * (k + 1));
130 /* --- Second stage --- */
137 if (MP_LEN(m) <= k + 1)
141 r = mp_create(k + 1);
142 mpx_umul(r->v, r->vl, q->v, q->vl, mb->m->v, mb->m->vl);
143 r->f = (q->f | mb->m->f) & MP_BURN;
145 mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
146 d->f = (m->f | r->f) & MP_BURN;
152 /* --- Final stage --- */
155 while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
156 mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
162 /* --- @mpbarrett_exp@ --- *
164 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
165 * @mp *d@ = fake destination
169 * Returns: Result, %$a^e \bmod m$%.
172 mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
186 dd = mp_sqr(spare, a);
187 dd = mpbarrett_reduce(mb, dd, dd);
191 dd = mp_mul(spare, x, a);
192 dd = mpbarrett_reduce(mb, dd, dd);
209 /*----- Test rig ----------------------------------------------------------*/
213 static int vmod(dstr *v)
215 mp *x = *(mp **)v[0].buf;
216 mp *n = *(mp **)v[1].buf;
217 mp *r = *(mp **)v[2].buf;
222 mpbarrett_create(&mb, n);
223 s = mpbarrett_reduce(&mb, MP_NEW, x);
225 if (MP_CMP(s, !=, r)) {
226 fputs("\n*** barrett reduction failure\n", stderr);
227 fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
228 fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
229 fputs("r = ", stderr); mp_writefile(r, stderr, 10); fputc('\n', stderr);
230 fputs("s = ", stderr); mp_writefile(s, stderr, 10); fputc('\n', stderr);
234 mpbarrett_destroy(&mb);
239 assert(mparena_count(MPARENA_GLOBAL) == 0);
243 static int vexp(dstr *v)
245 mp *m = *(mp **)v[0].buf;
246 mp *a = *(mp **)v[1].buf;
247 mp *b = *(mp **)v[2].buf;
248 mp *r = *(mp **)v[3].buf;
253 mpbarrett_create(&mb, m);
255 mr = mpbarrett_exp(&mb, MP_NEW, a, b);
257 if (MP_CMP(mr, !=, r)) {
258 fputs("\n*** barrett modexp failed", stderr);
259 fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
260 fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
261 fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
262 fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
263 fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
273 mpbarrett_destroy(&mb);
274 assert(mparena_count(MPARENA_GLOBAL) == 0);
278 static test_chunk tests[] = {
279 { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
280 { "mpbarrett-exp", vexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
284 int main(int argc, char *argv[])
287 test_run(argc, argv, tests, SRCDIR "/tests/mpbarrett");
293 /*----- That's all, folks -------------------------------------------------*/