3 * $Id: mpbarrett.c,v 1.1 1999/12/10 23:21:59 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 1.1 1999/12/10 23:21:59 mdw
34 * Barrett reduction support: works with even moduli.
38 /*----- Header files ------------------------------------------------------*/
41 #include "mpbarrett.h"
43 /*----- Main code ---------------------------------------------------------*/
45 /* --- @mpbarrett_create@ --- *
47 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
48 * @mp *m@ = modulus to work to
53 * Use: Initializes a Barrett reduction context ready for use.
56 void mpbarrett_create(mpbarrett *mb, mp *m)
60 /* --- Validate the arguments --- */
62 assert(((void)"Barrett modulus must be positive", (m->f & MP_NEG) == 0));
64 /* --- Compute %$\mu$% --- */
69 b = mp_lsl(MP_NEW, MP_ONE, 2 * MPW_BITS * mb->k);
74 /* --- @mpbarrett_destroy@ --- *
76 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
80 * Use: Destroys a Barrett reduction context releasing any resources
84 void mpbarrett_destroy(mpbarrett *mb)
90 /* --- @mpbarrett_reduce@ --- *
92 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
93 * @mp *d@ = destination for result
94 * @mp *m@ = number to reduce
96 * Returns: The residue of @m@ modulo the number in the reduction
99 * Use: Performs an efficient modular reduction. The argument is
100 * assumed to be positive.
103 mp *mpbarrett_reduce(mpbarrett *mb, mp *d, mp *m)
108 /* --- Special case if @m@ is too small --- */
116 /* --- First stage --- */
120 mp_build(&qq, m->v + (k - 1), m->vl);
121 q = mp_mul(MP_NEW, &qq, mb->mu);
122 q = mp_lsr(q, q, MPW_BITS * (k + 1));
125 /* --- Second stage --- */
132 if (MP_LEN(m) <= k + 1)
136 r = mp_create(k + 1);
137 mpx_umul(r->v, r->vl, q->v, q->vl, mb->m->v, mb->m->vl);
138 r->f = (q->f | mb->m->f) & MP_BURN;
140 mpx_usub(d->v, d->vl, m->v, mvl, r->v, r->vl);
141 d->f = (m->f | r->f) & MP_BURN;
147 /* --- Final stage --- */
150 while (MPX_UCMP(d->v, d->vl, >=, mb->m->v, mb->m->vl))
151 mpx_usub(d->v, d->vl, d->v, d->vl, mb->m->v, mb->m->vl);
157 /* --- @mpbarrett_exp@ --- *
159 * Arguments: @mpbarrett *mb@ = pointer to Barrett reduction context
160 * @mp *d@ = fake destination
164 * Returns: Result, %$a^e \bmod m$%.
167 mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
181 dd = mp_sqr(spare, a);
182 dd = mpbarrett_reduce(mb, dd, dd);
186 dd = mp_mul(spare, x, a);
187 dd = mpbarrett_reduce(mb, dd, dd);
204 /*----- Test rig ----------------------------------------------------------*/
208 static int vmod(dstr *v)
210 mp *x = *(mp **)v[0].buf;
211 mp *n = *(mp **)v[1].buf;
212 mp *r = *(mp **)v[2].buf;
217 mpbarrett_create(&mb, n);
218 s = mpbarrett_reduce(&mb, MP_NEW, x);
220 if (MP_CMP(s, !=, r)) {
221 fputs("\n*** barrett reduction failure\n", stderr);
222 fputs("x = ", stderr); mp_writefile(x, stderr, 10); fputc('\n', stderr);
223 fputs("n = ", stderr); mp_writefile(n, stderr, 10); fputc('\n', stderr);
224 fputs("r = ", stderr); mp_writefile(r, stderr, 10); fputc('\n', stderr);
225 fputs("s = ", stderr); mp_writefile(s, stderr, 10); fputc('\n', stderr);
229 mpbarrett_destroy(&mb);
234 assert(mparena_count(MPARENA_GLOBAL) == 0);
238 static int vexp(dstr *v)
240 mp *m = *(mp **)v[0].buf;
241 mp *a = *(mp **)v[1].buf;
242 mp *b = *(mp **)v[2].buf;
243 mp *r = *(mp **)v[3].buf;
248 mpbarrett_create(&mb, m);
250 mr = mpbarrett_exp(&mb, MP_NEW, a, b);
252 if (MP_CMP(mr, !=, r)) {
253 fputs("\n*** barrett modexp failed", stderr);
254 fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
255 fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
256 fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
257 fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
258 fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
268 mpbarrett_destroy(&mb);
269 assert(mparena_count(MPARENA_GLOBAL) == 0);
273 static test_chunk tests[] = {
274 { "mpbarrett-reduce", vmod, { &type_mp, &type_mp, &type_mp, 0 } },
275 { "mpbarrett-exp", vexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
279 int main(int argc, char *argv[])
282 test_run(argc, argv, tests, SRCDIR "/tests/mpbarrett");
288 /*----- That's all, folks -------------------------------------------------*/