git.distorted.org.uk Git - u/mdw/catacomb/blob - f-niceprime.c

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Prime fields with efficient reduction for special-form primes
   6  *
   7  * (c) 2004 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  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.
  18  *
  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.
  23  *
  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,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Header files ------------------------------------------------------*/
  31
  32 #include <mLib/sub.h>
  33
  34 #include "field.h"
  35 #include "mpreduce.h"
  36 #include "mprand.h"
  37
  38 /*----- Main code ---------------------------------------------------------*/
  39
  40 typedef struct fctx {
  41   field f;
  42   mpreduce r;
  43 } fctx;
  44
  45 /* --- Field operations --- */
  46
  47 static void fdestroy(field *ff)
  48   { fctx *f = (fctx *)ff; mpreduce_destroy(&f->r); DESTROY(f); }
  49
  50 static mp *frand(field *ff, mp *d, grand *r)
  51   { fctx *f = (fctx *)ff; return (mprand_range(d, f->r.p, r, 0)); }
  52
  53 static int fzerop(field *ff, mp *x) { return (MP_ZEROP(x)); }
  54
  55 static mp *fneg(field *ff, mp *d, mp *x)
  56   { fctx *f = (fctx *)ff; return (mp_sub(d, f->r.p, x)); }
  57
  58 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  59   fctx *f = (fctx *)ff; d = mp_add(d, x, y);
  60   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  61   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  62   return (d);
  63 }
  64
  65 static mp *fsub(field *ff, mp *d, mp *x, mp *y) {
  66   fctx *f = (fctx *)ff; d = mp_sub(d, x, y);
  67   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  68   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  69   return (d);
  70 }
  71
  72 static mp *fmul(field *ff, mp *d, mp *x, mp *y) {
  73   fctx *f = (fctx *)ff; d = mp_mul(d, x, y);
  74   return (mpreduce_do(&f->r, d, d));
  75 }
  76
  77 static mp *fsqr(field *ff, mp *d, mp *x) {
  78   fctx *f = (fctx *)ff; d = mp_sqr(d, x);
  79   return (mpreduce_do(&f->r, d, d));
  80 }
  81
  82 static mp *finv(field *ff, mp *d, mp *x)
  83   { fctx *f = (fctx *)ff; d = mp_modinv(d, x, f->r.p); return (d); }
  84
  85 static mp *freduce(field *ff, mp *d, mp *x)
  86   { fctx *f = (fctx *)ff; return (mpreduce_do(&f->r, d, x)); }
  87
  88 static mp *fsqrt(field *ff, mp *d, mp *x)
  89   { fctx *f = (fctx *)ff; return (mp_modsqrt(d, x, f->r.p)); }
  90
  91 static mp *fdbl(field *ff, mp *d, mp *x) {
  92   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 1);
  93   if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  94   return (d);
  95 }
  96
  97 static mp *ftpl(field *ff, mp *d, mp *x) {
  98   fctx *f = (fctx *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
  99   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3);
 100   while (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
 101   return (d);
 102 }
 103
 104 static mp *fqdl(field *ff, mp *d, mp *x) {
 105   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 2);
 106   while (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
 107   return (d);
 108 }
 109
 110 static mp *fhlv(field *ff, mp *d, mp *x) {
 111   fctx *f = (fctx *)ff;
 112   if (MP_ZEROP(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 113   if (x->v[0] & 1) { d = mp_add(d, x, f->r.p); x = d; }
 114   return (mp_lsr(d, x, 1));
 115 }
 116
 117 /* --- Field operations table --- */
 118
 119 static const field_ops fops = {
 120   FTY_PRIME, "niceprime",
 121   fdestroy, frand, field_stdsamep,
 122   freduce, field_id,
 123   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 124   0,
 125   fdbl, ftpl, fqdl, fhlv
 126 };
 127
 128 /* --- @field_niceprime@ --- *
 129  *
 130  * Arguments:   @mp *p@ = the characteristic of the field
 131  *
 132  * Returns:     A pointer to the field.
 133  *
 134  * Use:         Creates a field structure for a prime field of size %$p$%,
 135  *              using efficient reduction for nice primes.
 136  */
 137
 138 field *field_niceprime(mp *p)
 139 {
 140   fctx *f = CREATE(fctx);
 141   f->f.ops = &fops;
 142   f->f.zero = MP_ZERO;
 143   f->f.one = MP_ONE;
 144   f->f.nbits = mp_bits(p);
 145   f->f.noctets = (f->f.nbits + 7) >> 3;
 146   mpreduce_create(&f->r, p);
 147   f->f.m = f->r.p;
 148   return (&f->f);
 149 }
 150
 151 /*----- That's all, folks -------------------------------------------------*/