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 "field-guts.h"
  36 #include "mprand.h"
  37
  38 /*----- Main code ---------------------------------------------------------*/
  39
  40 /* --- Field operations --- */
  41
  42 static void fdestroy(field *ff) {
  43   fctx_niceprime *f = (fctx_niceprime *)ff;
  44   mpreduce_destroy(&f->r);
  45   DESTROY(f);
  46 }
  47
  48 static mp *frand(field *ff, mp *d, grand *r) {
  49   fctx_niceprime *f = (fctx_niceprime *)ff;
  50   return (mprand_range(d, f->r.p, r, 0));
  51 }
  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_niceprime *f = (fctx_niceprime *)ff;
  57   return (mp_sub(d, f->r.p, x));
  58 }
  59
  60 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  61   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_add(d, x, y);
  62   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  63   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  64   return (d);
  65 }
  66
  67 static mp *fsub(field *ff, mp *d, mp *x, mp *y) {
  68   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_sub(d, x, y);
  69   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  70   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  71   return (d);
  72 }
  73
  74 static mp *fmul(field *ff, mp *d, mp *x, mp *y) {
  75   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_mul(d, x, y);
  76   return (mpreduce_do(&f->r, d, d));
  77 }
  78
  79 static mp *fsqr(field *ff, mp *d, mp *x) {
  80   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_sqr(d, x);
  81   return (mpreduce_do(&f->r, d, d));
  82 }
  83
  84 static mp *finv(field *ff, mp *d, mp *x) {
  85   fctx_niceprime *f = (fctx_niceprime *)ff;
  86   d = mp_modinv(d, x, f->r.p);
  87   return (d);
  88 }
  89
  90 static mp *freduce(field *ff, mp *d, mp *x) {
  91   fctx_niceprime *f = (fctx_niceprime *)ff;
  92   return (mpreduce_do(&f->r, d, x));
  93 }
  94
  95 static mp *fsqrt(field *ff, mp *d, mp *x) {
  96   fctx_niceprime *f = (fctx_niceprime *)ff;
  97   return (mp_modsqrt(d, x, f->r.p));
  98 }
  99
 100 static mp *fdbl(field *ff, mp *d, mp *x) {
 101   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 1);
 102   if (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 103   return (d);
 104 }
 105
 106 static mp *ftpl(field *ff, mp *d, mp *x) {
 107   fctx_niceprime *f = (fctx_niceprime *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
 108   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3); d->f &= ~MP_UNDEF;
 109   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 110   return (d);
 111 }
 112
 113 static mp *fqdl(field *ff, mp *d, mp *x) {
 114   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 2);
 115   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 116   return (d);
 117 }
 118
 119 static mp *fhlv(field *ff, mp *d, mp *x) {
 120   fctx_niceprime *f = (fctx_niceprime *)ff;
 121   if (MP_ZEROP(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 122   if (x->v[0] & 1) { d = mp_add(d, x, f->r.p); x = d; }
 123   return (mp_lsr(d, x, 1));
 124 }
 125
 126 /* --- Field operations table --- */
 127
 128 static const field_ops fops = {
 129   FTY_PRIME, "niceprime",
 130   fdestroy, frand, field_stdsamep,
 131   freduce, field_id,
 132   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 133   0,
 134   fdbl, ftpl, fqdl, fhlv
 135 };
 136
 137 /* --- @field_niceprime@ --- *
 138  *
 139  * Arguments:   @mp *p@ = the characteristic of the field
 140  *
 141  * Returns:     A pointer to the field, or null.
 142  *
 143  * Use:         Creates a field structure for a prime field of size %$p$%,
 144  *              using efficient reduction for nice primes.
 145  */
 146
 147 field *field_niceprime(mp *p)
 148 {
 149   fctx_niceprime *f = CREATE(fctx_niceprime);
 150   f->f.ops = &fops;
 151   f->f.zero = MP_ZERO;
 152   f->f.one = MP_ONE;
 153   f->f.nbits = mp_bits(p);
 154   f->f.noctets = (f->f.nbits + 7) >> 3;
 155   if (mpreduce_create(&f->r, p)) {
 156     DESTROY(f);
 157     return (0);
 158   }
 159   f->f.m = f->r.p;
 160   f->f.q = f->r.p;
 161   return (&f->f);
 162 }
 163
 164 /*----- That's all, folks -------------------------------------------------*/