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

   1 /* -*-c-*-
   2  *
   3  * Prime fields with efficient reduction for special-form primes
   4  *
   5  * (c) 2004 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include <mLib/sub.h>
  31
  32 #include "field.h"
  33 #include "field-guts.h"
  34 #include "mprand.h"
  35
  36 /*----- Main code ---------------------------------------------------------*/
  37
  38 /* --- Field operations --- */
  39
  40 static void fdestroy(field *ff) {
  41   fctx_niceprime *f = (fctx_niceprime *)ff;
  42   mpreduce_destroy(&f->r);
  43   DESTROY(f);
  44 }
  45
  46 static mp *frand(field *ff, mp *d, grand *r) {
  47   fctx_niceprime *f = (fctx_niceprime *)ff;
  48   return (mprand_range(d, f->r.p, r, 0));
  49 }
  50
  51 static int fzerop(field *ff, mp *x) { return (MP_ZEROP(x)); }
  52
  53 static mp *fneg(field *ff, mp *d, mp *x) {
  54   fctx_niceprime *f = (fctx_niceprime *)ff;
  55   if (MP_ZEROP(x)) { if (d != x) mp_drop(d); return (MP_COPY(x)); }
  56   else return (mp_sub(d, f->r.p, x));
  57 }
  58
  59 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  60   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_add(d, x, y);
  61   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  62   else if (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
  63   return (d);
  64 }
  65
  66 static mp *fsub(field *ff, mp *d, mp *x, mp *y) {
  67   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_sub(d, x, y);
  68   if (MP_NEGP(d)) d = mp_add(d, d, f->r.p);
  69   else if (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
  70   return (d);
  71 }
  72
  73 static mp *fmul(field *ff, mp *d, mp *x, mp *y) {
  74   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_mul(d, x, y);
  75   return (mpreduce_do(&f->r, d, d));
  76 }
  77
  78 static mp *fsqr(field *ff, mp *d, mp *x) {
  79   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_sqr(d, x);
  80   return (mpreduce_do(&f->r, d, d));
  81 }
  82
  83 static mp *finv(field *ff, mp *d, mp *x) {
  84   fctx_niceprime *f = (fctx_niceprime *)ff;
  85   d = mp_modinv(d, x, f->r.p);
  86   return (d);
  87 }
  88
  89 static mp *freduce(field *ff, mp *d, mp *x) {
  90   fctx_niceprime *f = (fctx_niceprime *)ff;
  91   return (mpreduce_do(&f->r, d, x));
  92 }
  93
  94 static mp *fsqrt(field *ff, mp *d, mp *x) {
  95   fctx_niceprime *f = (fctx_niceprime *)ff;
  96   return (mp_modsqrt(d, x, f->r.p));
  97 }
  98
  99 static mp *fdbl(field *ff, mp *d, mp *x) {
 100   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 1);
 101   if (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 102   return (d);
 103 }
 104
 105 static mp *ftpl(field *ff, mp *d, mp *x) {
 106   fctx_niceprime *f = (fctx_niceprime *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
 107   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3); d->f &= ~MP_UNDEF;
 108   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 109   return (d);
 110 }
 111
 112 static mp *fqdl(field *ff, mp *d, mp *x) {
 113   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 2);
 114   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 115   return (d);
 116 }
 117
 118 static mp *fhlv(field *ff, mp *d, mp *x) {
 119   fctx_niceprime *f = (fctx_niceprime *)ff;
 120   if (MP_ZEROP(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 121   if (x->v[0] & 1) { d = mp_add(d, x, f->r.p); x = d; }
 122   return (mp_lsr(d, x, 1));
 123 }
 124
 125 /* --- Field operations table --- */
 126
 127 static const field_ops fops = {
 128   FTY_PRIME, "niceprime",
 129   fdestroy, frand, field_stdsamep,
 130   freduce, field_id,
 131   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 132   0,
 133   fdbl, ftpl, fqdl, fhlv
 134 };
 135
 136 /* --- @field_niceprime@ --- *
 137  *
 138  * Arguments:   @mp *p@ = the characteristic of the field
 139  *
 140  * Returns:     A pointer to the field, or null.
 141  *
 142  * Use:         Creates a field structure for a prime field of size %$p$%,
 143  *              using efficient reduction for nice primes.
 144  */
 145
 146 field *field_niceprime(mp *p)
 147 {
 148   fctx_niceprime *f = CREATE(fctx_niceprime);
 149   f->f.ops = &fops;
 150   f->f.zero = MP_ZERO;
 151   f->f.one = MP_ONE;
 152   f->f.nbits = mp_bits(p);
 153   f->f.noctets = (f->f.nbits + 7) >> 3;
 154   if (mpreduce_create(&f->r, p)) {
 155     DESTROY(f);
 156     return (0);
 157   }
 158   f->f.m = f->r.p;
 159   f->f.q = f->r.p;
 160   return (&f->f);
 161 }
 162
 163 /*----- That's all, folks -------------------------------------------------*/