git.distorted.org.uk Git - u/mdw/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   return (mp_sub(d, f->r.p, x));
  56 }
  57
  58 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  59   fctx_niceprime *f = (fctx_niceprime *)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_niceprime *f = (fctx_niceprime *)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_niceprime *f = (fctx_niceprime *)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_niceprime *f = (fctx_niceprime *)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_niceprime *f = (fctx_niceprime *)ff;
  84   d = mp_modinv(d, x, f->r.p);
  85   return (d);
  86 }
  87
  88 static mp *freduce(field *ff, mp *d, mp *x) {
  89   fctx_niceprime *f = (fctx_niceprime *)ff;
  90   return (mpreduce_do(&f->r, d, x));
  91 }
  92
  93 static mp *fsqrt(field *ff, mp *d, mp *x) {
  94   fctx_niceprime *f = (fctx_niceprime *)ff;
  95   return (mp_modsqrt(d, x, f->r.p));
  96 }
  97
  98 static mp *fdbl(field *ff, mp *d, mp *x) {
  99   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 1);
 100   if (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 101   return (d);
 102 }
 103
 104 static mp *ftpl(field *ff, mp *d, mp *x) {
 105   fctx_niceprime *f = (fctx_niceprime *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
 106   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3); d->f &= ~MP_UNDEF;
 107   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 108   return (d);
 109 }
 110
 111 static mp *fqdl(field *ff, mp *d, mp *x) {
 112   fctx_niceprime *f = (fctx_niceprime *)ff; d = mp_lsl(d, x, 2);
 113   while (MP_CMP(d, >=, f->r.p)) d = mp_sub(d, d, f->r.p);
 114   return (d);
 115 }
 116
 117 static mp *fhlv(field *ff, mp *d, mp *x) {
 118   fctx_niceprime *f = (fctx_niceprime *)ff;
 119   if (MP_ZEROP(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 120   if (x->v[0] & 1) { d = mp_add(d, x, f->r.p); x = d; }
 121   return (mp_lsr(d, x, 1));
 122 }
 123
 124 /* --- Field operations table --- */
 125
 126 static const field_ops fops = {
 127   FTY_PRIME, "niceprime",
 128   fdestroy, frand, field_stdsamep,
 129   freduce, field_id,
 130   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 131   0,
 132   fdbl, ftpl, fqdl, fhlv
 133 };
 134
 135 /* --- @field_niceprime@ --- *
 136  *
 137  * Arguments:   @mp *p@ = the characteristic of the field
 138  *
 139  * Returns:     A pointer to the field, or null.
 140  *
 141  * Use:         Creates a field structure for a prime field of size %$p$%,
 142  *              using efficient reduction for nice primes.
 143  */
 144
 145 field *field_niceprime(mp *p)
 146 {
 147   fctx_niceprime *f = CREATE(fctx_niceprime);
 148   f->f.ops = &fops;
 149   f->f.zero = MP_ZERO;
 150   f->f.one = MP_ONE;
 151   f->f.nbits = mp_bits(p);
 152   f->f.noctets = (f->f.nbits + 7) >> 3;
 153   if (mpreduce_create(&f->r, p)) {
 154     DESTROY(f);
 155     return (0);
 156   }
 157   f->f.m = f->r.p;
 158   f->f.q = f->r.p;
 159   return (&f->f);
 160 }
 161
 162 /*----- That's all, folks -------------------------------------------------*/