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

   1 /* -*-c-*-
   2  *
   3  * $Id: f-prime.c,v 1.12 2004/04/08 01:36:15 mdw Exp $
   4  *
   5  * Prime fields with Montgomery arithmetic
   6  *
   7  * (c) 2001 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 "mpmont.h"
  36 #include "mprand.h"
  37
  38 /*----- Main code ---------------------------------------------------------*/
  39
  40 typedef struct fctx {
  41   field f;
  42   mpmont mm;
  43 } fctx;
  44
  45 /* --- Field operations --- */
  46
  47 static void fdestroy(field *ff)
  48   { fctx *f = (fctx *)ff; mpmont_destroy(&f->mm); DESTROY(f); }
  49
  50 static mp *frand(field *ff, mp *d, grand *r)
  51   { fctx *f = (fctx *)ff; return (mprand_range(d, f->mm.m, r, 0)); }
  52
  53 static mp *fin(field *ff, mp *d, mp *x) {
  54   fctx *f = (fctx *)ff;
  55   mp_div(0, &d, x, f->mm.m);
  56   return (mpmont_mul(&f->mm, d, d, f->mm.r2));
  57 }
  58
  59 static mp *fout(field *ff, mp *d, mp *x)
  60   { fctx *f = (fctx *)ff; return (mpmont_reduce(&f->mm, d, x)); }
  61
  62 static int fzerop(field *ff, mp *x) { return (!MP_LEN(x)); }
  63
  64 static mp *fneg(field *ff, mp *d, mp *x)
  65   { fctx *f = (fctx *)ff; return (mp_sub(d, f->mm.m, x)); }
  66
  67 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  68   fctx *f = (fctx *)ff; d = mp_add(d, x, y);
  69   if (d->f & MP_NEG) d = mp_add(d, d, f->mm.m);
  70   else if (MP_CMP(d, >, f->mm.m)) d = mp_sub(d, d, f->mm.m);
  71   return (d);
  72 }
  73
  74 static mp *fsub(field *ff, mp *d, mp *x, mp *y) {
  75   fctx *f = (fctx *)ff; d = mp_sub(d, x, y);
  76   if (d->f & MP_NEG) d = mp_add(d, d, f->mm.m);
  77   else if (MP_CMP(d, >, f->mm.m)) d = mp_sub(d, d, f->mm.m);
  78   return (d);
  79 }
  80
  81 static mp *fmul(field *ff, mp *d, mp *x, mp *y)
  82   { fctx *f = (fctx *)ff; return (mpmont_mul(&f->mm, d, x, y)); }
  83
  84 static mp *fsqr(field *ff, mp *d, mp *x) {
  85   fctx *f = (fctx *)ff; d = mp_sqr(d, x);
  86   return (mpmont_reduce(&f->mm, d, d));
  87 }
  88
  89 static mp *finv(field *ff, mp *d, mp *x) {
  90   fctx *f = (fctx *)ff; d = mpmont_reduce(&f->mm, d, x);
  91   d = mp_modinv(d, d, f->mm.m); return (mpmont_mul(&f->mm, d, d, f->mm.r2));
  92 }
  93
  94 static mp *freduce(field *ff, mp *d, mp *x)
  95   { fctx *f = (fctx *)ff; mp_div(0, &d, x, f->mm.m); return (d); }
  96
  97 static mp *fsqrt(field *ff, mp *d, mp *x) {
  98   fctx *f = (fctx *)ff; d = mpmont_reduce(&f->mm, d, x);
  99   d = mp_modsqrt(d, d, f->mm.m); if (!d) return (d);
 100   return (mpmont_mul(&f->mm, d, d, f->mm.r2));
 101 }
 102
 103 static mp *fdbl(field *ff, mp *d, mp *x) {
 104   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 1);
 105   if (MP_CMP(d, >, f->mm.m)) d = mp_sub(d, d, f->mm.m);
 106   return (d);
 107 }
 108
 109 static mp *ftpl(field *ff, mp *d, mp *x) {
 110   fctx *f = (fctx *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
 111   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3);
 112   while (MP_CMP(d, >, f->mm.m)) d = mp_sub(d, d, f->mm.m);
 113   return (d);
 114 }
 115
 116 static mp *fqdl(field *ff, mp *d, mp *x) {
 117   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 2);
 118   while (MP_CMP(d, >, f->mm.m)) d = mp_sub(d, d, f->mm.m);
 119   return (d);
 120 }
 121
 122 static mp *fhlv(field *ff, mp *d, mp *x) {
 123   fctx *f = (fctx *)ff;
 124   if (!MP_LEN(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 125   if (x->v[0] & 1) { d = mp_add(d, x, f->mm.m); x = d; }
 126   return (mp_lsr(d, x, 1));
 127 }
 128
 129 /* --- Field operations table --- */
 130
 131 static const field_ops fops = {
 132   FTY_PRIME, "prime",
 133   fdestroy, frand, field_stdsamep,
 134   fin, fout,
 135   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 136   0,
 137   fdbl, ftpl, fqdl, fhlv
 138 };
 139
 140 /* --- @field_prime@ --- *
 141  *
 142  * Arguments:   @mp *p@ = the characteristic of the field
 143  *
 144  * Returns:     A pointer to the field or null.
 145  *
 146  * Use:         Creates a field structure for a prime field of size %$p$%,
 147  *              using Montgomery reduction for arithmetic.
 148  */
 149
 150 field *field_prime(mp *p)
 151 {
 152   fctx *f;
 153
 154   if (!MP_ISPOS(p) || !MP_ISODD(p))
 155     return (0);
 156   f = CREATE(fctx);
 157   f->f.ops = &fops;
 158   mpmont_create(&f->mm, p);
 159   f->f.zero = MP_ZERO;
 160   f->f.one = f->mm.r;
 161   f->f.m = f->mm.m;
 162   f->f.nbits = mp_bits(p);
 163   f->f.noctets = (f->f.nbits + 7) >> 3;
 164   return (&f->f);
 165 }
 166
 167 /*----- That's all, folks -------------------------------------------------*/