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

   1 /* -*-c-*-
   2  *
   3  * $Id: f-niceprime.c,v 1.5 2004/04/02 01:03:49 mdw Exp $
   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 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: f-niceprime.c,v $
  33  * Revision 1.5  2004/04/02 01:03:49  mdw
  34  * Miscellaneous constification.
  35  *
  36  * Revision 1.4  2004/04/01 21:28:41  mdw
  37  * Normal basis support (translates to poly basis internally).  Rewrite
  38  * EC and prime group table generators in awk, so that they can reuse data
  39  * for repeated constants.
  40  *
  41  * Revision 1.3  2004/04/01 12:50:09  mdw
  42  * Add cyclic group abstraction, with test code.  Separate off exponentation
  43  * functions for better static linking.  Fix a buttload of bugs on the way.
  44  * Generally ensure that negative exponents do inversion correctly.  Add
  45  * table of standard prime-field subgroups.  (Binary field subgroups are
  46  * currently unimplemented but easy to add if anyone ever finds a good one.)
  47  *
  48  * Revision 1.2  2004/03/27 17:54:11  mdw
  49  * Standard curves and curve checking.
  50  *
  51  * Revision 1.1  2004/03/27 00:04:46  mdw
  52  * Implement efficient reduction for pleasant-looking primes.
  53  *
  54  */
  55
  56 /*----- Header files ------------------------------------------------------*/
  57
  58 #include <mLib/sub.h>
  59
  60 #include "field.h"
  61 #include "mpreduce.h"
  62 #include "mprand.h"
  63
  64 /*----- Main code ---------------------------------------------------------*/
  65
  66 typedef struct fctx {
  67   field f;
  68   mpreduce r;
  69 } fctx;
  70
  71 /* --- Field operations --- */
  72
  73 static void fdestroy(field *ff)
  74   { fctx *f = (fctx *)ff; mpreduce_destroy(&f->r); DESTROY(f); }
  75
  76 static mp *frand(field *ff, mp *d, grand *r)
  77   { fctx *f = (fctx *)ff; return (mprand_range(d, f->r.p, r, 0)); }
  78
  79 static int fzerop(field *ff, mp *x) { return (!MP_LEN(x)); }
  80
  81 static mp *fneg(field *ff, mp *d, mp *x)
  82   { fctx *f = (fctx *)ff; return (mp_sub(d, f->r.p, x)); }
  83
  84 static mp *fadd(field *ff, mp *d, mp *x, mp *y) {
  85   fctx *f = (fctx *)ff; d = mp_add(d, x, y);
  86   if (d->f & MP_NEG) d = mp_add(d, d, f->r.p);
  87   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  88   return (d);
  89 }
  90
  91 static mp *fsub(field *ff, mp *d, mp *x, mp *y) {
  92   fctx *f = (fctx *)ff; d = mp_sub(d, x, y);
  93   if (d->f & MP_NEG) d = mp_add(d, d, f->r.p);
  94   else if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
  95   return (d);
  96 }
  97
  98 static mp *fmul(field *ff, mp *d, mp *x, mp *y) {
  99   fctx *f = (fctx *)ff; d = mp_mul(d, x, y);
 100   return (mpreduce_do(&f->r, d, d));
 101 }
 102
 103 static mp *fsqr(field *ff, mp *d, mp *x) {
 104   fctx *f = (fctx *)ff; d = mp_sqr(d, x);
 105   return (mpreduce_do(&f->r, d, d));
 106 }
 107
 108 static mp *finv(field *ff, mp *d, mp *x)
 109   { fctx *f = (fctx *)ff; mp_gcd(0, 0, &d, f->r.p, x); return (d); }
 110
 111 static mp *freduce(field *ff, mp *d, mp *x)
 112   { fctx *f = (fctx *)ff; return (mpreduce_do(&f->r, d, x)); }
 113
 114 static mp *fsqrt(field *ff, mp *d, mp *x)
 115   { fctx *f = (fctx *)ff; return (mp_modsqrt(d, x, f->r.p)); }
 116
 117 static mp *fdbl(field *ff, mp *d, mp *x) {
 118   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 1);
 119   if (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
 120   return (d);
 121 }
 122
 123 static mp *ftpl(field *ff, mp *d, mp *x) {
 124   fctx *f = (fctx *)ff; MP_DEST(d, MP_LEN(x) + 1, x->f);
 125   MPX_UMULN(d->v, d->vl, x->v, x->vl, 3);
 126   while (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
 127   return (d);
 128 }
 129
 130 static mp *fqdl(field *ff, mp *d, mp *x) {
 131   fctx *f = (fctx *)ff; d = mp_lsl(d, x, 2);
 132   while (MP_CMP(d, >, f->r.p)) d = mp_sub(d, d, f->r.p);
 133   return (d);
 134 }
 135
 136 static mp *fhlv(field *ff, mp *d, mp *x) {
 137   fctx *f = (fctx *)ff;
 138   if (!MP_LEN(x)) { MP_COPY(x); MP_DROP(d); return (x); }
 139   if (x->v[0] & 1) { d = mp_add(d, x, f->r.p); x = d; }
 140   return (mp_lsr(d, x, 1));
 141 }
 142
 143 /* --- Field operations table --- */
 144
 145 static const field_ops fops = {
 146   FTY_PRIME, "niceprime",
 147   fdestroy, frand, field_stdsamep,
 148   freduce, field_id,
 149   fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
 150   0,
 151   fdbl, ftpl, fqdl, fhlv
 152 };
 153
 154 /* --- @field_niceprime@ --- *
 155  *
 156  * Arguments:   @mp *p@ = the characteristic of the field
 157  *
 158  * Returns:     A pointer to the field.
 159  *
 160  * Use:         Creates a field structure for a prime field of size %$p$%,
 161  *              using efficient reduction for nice primes.
 162  */
 163
 164 field *field_niceprime(mp *p)
 165 {
 166   fctx *f = CREATE(fctx);
 167   f->f.ops = &fops;
 168   f->f.zero = MP_ZERO;
 169   f->f.one = MP_ONE;
 170   f->f.nbits = mp_bits(p);
 171   f->f.noctets = (f->f.nbits + 7) >> 3;
 172   mpreduce_create(&f->r, p);
 173   f->f.m = f->r.p;
 174   return (&f->f);
 175 }
 176
 177 /*----- That's all, folks -------------------------------------------------*/