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

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