3 * $Id: f-niceprime.c,v 1.3 2004/04/01 12:50:09 mdw Exp $
5 * Prime fields with efficient reduction for special-form primes
7 * (c) 2004 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
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.
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.
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,
30 /*----- Revision history --------------------------------------------------*
32 * $Log: f-niceprime.c,v $
33 * Revision 1.3 2004/04/01 12:50:09 mdw
34 * Add cyclic group abstraction, with test code. Separate off exponentation
35 * functions for better static linking. Fix a buttload of bugs on the way.
36 * Generally ensure that negative exponents do inversion correctly. Add
37 * table of standard prime-field subgroups. (Binary field subgroups are
38 * currently unimplemented but easy to add if anyone ever finds a good one.)
40 * Revision 1.2 2004/03/27 17:54:11 mdw
41 * Standard curves and curve checking.
43 * Revision 1.1 2004/03/27 00:04:46 mdw
44 * Implement efficient reduction for pleasant-looking primes.
48 /*----- Header files ------------------------------------------------------*/
56 /*----- Data structures ---------------------------------------------------*/
63 /*----- Main code ---------------------------------------------------------*/
65 /* --- Field operations --- */
67 static void fdestroy(field *ff)
70 mpreduce_destroy(&f->r);
74 static mp *frand(field *ff, mp *d, grand *r)
77 return (mprand_range(d, f->r.p, r, 0));
80 static int fzerop(field *ff, mp *x)
85 static mp *fneg(field *ff, mp *d, mp *x)
88 return (mp_sub(d, f->r.p, x));
91 static mp *fadd(field *ff, mp *d, mp *x, mp *y)
96 d = mp_add(d, d, f->r.p);
97 else if (MP_CMP(d, >, f->r.p))
98 d = mp_sub(d, d, f->r.p);
102 static mp *fsub(field *ff, mp *d, mp *x, mp *y)
104 fctx *f = (fctx *)ff;
107 d = mp_add(d, d, f->r.p);
108 else if (MP_CMP(d, >, f->r.p))
109 d = mp_sub(d, d, f->r.p);
113 static mp *fmul(field *ff, mp *d, mp *x, mp *y)
115 fctx *f = (fctx *)ff;
117 return (mpreduce_do(&f->r, d, d));
120 static mp *fsqr(field *ff, mp *d, mp *x)
122 fctx *f = (fctx *)ff;
124 return (mpreduce_do(&f->r, d, d));
127 static mp *finv(field *ff, mp *d, mp *x)
129 fctx *f = (fctx *)ff;
130 mp_gcd(0, 0, &d, f->r.p, x);
134 static mp *freduce(field *ff, mp *d, mp *x)
136 fctx *f = (fctx *)ff;
137 return (mpreduce_do(&f->r, d, x));
140 static mp *fsqrt(field *ff, mp *d, mp *x)
142 fctx *f = (fctx *)ff;
143 return (mp_modsqrt(d, x, f->r.p));
146 static mp *fdbl(field *ff, mp *d, mp *x)
148 fctx *f = (fctx *)ff;
150 if (MP_CMP(d, >, f->r.p))
151 d = mp_sub(d, d, f->r.p);
155 static mp *ftpl(field *ff, mp *d, mp *x)
157 fctx *f = (fctx *)ff;
158 MP_DEST(d, MP_LEN(x) + 1, x->f);
159 MPX_UMULN(d->v, d->vl, x->v, x->vl, 3);
160 while (MP_CMP(d, >, f->r.p))
161 d = mp_sub(d, d, f->r.p);
165 static mp *fqdl(field *ff, mp *d, mp *x)
167 fctx *f = (fctx *)ff;
169 while (MP_CMP(d, >, f->r.p))
170 d = mp_sub(d, d, f->r.p);
174 static mp *fhlv(field *ff, mp *d, mp *x)
176 fctx *f = (fctx *)ff;
183 d = mp_add(d, x, f->r.p);
186 return (mp_lsr(d, x, 1));
189 /* --- Field operations table --- */
191 static field_ops fops = {
192 FTY_PRIME, "niceprime",
193 fdestroy, frand, field_stdsamep,
195 fzerop, fneg, fadd, fsub, fmul, fsqr, finv, freduce, fsqrt,
197 fdbl, ftpl, fqdl, fhlv
200 /* --- @field_niceprime@ --- *
202 * Arguments: @mp *p@ = the characteristic of the field
204 * Returns: A pointer to the field.
206 * Use: Creates a field structure for a prime field of size %$p$%,
207 * using efficient reduction for nice primes.
210 field *field_niceprime(mp *p)
212 fctx *f = CREATE(fctx);
216 f->f.nbits = mp_bits(p);
217 f->f.noctets = (f->f.nbits + 7) >> 3;
218 mpreduce_create(&f->r, p);
223 /*----- That's all, folks -------------------------------------------------*/