[u/mdw/catacomb] / field-exp.c

/* -*-c-*-
 *
 * $Id$
 *
 * Exponentiation in finite fields
 *
 * (c) 2004 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of Catacomb.
 *
 * Catacomb is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * Catacomb is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with Catacomb; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include "field.h"
#include "field-exp.h"

/*----- Main code ---------------------------------------------------------*/

/* --- @field_exp@ --- *
 *
 * Arguments:	@field *f@ = pointer to field
 *		@mp *d@ = fake destination
 *		@mp *a@ = base
 *		@mp *e@ = exponent
 *
 * Returns:	Result, %$a^e$%.
 *
 * Use:		Exponentiation in a finite field.  Note that all quantities
 *		are in internal format.
 */

mp *field_exp(field *f, mp *d, mp *a, mp *e)
{
  mp *x = MP_COPY(f->one);
  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;

  MP_COPY(a);
  MP_SHRINK(e);
  if (MP_ZEROP(e))
    ;
  else {
    if (MP_NEGP(e))
      a = F_INV(f, a, a);
    if (MP_LEN(e) < EXP_THRESH)
      EXP_SIMPLE(x, a, e);
    else
      EXP_WINDOW(x, a, e);
  }
  mp_drop(d);
  mp_drop(spare);
  mp_drop(a);
  return (x);
}

/*----- That's all, folks -------------------------------------------------*/
Commit	Line	Data
1d7857e7	1	/* --c--
	2	*
	3	* $Id$
	4	*
	5	* Exponentiation in finite fields
	6	*
	7	* (c) 2004 Straylight/Edgeware
	8	*/
	9
45c0fd36	10	/----- Licensing notice --------------------------------------------------
1d7857e7	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.
45c0fd36	18	*
1d7857e7	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.
45c0fd36	23	*
1d7857e7	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 "field.h"
	33	#include "field-exp.h"
	34
	35	/----- Main code ---------------------------------------------------------/
	36
	37	/* --- @field_exp@ --- *
	38	*
	39	* Arguments: @field *f@ = pointer to field
	40	* @mp *d@ = fake destination
	41	* @mp *a@ = base
	42	* @mp *e@ = exponent
	43	*
	44	* Returns: Result, %$a^e$%.
	45	*
	46	* Use: Exponentiation in a finite field. Note that all quantities
	47	* are in internal format.
	48	*/
	49
	50	mp field_exp(field f, mp d, mp a, mp *e)
	51	{
	52	mp *x = MP_COPY(f->one);
	53	mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
	54
	55	MP_COPY(a);
	56	MP_SHRINK(e);
	57	if (MP_ZEROP(e))
	58	;
	59	else {
	60	if (MP_NEGP(e))
	61	a = F_INV(f, a, a);
	62	if (MP_LEN(e) < EXP_THRESH)
	63	EXP_SIMPLE(x, a, e);
	64	else
	65	EXP_WINDOW(x, a, e);
	66	}
	67	mp_drop(d);
	68	mp_drop(spare);
	69	mp_drop(a);
	70	return (x);
	71	}
	72
	73	/----- That's all, folks -------------------------------------------------/