Fix behaviour with bogus trailing attributes.

[u/mdw/catacomb] / ec-prime.c

/* -*-c-*-
 *
 * $Id: ec-prime.c,v 1.2 2002/01/13 13:48:44 mdw Exp $
 *
 * Elliptic curves over prime fields
 *
 * (c) 2001 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.
 */

/*----- Revision history --------------------------------------------------* 
 *
 * $Log: ec-prime.c,v $
 * Revision 1.2  2002/01/13 13:48:44  mdw
 * Further progress.
 *
 * Revision 1.1  2001/04/29 18:12:33  mdw
 * Prototype version.
 *
 */

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

#include "ec.h"

/*----- Data structures ---------------------------------------------------*/

typedef struct ecctx {
  ec_curve c;
  mp *a, *b;
} ecctx;

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

static ec *ecneg(ec_cuvrve *c, ec *d, const ec *p)
{
  EC_COPY(d, p);
  d->y = F_NEG(c->f, d->y, d->y);
  return (d);
}

static ec *ecdbl(ec_curve *c, ec *d, const ec *a)
{
  if (EC_ATINF(a))
    EC_SETINF(d);
  else if (!MP_LEN(a->y))
    EC_COPY(d, a);
  else {
    field *f = c->f;
    ecctx *cc = (ecctx *)c;
    mp *lambda;
    mp *dy, *dx;

    dx = F_SQR(f, MP_NEW, a->x);
    dy = F_DBL(f, MP_NEW, a->y);
    dx = F_TPL(f, dx, dx);
    dx = F_ADD(f, dx, dx, cc->a);
    dy = F_INV(f, dy, dy);
    lambda = F_MUL(d, MP_NEW, dx, dy);

    dx = F_SQR(f, dx, lambda);
    dy = F_DBL(d, dy, a->x);
    dx = F_SUB(f, dx, dx, dy);
    dy = F_SUB(f, dy, a->x, dx);
    dy = F_MUL(f, dy, lambda, dy);
    dy = F_SUB(f, dy, dy, a->y);

    EC_DESTROY(d);
    d->x = dx;
    d->y = dy;
    d->z = 0;
    MP_DROP(lambda);
  }
  return (d);
}

static ec *ecadd(ec_curve *c, ec *d, const ec *a, const ec *b)
{
  if (a == b)
    ecdbl(c, d, a);
  else if (EC_ATINF(a))
    EC_COPY(d, b);
  else if (EC_ATINF(b))
    EC_COPY(d, a);
  else {
    field *f = c->f;
    mp *lambda;
    mp *dy, *dx;

    if (!MP_EQ(a->x, b->x)) {
      dy = F_SUB(f, MP_NEW, a->y, b->y);
      dx = F_SUB(f, MP_NEW, a->x, b->x);
      dx = F_INV(f, dx, dx);
      lambda = F_MUL(f, MP_NEW, dy, dx);
    } else if (!MP_LEN(a->y) || !MP_EQ(a->y, b->y)) {
      EC_SETINF(d);
      return (d);
    } else {
      ecctx *cc = (ecctx *)c;
      dx = F_SQR(f, MP_NEW, a->x);
      dx = F_TPL(f, dx, dx);
      dx = F_ADD(f, dx, dx, cc->a);
      dy = F_DBL(f, MP_NEW, a->y);
      dy = F_INV(f, dy, dy);
      lambda = F_MUL(d, MP_NEW, dx, dy);
    }

    dx = F_SQR(f, dx, lambda);
    dx = F_SUB(f, dx, dx, a->x);
    dx = F_SUB(f, dx, dx, b->x);
    dy = F_SUB(f, dy, b->x, dx);
    dy = F_MUL(f, dy, lambda, dy);
    dy = F_SUB(f, dy, dy, b->y);

    EC_DESTROY(d);
    d->x = dx;
    d->y = dy;
    d->z = 0;
    MP_DROP(lambda);
  }
  return (d);
}

/*----- That's all, folks -------------------------------------------------*/