pub/dh-kcdsa.c: Choose the starting point for the right result size.

[catacomb] / pub / dh-kcdsa.c

/* -*-c-*-
 *
 * Generate KCDSA prime groups
 *
 * (c) 2006 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 <mLib/macros.h>

#include "dh.h"
#include "mprand.h"
#include "pgen.h"
#include "prim.h"

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

/* --- @dh_kcdsagen@ --- *
 *
 * Arguments:	@dh_param *dp@ = pointer to output parameter block
 *		@unsigned ql@ = size of small factor of %$(p - 1)/2$%
 *		@unsigned pl@ = size of %$p$% in bits
 *		@unsigned flags@ = other generation flags
 *		@unsigned steps@ = number of steps to go
 *		@grand *r@ = random number source
 *		@pgen_proc *ev@ = event handler function
 *		@void *ec@ = context for the event handler
 *
 * Returns:	@PGEN_DONE@ if it worked, @PGEN_ABORT@ if it failed.
 *
 * Use:		Generates a KCDSA prime group.  That is, it chooses a prime
 *		%$p$%, such that $%p = 2 q v + 1$%, for primes %$q$% and
 *		%$v$%.  The actual group of interest is the subgroup of order
 *		%$q$%.
 */

int dh_kcdsagen(dh_param *dp, unsigned ql, unsigned pl,
		unsigned flags, unsigned steps, grand *r,
		pgen_proc *ev, void *ec)
{
  pgen_filterctx pf;
  pgen_simulprime sp[2];
  pgen_simulctx ss;
  prim_ctx pc;
  rabin rb;
  int rc = PGEN_ABORT;
  int i;
  mp *x = MP_NEW, *t = MP_NEW;

  /* --- First trick: find %$v$% --- */

  pf.step = 2;
  x = mprand(x, pl - ql - 1, r, 1);
  x = pgen("v", x, x, ev, ec,
	   steps, pgen_filter, &pf,
	   rabin_iters(pl - ql), pgen_test, &rb);
  if (!x)
    goto fail_0;

  /* --- Second trick: find %$p$% and %$q$% --- */

  x = mp_lsl(x, x, 1);
  sp[0].add = MP_ZERO; sp[0].mul = MP_ONE; sp[0].f = 0;
  sp[1].add = MP_ONE; sp[1].mul = x; sp[1].f = PGENF_KEEP; x = MP_NEW;
  ss.step = MP_TWO; ss.v = sp; ss.n = N(sp);
  do {
    x = mprand(x, ql, r, 1);
    t = mp_mul(t, x, sp[1].mul);
  } while (mp_bits(t) != pl);
  dp->q = pgen("p", MP_NEW, x, ev, ec,
	       steps, pgen_simulstep, &ss,
	       rabin_iters(ql), pgen_simultest, &ss);
  mp_drop(sp[1].mul);
  dp->p = sp[1].u.x;
  if (!dp->q)
    goto fail_1;

  /* --- Third trick: find a generator --- */

  mpmont_create(&pc.mm, dp->p);
  mp_div(&x, 0, dp->p, dp->q);
  i = 0;
  pc.exp = x;
  pc.n = 0;
  dp->g = pgen("g", MP_NEW, MP_NEW, ev, ec,
	       0, prim_step, &i, 1, prim_test, &pc);
  mpmont_destroy(&pc.mm);
  if (!dp->g)
    goto fail_1;

  rc = PGEN_DONE;
  goto done;

  /* --- Tidying up and going home --- */

fail_1:
  mp_drop(dp->p);
  mp_drop(dp->q);
fail_0:
done:
  mp_drop(x);
  mp_drop(t);
  return (rc);
}

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